skf rolling bearings information

Rolling bearings in industrial gearboxes

Copyright SKF 1997The contents of this publication are thecopyright of the publisher and may notbe reproduced (even extracts) unlesspermission is granted. Every care hasbeen taken to ensure the accuracy ofthe information contained in this publi-cation but no liability can be acceptedfor any loss or damage whether direct,indirect or consequential arising out ofthe use of the information containedherein.

Publication 4560 E

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1 Industrial gearboxes overview

2 Bearing types for industrial gearboxes

3 Design of bearing arrangements

4 Dimensioning the bearing arrangement

5 Lubrication and maintenance

6 Recommended fits

7 Mounting and dismounting bearings

8 Application examples

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Rolling bearings inindustrial gearboxes

Handbook for the gearbox designer

Foreword

This Handbook is intended to provide the gearbox designer

with the knowledge required to select bearings for gearboxes

and to correctly design gearbox bearing arrangements. Recom-

mendations are given based on experience gained by SKF

during decades of cooperation with gearbox manufacturers the

world over.

General information regarding the selection, calculation,

mounting and maintenance of ball and roller bearings is given

in the SKF General Catalogue. The questions arising from the

use of rolling bearings in industrial gearboxes are dealt with

here. Data from the General Catalogue are only repeated here

when it has been thought necessary for the sake of clarity.

The application examples described comprise proven

gearbox designs from major manufacturers which are worthy

of note.

Grateful thanks are extended to the companies concerned

for the provision of the detailed information about their prod-

ucts and the permission to publish.

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41 Industrial gearboxes overview............................... 9Types of gearbox............................................................ 9

Geared transmissions.................................................... 10

Demands made on gearboxes ...................................... 14

Selecting the gears ........................................................ 14

Designing the casing ..................................................... 15

2 Bearing types for industrial gearboxes.................. 17Deep groove ball bearings ............................................ 18

Angular contact ball bearings....................................... 20

Cylindrical roller bearings ............................................. 22

CARB roller bearings ................................................. 24

Spherical roller bearings ............................................... 26

Taper roller bearings ...................................................... 28

Spherical roller thrust bearings .................................... 30

3 Design of bearing arrangements............................. 33Shafts and gear wheels in spur gearboxes ................. 33

Shafts in bevel gearboxes ............................................. 44

Shafts in worm gearboxes............................................. 50

Shafts and gear wheels for planetary gearboxes........ 56

Made by SKF stands for excellence. It symbolises our consistent endeavour to achieve total quality in everything we do. For those who use our products,Made by SKF implies three main benefits.

Reliability thanks to modern, efficient products, basedon our worldwide application know-how, optimised materials, forward-looking designs and the most advanced production techniques.

Cost effectiveness resulting from the favourable ratiobetween our product quality plus service facilities, andthe purchase price of the product.

Market lead which you can achieve by taking advantage of our products and services. Increased operating time and reduced down-time, as well as improved output and product quality are the key to a successful partnership.

Contents

54 Calculation of bearing arrangement ....................... 65Bearing loads ................................................................. 65

Determination of external forces .................................. 66

Calculation of bearing loads ......................................... 74

Dimensioning the bearing arrangement ...................... 76

5 Lubrication and maintenance.................................. 91Grease lubrication.......................................................... 92

Oil lubrication ................................................................. 95

Maintenance ................................................................... 98

6 Recommended fits..................................................103

7 Mounting and dismounting bearings....................109Adjustment of angular contact bearings....................109

8 Application examples ............................................. 115


6SKF is an international industrial Groupoperating in some 130 countries and isworld leader in bearings.

The company was founded in 1907following the invention of the self-align-ing ball bearing by Sven Wingquist and,after only a few years, SKF began toexpand all over the world.

Today, SKF has some 43 000 em-ployees and more than 80 manufactur-ing facilities spread throughout theworld. An international sales network includes a large number of sales com-panies and some 20 000 distributorsand retailers. Worldwide availability ofSKF products is supported by a com-prehensive technical advisory service.

The key to success has been a con-sistent emphasis on maintaining the

highest quality of its products and services. Continuous investment inresearch and development has alsoplayed a vital role, resulting in manyexamples of epoch-making innovations.

The business of the Group consistsof bearings, seals, special steel and acomprehensive range of other high-tech industrial components. The ex-perience gained in these various fieldsprovides SKF with the essential know-ledge and expertise required in orderto provide the customers with the mostadvanced engineering products andefficient service.

The SKF Group a worldwide corporation

SKF manufacturesball bearings, roller bearings and plainbearings. The smal-lest are just a fewmillimetres (a frac-tion of an inch) indiameter, the largestseveral metres. Inorder to protect thebearings effectivelyagainst the ingressof contaminationand the escape oflubricant, SKF alsomanufactures oiland bearing seals.SKF's subsidiariesCR and RFT S.p.A.are among the world's largest pro-ducers of seals.

7The SKF house colours are blue and red,but the thinking is green. The latest exampleis the new factory in Malaysia, where thebearing component cleaning process con-forms to the strictest ecological standards.Instead of trichloroethylene, a water-basedcleaning fluid is used in a closed system.The cleaning fluid is recycled in the factory'sown treatment plant.

SKF has developed the Channel concept infactories all over the world. This drasticallyreduces the lead time from raw material toend product as well as work in progress and finished goods in stock. The conceptenables faster and smoother informationflow, eliminates bottlenecks and bypassesunnecessary steps in production. TheChannel team members have the know-ledge and commitment needed to share theresponsibility for fulfilling objectives in areassuch as quality, delivery time, productionflow etc.

The SKF Engineering & Research Centre is situated just outside Utrecht in TheNetherlands. In an area of 17 000 squaremetres (185 000 sq.ft) some 150 scientists,engineers and support staff are engaged inthe further improvement of bearing perform-ance. They are developing technologiesaimed at achieving better materials, betterdesigns, better lubricants and better seals together leading to an even better unders-tanding of the operation of a bearing in itsapplication. This is also where the SKF New Life Theory was evolved, enabling thedesign of bearings which are even morecompact and offer even longer operationallife.

1 Industrial gearboxes overview

Types of gearbox . . . . . . . . . . 9

Geared transmission . . . . . . 10

Demands on gearboxes . . . 14

Selecting the gears . . . . . . . 14

Designing the casing . . . . . . 15

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Industrial gearboxes overview

Gearboxes are devices for the transmission ortranslation of movement. In industry gearboxesare used to transform the speeds and torquesproduced by the prime mover in order thatthey are appropriate to the machine which is tobe driven. The speeds and torques required bythe machine are dictated by its use. Primemovers can generally only meet these require-ments when combined with gears.

Types of gearboxGearboxes are characterised by havingat least three members: the power in-put, power take-off and the casing. Thecasing transmits the support momentto the base.

In contrast, a coupling has only twomembers: the power input and power

1 Industrial gearboxes overviewTypes of gearbox

P1

M1

PV

P2

M2

n2

n1

P2P1M2

Pv

M1 n1 n2

take-off. The coupling housing has nopart in the flow of force.

The symbols used for powertransmission by gearboxes and coup-lings are shown in figs and .21

Fig 1 Fig 2GearTorqueM1 < M2>

Rotational speedn1 n2>

PowerP1 = P2 + Pv

CouplingTorqueM1 = M2

Rotational speedn1 n2

PowerP1 = P2 + Pv

(with slip)

Geared transmissionsGeared transmissions are the mostcommonly used. They transmit powerwithout slip, have high operational re-liability and long life, require little main-tenance and are characterised by theability to accept overloading, small sizeand high efficiency.

Spur gearsThe spur gear is the most well-knownand commonly used design of gearedtransmission. The dimensioning andmanufacture of the gear wheels arethe easiest to control. Their kinematicbehaviour also forms the basis of plan-etary gears. Spur gears are in rollingcontact and, irrespective of tooth type,have parallel axes.

1 Industrial gearboxes overviewGeared transmissions

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The main types ofpower transmis-sion equipmentare shown in thefollowing.In addition, thereare many com-binations, for ex-ample bevel/spurgears, spur gearswith belt drive input,or variable tractiondrives combinedwith a planetarygear.

Types of gearbox

Fixed ratio transmissions, shift transmission

Geared transmissions Spur gears Planetary gears Bevel gears Worm gears Hypoid gears Helical gears

Eccentric drives Cyclo drives Harmonic drives

Traction drives Belt drives Chain drives

Infinitely variable transmissions

Mechanical transmissions Belt drives Roller drives Ratchet gears

Hydraulic transmissions Hydrostatic transmissions Hydrodyanmic transmissions

Gear wheels with straight cut teeth( fig a) are simple in designand can be accurately produced.The axial forces generated by in-accuracies and deformations (twisting) are negligible.

Gear wheels with helical teeth ( fig b) run more smoothly andcan carry heavier loads than thosewith straight cut teeth. A more elab-orate bearing arrangement is re-quired because of the axial forces.

The double helix or herringbone ( fig c) allows for large toothwidths and can carry particularlyheavy loads. The axial forces canceleach other out. Deviations in thehelix angle cause axial vibrations.

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Internal gearing ( fig d) hasgreater load carrying capacity thanexternal because of the favourableosculation, but is more difficult toproduce. The bearing arrangementis more complicated. The most fre-quent use is in planetary gears.

Bevel gearsThe common characteristic of this type of rolling contact gearing is thatthe axes of the wheels intersect eachother. There are three basic designscategorised by the form of the flank.

With straight cut teeth ( fig a),the mesh begins and ends acrossthe total tooth width. The noise pro-duced considerably limits the use-fulness of straight cut bevel gears.

Bevel gears with helical teeth ( fig b) have straight flanks.The teeth are usually ground andthe mesh is gradual. The total over-lap is bigger and the noise behav-iour better than with straight cutteeth.

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3 Bevel gears having spirally cut teeth( fig c) with curved flanks haveclear advantages in respect of loadcarrying capacity. Particularly thosewith ground teeth are quieter thanthe types described above. Forbevel gears which have to transmithigh power, the spiral bevel gearsare the most frequently used.

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Spur gear unita) straight cut teethb) helical teethc) double helixd) internal gearing

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Fig 3

Fig 4

a b c d

Bevel gear unita) straight cut teethb) helical teethc) spirally cut teeth

a b c


Worm gearsThe worm and wheel axes cross eachother at a considerable distance andusually at an angle < 90 ( fig ).Worm gears are suitable for large single stage speed reduction. Theiroperation is quiet and vibration damp-ing. The efficiency is lower than that ofcompeting bevel/spur and planetarygears, because of the higher propor-tion of sliding motion. To reduce thefriction, the use of synthetic lubricantsis favoured.

The most commonly used design isthe cylindrical worm paired with a glob-oid wheel ( fig a). The cylindricalworm can be hardened and groundwhich improves load carrying capacity;it is also freely adjustable in the axialdirection so that bearing arrangementand mounting can be simplified. Twoother designs globoid worm with spurwheel ( fig b) and globoid wormwith globoid wheel ( fig c) arealso used.

Depending on the flank form, theworm types are classified as follows:

ZA worm: trapezoidal worm threadin the axial cross section;

ZN worm: trapezoidal worm threadin the normal cross section;

ZK worm; trapezoidal tool (in normalcross section);

ZI worm; evolvent thread in end facecross section;

ZC worm: concave worm flanks

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Hypoid gearsThe pinion axis is displaced so that theaxes of this type of bevel gear do notintersect but are crossed ( fig ).

The wheels of hypoid gears are usu-ally spirally cut. The advantages of thistype of gear derive from the larger pin-ion and thus the smaller circumferentialforce for the same torque, as well asfrom the axis displacement which oftenallows the pinion to be supported atboth sides so that the bearing arrange-ment is stiffer. The noise behaviour isalso improved by the sliding motion inthe longitudinal direction of the teeth.However, the additional sliding motionincreases the friction, wear and risk of smearing and requires the use ofhypoid oils with high additive content.

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Fig 5Hypoid gear unit

Fig 6

a b c

Worm gear unita) cylindrical worm

with globoidwheel

b) globoid wormwith spur wheel

c) globoid wormwith globoidwheel

HSP

Z

The ZI and ZC designs are the mostpopular. The ZI worm can be very ac-curately ground whilst the favourableosculation conditions of the ZC worm(concave worm, convex wheel) bringload carrying advantages.

Planetary gearsFrom the point of view of the toothflanks, planetary gears are mostly spurgears. In contrast to the spur gear unitsso far described, the shafts of whichare supported in stationary casings,the planetary gear unit has gear wheelswhich circulate. They are also referredto as epicyclic gears.

In the simplest design ( fig ),which is that most commonly used inindustry, the sun wheel drives the plan-etary wheels (when acting as a speedreducer). These are supported in thehollow wheel and drive the planetarycarrier from which the power is takenoff.

Planetary gears have the followingimportant advantages compared withconventional spur gear units:

the volume, weight and centrifugalmass are smaller;

the rolling and sliding velocities inthe mesh are lower, so that noise isreduced;

some of the power is transmitted ascoupling power, so that efficiency ishigher.

These advantages have led to a continuous increase in the economic importance of planetary gear units inspite of their disadvantages whichinclude more difficult inspection, main-tenance and repairs.

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Simple planetarygear unit (prin-ciple)Z sun wheelP planetary wheelH hollow wheelS planetary carrier

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Fig 7

Demands made on gearboxesThe most important demands whichmust be fulfilled are:

there must be a sufficient safetymargin in respect of fatigue and/orrequisite life for all components sothat the torques and speeds can bereliably transmitted;

there must be sufficient cooling evenunder maximum power transmissionconditions;

noise emission should not exceedthe permitted limits.

In addition to these demands, specialrequirements in respect of operationand design are dictated by the variousapplications. Some examples:

radial and/or axial forces on the in-put and output shafts, e.g. for ex-truders;

external forces on the casing, e.g. inmining;

heavy impacts, torque peaks, e.g.when driven by single cylinder com-bustion engines or when drivingbucket excavators;

vibrations, e.g. in wire drawing; extreme environmental influences

in respect of temperature, dirt, dust,water, e.g. in arctic or tropical opencast mining and in continuous cast-ing plant;

seals subjected to pressure, e.g. insubmerged gearboxes of dredgersor in mixing equipment in the chemi-cal industry;

reversing operation, e.g. for rollingmills;

return stop, e.g. for conveyors; operation with little or no clearance

and torsional stiffness, e.g. for posi-tioning antennae and for robots;

precision, e.g. for printing presses; lubrication with non-flammable lub-

ricants, e.g. in mining; minimum maintenance, e.g. in wind

power plant; arrangement, e.g. slip-on gears for

converters; accessibility of measuring points to

monitor lubrication, temperature,vibrations or torque, e.g. for largeplastic extruders.

Selecting the gearsTo avoid either under or over-dimen-sioning a gear unit the load and theload carrying capacity of the gear mustbe able to be determined as accuratelyand reliably as possible. The size iscorrectly chosen when a comparisonof the load spectrum and the load carrying capacity gives the desiredservice life. The determination of theload spectrum is a time-consumingand costly exercise calling for con-siderable measurements. Therefore,dimensioning is usually based on therated torque of the driven machine, i.e. the operating torque for the mostarduous work conditions. For a rollingmill, for example, this is the maximumcontinuous rolling torque (not the initialentry). The actual loads are higherbecause of additional external forces,produced by accelerations and vibra-tions, for example. When calculatingthe load carrying capacity of the gearwheels, these additional loads are considered by an application factor KA according to DIN 3990.

One standard work on the subjectlists the following criteria for evaluatingthe load carrying capacity of gear whe-els:

resistance to pitting (tooth flank fatigue),

root strength (tooth fracture fromfatigue),

resistance to scuffing (hot toothflank welding),

wear strength (slow wear of toothflanks),

grey spot resistance (fatigue frommicro pores on the tooth flanks, and

lubricant film formation.

The load carrying capacity which isused as the basis for dimensioninggear wheels is determined in rig testsunder standard conditions (partly stand-ardised: FZG test to DIN 51 354).

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1 Industrial gearboxes overviewDemands made on gearboxes/Selecting the gears

Designing the casingThe following functions have a de-cisive influence on the design of thecasing:

forces and supporting momentsmust be taken up and transmiitted atthe same time as the position of thegear wheels and the form of thebearing seatings must be accuratelymaintained;

there must be adequate heat removal;

noise radiation must be at a min-imum;

gear wheels and bearings must beprotected against contamination byforeign matter;

lubricant loss must be prevented.

The increase in load carrying capacityof gear wheels and rolling bearingsresulting from design improvements,improved materials and enhancedquality has enabled gearboxes to bedownsized or uprated. The higher specific loads, frictional losses and in-creased noise resulting from this trendmean that the casings must be morestable so as to keep deformations to aminimum, but also that they shouldhave a sufficiently large surface to pre-vent inadmissible heating and prema-ture lubricant ageing, and should beproperly designed with respect to mini-mising noise so as not to exceed thenoise emission limits.

1 Industrial gearboxes overviewDesigning tha casing

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2 Bearing types for industrialgearboxes

Deep groove ball bearings . 18

Angular contact ball bearings . . . . . . . . . . . . . . . . 20

Cylindrical roller bearings . . 22

CARB roller bearings . . . . 24

Spherical roller bearings . . . 26

Taper roller bearings . . . . . . 28

Spherical roller thrust bearings . . . . . . . . . . . . . . . . 30

For the support of the shafts and gear wheels ofindustrial gearboxes, rolling bearings are usedalmost exclusively. The exceptions are in somespecialised areas, such as turbo drives, wherehydrodynamic plain bearings are used.

There are many good reasons for thisdominance of rolling bearings:

good location with minimum radialand axial play enables optimummeshing to be achieved;

high specific load carrying capacitywith low friction;

wide range of internationally stand-ardised products produced in highvolumes at reasonable prices andhaving good availability;

can be calculated using reliable loadcarrying capacity values;

little design work for the user; simple arrangement; axially compact so that short and

stiff shafts can be used; normal tolerances and surface fin-

ishes for shaft and housing seatings; less sensitive to misalignment than

plain bearings; ability of radial bearings to accept

axial loads; not influenced by direction of load or

rotation; low starting torque; no starting problems in intermittent

operation; relatively easy to lubricate; favourable behaviour under emer-

gency conditions; economic maintenance.

Almost all bearing types are used inindustrial gearboxes and almost all theavailable sizes. In the majority of appli-cations, standard catalogue bearingscan be used; any variants with respectto clearance or cage design are alsogenerally common, so that the com-prehensive range of SKF cataloguebearings for general engineering appli-cations covers the needs of gearboxesvery well and enables the designer tomake an optimum selection. The mostimportant bearing types for gearboxesare described in more detail in the following.

Bearing types forindustrial gearboxes

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2 Bearing types for industrial gearboxesDeep groove ball bearings

2 Bearing types for industrial gearboxesDeep groove ball bearings

Deep groove ball bearingsDeep groove ball bearings are themost popular of all bearing types andthis also applies for gearboxes. Themost important characteristics whichmake them so popular are

they are able to carry radial loads as well as axial loads acting in bothdirections;

they are suitable for high and veryhigh speed operation as their frictionis low;

they have practically no tendency tosmear, i.e. cold welding when theballs are accelerated;

they run quietly, particularly if theyare lightly preloaded by axial force;

they are robust in operation andrequire little maintenance;

they are favourably priced.

The dominant role for deep groove ballbearings is where shafts have to belocated axially and loads are relativelylight. This is the case in

spur gear units (drive shaft and hol-low take-off shaft),

multi-ratio gear units (switching spurgear wheels),

geared motors worm gear units (worm wheels), planetary gears (drive shaft, planet-

ary carrier) and coupling shafts.

These improvements also bring ad-vantages when the bearings are usedin gearboxes. In particular the reducedsensitivity to misalignment means thatthere is no reduction in bearing lifeunder the slight misalignments of up toapproximately 3 minutes of arc whichare normally encountered. The im-proved surfaces reduce friction lead-ing to lower running temperatures sothat lubrication conditions are im-proved and bearing life extended.

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Benefits offeredby SKF In recent years SKF has made a

number of improvements to deepgroove ball bearings which haveresulted in further performanceenhancements. The more import-ant include

optimised raceway geometryand finish, reducing friction, run-ning noise and sensitivity tomisalignment;

improved cages which are morestable, thus increasing reliabilityat high speeds;

improved seals, thus enhancingthe sealing efficiency of sealedbearings.

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2 Bearing types for industrial gearboxesAngular contact ball bearings

Angular contact ballbearingsThe raceways of these bearings arearranged at an angle to the bearingaxis (contact angle), so that they areable to carry heavier axial loads thandeep groove ball bearings. Slidingmovements of the balls are superim-posed on their rolling motion, so thatthe single row bearings require ac-curate adjustment or a minimum axial load to function properly.

Angular contact ball bearings areavailable in the following designs:

single row, single direction angularcontact ball bearings,

double row, double direction andpaired single row angular contactball bearings and

four-point contact ball bearings, i.e.single row, double direction ball bearings.

Single direction implies that axial loads acting in one direction only canbe accommodated, whereas doubledirection bearings (and paired singledirection bearings, depending on thearrangement) can take axial loadsacting in both directions.

The single and double row angularcontact ball bearings are preferred as locating bearings for worm shafts.Four-point contact ball bearings areused primarily as thrust bearings inhigh speed spur gear units, where theouter ring is radially free.

The improvements made by SKFto single and double row angularcontact ball bearings, e.g. reinfor-cement of the ball set (single row BE design, double row A and Edesigns) to give higher load carry-ing capacity means that worm gearunits can transmit more powerand, at the same time, the reduc-tion in friction means that bearingtemperature can be lowered. Thereduced tolerances for axial clear-ance and for dimensional and run-ning accuracy which are standardfor SKF single row angular contactball bearings for paired mountingof the CB design, because of theimproved location and reducedrunning noise, are advantageousin low-noise worm gear units suchas those required for lifts andescalators.

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2 Bearing types for industrial gearboxesAngular contact ball bearings

Benefits offeredby SKF

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2 Bearing types for industrial gearboxesCylindrical roller bearings

2 Bearing types for industrial gearboxesCylindrical roller bearings

Cylindrical roller bearingsThe special properties of cylindricalroller bearings make them a popularchoice for gearboxes and include:

high radial load carrying capacity; low friction the lowest of any roller

bearing under purely radial load; suitable for a wide range of operating

speeds, including very high speeds,as the cage has the correct combina-tion of roller guidance, strength andsliding friction properties;

ability to accommodate moderateaxial loads, when they are simulta-neously under radial load, via theslid-ing surfaces of the rollerend/flange contact, although the inc-reasedfriction means that lubrication andcooling must be adapted to the conditions;

the ease with which lateral displace-ment can take place within the bear-ing makes them ideal as non-locat-ing bearings;

proven good performance underexternal radial accelerations;

most designs are separable so thatmounting and dismounting are simple.

These characteristics make cylindricalroller bearings ideal for the followingapplications:

as the non-locating bearings of allhigh-performance units; the NUdesign with its flangeless inner ringis perhaps the most used, but alsothe NJ, NJG and NCF find applica-tion; the rings of these bearingsneed only be axially located at oneside, and by mounting the rings withrelative axial displacement the bear-ings can accommodate lateraldisplacement in both directions.

in spur gear units, even where com-bined radial and axial loads are pro-duced by the helical teeth; the mostpopular positions are those on theintermediate shaft, as the axial forcesfrom the driven and driving wheelsgenerally act in opposite directionsso that the resultant axial load islight.

Practically all improvements madeto cylindrical roller bearings by SKFcould be considered as tailored togearbox needs, so that they makean appreciable contribution to in-creased performance. The maincharacteristics are

the reinforced roller complementsand opened flanges of the ECdesign give increased radial andaxial load carrying capacity;

the logarithmic roller profile en-sures an optimum stress distribu-tion over the whole roller lengthso that edge stresses are avoid-ed even under heavy loads andthe permissible misalignments;

the refined raceway micro-geo-metry reduces friction and im-proves lubricant film formation;

newly developed cages ensureproper bearing function over theincreased performance range; the standard polyamide cages(designation suffix P) of smallbearings have low friction, areelastic and have good slidingproperties;the steel window-type cages(designation suffix J) which arestandard for medium-sized bear-ings and can also be fitted to thesmaller sizes (to special order)withstand high temperaturesand also medium to strong vib-rations;the machined brass cages (forgearbox bearings preferably out-er ring centred and in two parts,designation suffix MA, or in onepiece, suffix MP or ML) are stan-dard for large bearings and canbe fitted to other sizes to specialorder; they can tolerate highspeeds and are resistant to vib-rations and accelerations.

The range of cylindrical roller bearingsis large compared with other bearingtypes. The various flange configurations(NU, NJ, NUP, N and NCF designs)make the bearings suitable for a multi-tude of applications and the differentcage designs extend the usefulness ofthese bearings.

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2 Bearing types for industrial gearboxesCARB roller bearings

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CARB roller bearingsCARB is a completely new type ofbearing: a Compact Aligning RollerBearing. This single row roller bearing,developed by SKF, is characterised bya combination of properties whichmake it interesting for a multitude ofapplications:

the ability to compensate for angularmisalignments or initial errors ofalignment typical of spherical rollerbearings;

the ability to take up axial displace-ments in the bearing itself typical ofcylindrical roller bearings;

the low cross section typical ofneedle roller bearings;

the high radial load carrying capacityimparted by long sphered rollers;

the low friction obtained from optim-ally matched raceway profiles;

the quietness of operation.

Because of its many advantages, theCARB makes an ideal non-locatingbearing. The points in favour of its usein industrial gearboxes include, in addi-tion its compact design and high radialload carrying capacity even whenmisaligned, the potential for downsiz-ing or increasing operational reliabilityor the power rating. The CARB is par-ticularly suitable for the bearingarrange-ments of

heavily loaded shafts in spur gearboxes,

pinion shafts in bevel gearboxes,and

planetary gears.

Two versions of CARB are available: a bearing with cage and a full comple-ment bearing.

SKF has introduced a completelynew roller bearing, the CARB. It isthe only bearing available whichcombines the advantages of threedifferent bearing types without, atthe same time, incorporating theirdisadvantages. For gearbox ap-plications, these advantages trans-late into the following opportunit-ies for enhanced performance.

Up to 30 % higher load carryingcapacity at the bearing positioncombined with small radialspace requirements

The low cross section allowsdownsizing or increased per-formance

Compensation for errors of po-sition and also form of bearingseatings in housings thus allow-ing machining costs to be reduced

Both bearing rings can bemounted with an interference fitso that there will be no wear inthe bore and no additional axialloads under conditions of axialdisplacement

Quiet running and little vibration


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2 Bearing types for industrial gearboxesCARB roller bearings

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2 Bearing types for industrial gearboxesSpherical roller bearings

Spherical roller bearingsThe self-aligning capability (also inoperation) of spherical roller bearingsmakes their use advantageous whereshaft bending occurs or where thereare errors of alignment between shaftand housing (casing). They are there-fore used in all cases where misalign-ment of the bearing rings would pro-duce inadmissible edge stresses if rigidbearings were used. Additional import-ant characteristics make the sphericalroller bearing a reliable all-rounder forgearbox applications. These include

the high radial load carrying capacityand the ability to accommodate axialloads acting in both directions;

the wide range of dimension seriesand very wide range of sizes

even very large sizes.

The many successful developmentrefinements and the improved charac-teristics resulting from them explainthe popularity of spherical roller bear-ings for gearboxes (particularly in spur,bevel and planetary gear units).

The design and functional charac-teristics substantiate the leadingposition of SKF spherical rollerbearings:

long, symmetrical rollers givevery high load carrying capacity;

the floating guide ring betweenthe rows of rollers ensures thatthe rollers are properly guided(without wobble) into the load-ed zone and, in cases whereaxial loads predominate, that theload is correctly carried by therollers and symmetrically distrib-uted over the roller length;

the special form and optimumsurface finish of the racewaysminimise friction and operatingtemperature enabling highspeed operation;

the latest development the Edesign has even higher loadcarrying capacity as the bearingsection is more efficiently ex-ploited;

the position of the guide ringabove the pitch diameter in theE design favours lubricant filmformation between the rollersand guide ring;

all SKF spherical roller bearingsare fitted with robust metalliccages which perform well evenunder arduous conditions.

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2 Bearing types for industrial gearboxesSpherical roller bearings


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2 Bearing types for industrial gearboxesTaper roller bearings

Taper roller bearingsThe tapered form of the racewaysmakes these bearings eminently suit-able for combined radial and axialloads. There is a choice of contactangles so that the appropriate bearingfor the particular combination of radialand axial loads can be found. Thenecessity for functional reasons to usetwo bearings adjusted against eachother enables the force distribution onthe rollers to be controlled so that maxi-mum life can be obtained at the sametime as the stiffness and guidance ofgear shafts can be optimised. Themain gearbox applications are

spur gear units with helical teeth, bevel and bevel/spur units and worm gear units.

As taper roller bearings can supportvery heavy loads, they are alwaysused when the load carrying capacityof other bearings for combined loadconditions (deep groove and angularcontact ball bearings) is inadequate.

Because the raceways are at anangle to the bearing axis, an internalaxial force is produced when the bear-ing is radially loaded, which acts on the housing via the outer ring and can deform it. With larger units (fromapproximately 90 mm shaft diameter)and specifically high performancerequirements, the casing walls areoften not sufficiently stiff, so that theuse of double row or paired single rowtaper roller bearings (or spherical rollerbearings) is recommended, becausethe internal axial forces cancel outeach other and the casing walls willnot be deformed.

Paired single row taper roller bear-ings in a face-to-face arrangement(designation suffix DF) are always usedwhen the preset axial play can be ex-ploited and when adjustment duringmounting is to be avoided.

SKF taper roller bearings have anumber of advantages which makethem suitable for industrial gear-boxes. These include

the ideal form and optimumfinish of the roller end/guideflange contact enable hydrody-namic lubrication to be achievedand mixed lubrication conditionsavoided, so that the critical run-ning-in process normally re-quired when commissioning a gearbox is not needed;

the logarithmic raceway profilesguarantee optimum stress dis-tribution over the whole rollerlength and prevent edge stresses;

the improved surface topographyof the raceways enhances lubric-ant film formation and reducesbearing noise.

29

2

2 Bearing types for industrial gearboxesTaper roller bearings


machine, e.g. in extruder gearing andwater turbine gearboxes. The bearingsare used successfully as thrust bear-ings for the pinion and worm shafts oflarge and very heavily loaded beveland worm gear units.

Spherical roller thrustbearingsThe special feature of these bearingsis their self-aligning capability. Thismeans that their full load carryingcapacity can be utilised, in contrast tothe very stiff cylindrical roller thrustbearings, even when the bearing washers are slightly out of alignmentwith each other. The even distributionof load is still maintained when thereare small angular misalignments of theseating surfaces. Such misalignmentswould considerably shorten the life ofcylindrical roller thrust bearings.

Spherical roller thrust bearings areused in gearboxes, particularly whereaxial forces are produced by the driven

30

2 Bearing types for industrial gearboxesSpherical roller thrust bearings

SKF spherical roller thrust bearingshave particularly low friction thanksto the special roller end/flangecontact geometry.


Marine gearboxwith spherical roller bearings,cylindrical rollerbearings, four-point contact ballbearings andspherical rollerthrust bearings

31

2


3 Design of bearing arrangements

Shafts and gear wheels in spur gearboxes . . . . . . . . 33

Shafts in bevel gearboxes . . 44

Shafts in worm gearboxes . 50

Shafts and gear wheels for planetary gearboxes . . . . . . 56

It is quite possible that several different bearingtypes are used in one gearbox, and where com-bined gear units are concerned, there are sev-eral types of gearing. A stepwise approach is therefore appropriate when selecting bearings,taking each shaft in turn so that the differentconditions for the individual shafts and gearwheels can be fully considered. The bearingarrangements described in the following arewell proven and the conditions specific to a certain shaft are covered. A presentation of themost commonly used bearing series facilitatesthe initial selection.

Design of bearingarrangements

Shafts and gear wheelsin spur gearboxesSpur gearboxes are generally used toreduce speed. There are three maintypes which differ in the way they aremounted: stationary units (mounted onthe machine base), cartridge units(mounted on the drive shaft of the driven machine) and flanged units(flanged to the casing of the primemover and/or driven machine).

The drive from the prime mover isvia a coupling or a belt. The drive istransmitted to the driven machine via acoupling, a quill shaft connection or viaa pinion.

Input shaftsThe input (drive) shafts have the high-est speeds and lightest loads providedno additional external loads have to beconsidered, e.g. belt tension forces.Vibrations and imbalance forces maybe produced by the prime mover. It isalso necessary to consider the prob-

33

3

3 Design of bearing arrangementsShafts and gear wheels in spur gearboxes

lems of high angular accelerationswhen starting without load as well asoperation without load at maximumspeed in order to prevent bearing dam-age caused by the rolling elements sliding on the raceways. There is adanger of this occurring when loadsare suddenly applied. The temperaturedifferences and the associated thermalexpansions in the radial and axialdirections are high for input shafts, asthe speed related large power loss andrelatively small masses as well as therelatively small surface of the pinionshaft mean that there is insufficientheat removal. The distance betweenbearings is dictated by the casing andthe low torque often means that slimshafts are used. This means that shaft

bending and bearing misalignment mustbe taken into account, particularly if abelt drive is used.

Two deep groove ball bearingsarranged for cross location ( fig )provide a cost-favourable bearing ar-rangement for moderate power require-ments. Deep groove ball bearings aresuitable for high-speed operation. Be-cause of the low friction, small quanti-ties of oil are adequate for lubricationand cooling so that the collected oilsplashed by the gear wheels dippinginto the oil bath is generally sufficient.

In order to prevent axial clamping ofthe bearings being caused by thermalexpansion of the shaft there should besufficient axial clearance between theouter ring and the cover.

For shaft diameters of up to approx-imately 90 mm, two taper roller bear-ings arranged face-to-face ( fig )are advantageous both from technicaland cost considerations. The taper roller bearings are adjusted againsteach other via the cover so that theywill have zero clearance when at theoperating temperature or, for reasonsof stiffness, they may have a slight pre-load. When determining the initial axialclearance it is necessary not only toconsider the temperature differentialbetween shaft and casing but also thedeformation of the shaft and, above all,the casing. The casings of larger unitsare often not stiff enough with respectto the axial forces (tooth force + in-ternal axial forces in the bearings). Insuch cases bearing adjustment is dif-

2

1

Bearing arrange-ment for an inputshaft with twocross-locateddeep groove ballbearings

Bearing arrange-ment for an inputshaft with two cylindrical rollerbearings

Bearing arrange-ment for an inputshaft with twotaper roller bear-ings arrangedface-to-face

34


ficult and shaft guidance is not suffici-ent-ly accurate. The taper roller bear-ing arrangement shown is, therefore,not always suitable.

Cylindrical roller bearings ( fig )have a high radial stiffness and guidethe shaft very accurately without havingto be adjusted as taper roller bearings.Axial forces are transmitted via theflanges and roller ends. Because thiscauses more frictional heat, lubricationand cooling must be particularly good.

In order to prevent axial clamping ofthe bearings when thermal expansionof the shaft takes place, there shouldbe adequate axial play between theflanges.

The classical locating/non-locatingarrangement ( fig ) is more com-plicated from a design point of viewthan the cross-located arrangementsdescribed above, as the inner andouter rings must be axially located atboth sides. However, it has advant-ages with regard to dimensioning asthe axial force is always taken up by agiven bearing in this case the spher-ical roller bearing irrespective of thedirection of the load. Additionally,displacement of the non-locating bear-ing is always assured so that there isno risk of axial clamping occurringwhen the shaft expands.

Two NU-design cylindrical roller bear-ings as radial bearings together with afour-point contact ball bearing as thethrust bearing ( fig ) have provedsuitable for very high-speed operation(up to n dm 1 000 000). For such

5

4

3

high-speed operation the bearingsmust have

machined brass cages, centred inthe outer ring,

increased internal clearance: C3 forthe cylindrical roller bearings and C4 for the four-point contact ballbearing, and

seatings having increased accuracyof form and position (IT4/2).

At high circumferential speeds thebearings will reject normal oil supplies.Therefore, it is necessary to inject oilat high speed (v 15 m/s) into the gap between cage and inner ring. Oildrainage facilities should be providedat the injection side of the bearings.

35

3


Bearing arrange-ment for an inputshaft with two cylindrical rollerbearings as theradial bearingsand a four-pointcontact ball bear-ing as the thrustbearing

Classiclocating/non-locating bearingarrangement witha spherical rollerbearing and a cylindrical rollerbearing

other bearing types or arrangementswhich are less unfavourable in respectof casing deformation.

In comparison with input shafts, the axial loading of cylindrical rollerbearings used to support intermediateshafts ( fig ) is less critical. Theaxial forces at the gears act in oppos-ite directions and cancel each otherout, at least partially, so that the axialload on the bearings is light. Also thespeeds are lower so that frictional losses deriving from the axial loadremain small.

The high radial load carrying capa-city of the cylindrical roller bearings isan advantage as the intermediate shaftbearings are heavily loaded. The choicebetween caged or full complement cylindrical roller bearings is determinedprimarily by the factors load, speed,lubrication conditions, friction and cost.

Compared with the input shaft, thereis only a small temperature gradientbetween the intermediate shaft and thecasing. This makes it possible to usespherical roller bearings in a cross-located arrangement as shown in fig which is simple in design andtherefore cost-favourable.

There is a wide range of sphericalroller bearings available, particularlyfor medium and large shaft diameters,and there is also a choice of severalcross sections for each diameter. It isthus possible to easily find bearingswhich can support the heavy loadsacting on the intermediate shaft but

8

7

Intermediate shaftsIntermediate shafts are the most heav-ily loaded as they are subjected to theforces from two gear meshes. Thespeeds are moderate. The axial forceson pinion and wheel oppose eachother when the direction of the teeth isthe same so that they partially balanceeach other. There are no additionalexternal forces but vibrations may betransmitted from the input or outputshafts. As there is no torque acting atthe shaft ends, reasonably small dia-meters can be used enabling a rela-tively large bearing section to be util-ised for the accommodation of the high radial forces. Design limits for thebearing outside diameter are set bythe distance between input and outputshafts.

When using taper roller bearings ( fig ) it should be rememberedthat axial forces are produced eventhough the load is purely radial. Thismay lead to axial deformation of thecasing. These deformations occur in the central, less stiff region of thecasing because of the position of theintermediate shaft, and are larger than for the input shaft. They lead to achange in position of the shaft and cantherefore cause inadmissibly high mis-alignment of the bearings and themesh.

Experience shows that the casingdeformations occurring in smaller unitswith shaft diameters up to 90 mm aregenerally within acceptable limits. Forlarger units it is necessary to resort to

6

36

Bearing arrange-ment for an inter-mediate shaft withtwo taper rollerbearings arrangedface-to-face

Bearing arrange-ment for an inter-mediate shaft withtwo cylindrical roller bearings


which have outside diameters withinthe limits set by the distance betweenthe shafts.

A locating/non-locating bearingarrangement as per fig with a spherical roller bearing at the locatingside and a CARB as the non-locatingbearing offers the possibility of reduc-ing the cross section of the non-locat-ing bearing arrangement, because ofthe high load carrying capacity of theCARB, so that the available space canbe better exploited. In many applica-tions there is a risk that the bearingseating in the housing will be ham-mered out so that an intermediatesleeve must be incorporated. By usinga CARB bearing this is no longer a problem as the outer ring is mountedwith an interference fit in the housing,so that a sleeve is not needed.

9

37

3

Bearing arrange-ment for an inter-mediate shaft withtwo spherical roller bearings

Bearing arrange-ment for an inter-mediate shaft with one spherical roller bearing(locating) and one CARB (non-locating bearing)


Fig 9

(EHD) lubrication, i.e. the formation of a separating lubricant film between rolling elements and raceways, cannotbe achieved. Operating bearings under conditions of mixed friction orboundary lubrication will result in wearand shorter bearing life. Besides rota-tional speed, operating temperatureand lubricant viscosity are the mostimportant factors determining EHDlubrication.

There is a limit to how high the viscosity of the oil can be becauseconsideration must be paid to the high-speed gears and bearings in theunit. Therefore, a cooling of the gear-box in the region where the low-speedbearings of the drive shaft are situatedis often the most effective means ofincreasing bearing life. Suitable ad-ditives in the oil can also contribute to a reduction in wear.

Other factors influencing drive shaftbearings depend on the gearbox design:

In stationary, base-mounted gear-boxes, depending on the type ofpower take-off, it is necessary toconsider the forces of the coupling,the propeller shaft, a pinion or of thedirectly coupled driven machine(e.g. extruders).

Bearing arrange-ment for an outputshaft with twospherical rollerbearings

Locating/non-locating bearingarrangement foran intermediateshaft with twomatched singlerow taper rollerbearings and onecylindrical rollerbearing

The locating/non-locating arrange-ment shown in fig can carry veryheavy radial as well as axial loads.Two matched single row taper rollerbearings (DF execution) are used forthe locating arrangement. In contrastto the cross-located bearing arrange-ments shown in figs 2 and 6, the inter-nal axial forces of the taper roller bear-ings compensate each other within thebearing pair and do not deform thecasing. The intermediate ring suppliedwith the bearing pair ensures that thereis a minimum axial clearance within thebearings. This is adequate for temper-ature differentials between shaft andcasing of up to 20 C. To avoid deform-ation of the thin-walled inner ring asthe cover screws are tightened, thelength of the centring surface (spigot)of the cover should be chosen to givea preload of approximately 0,01 mm.

Drive (output) shaftsThe conditions for the drive shafts are characterised by high torques and low speeds. The torque calls for a large shaft diameter so that therequisite load carrying capacity can beobtained even when using bearingswith low cross sections. There arepotential problems with lubrication ofthe rolling contacts if, because of thelow speeds, elastohydrodynamic

10

38


The bearings in cartridge-type gear-boxes are subjected to the reac-tionary forces of the torque support.Additional forces may also be pro-duced as a result of casing deforma-tion.

The casings of flanged gearboxesare bolted to the driven machine.The shafts are generally rigidlycoupled so that the double supportof the output shaft becomes a mul-tiple support in practice. Centringerrors of the coupled componentsproduce additional forces in the bearings so that narrower tolerancesfor the centring should ensure theaccuracy of alignment of the bearingarrangement.

The arrangement with spherical rollerbearings ( fig ) is especially suit-able for applications where rough operation, external additional forces,misalignments and shock loads placeheightened demands on the bearings.Axial shock loads are taken up by theless sensitive raceways in the absenceof flanges on the rings.

For cartidge-type gearboxes, therelatively large diameters of the hollowshaft mean that bearings having lowcross section are suitable. Figshows a well-proven bearing arrange-ment incorporating full complement

12

11

cylindrical roller bearings of seriesNCF 29 V. For lighter loads but withsimilar diameters, deep groove ballbearings of series 619 can be used in the same arrangement. For heavierloads as well as larger deformations,but still with the same diameters andarrangement, spherical roller bearingsof series 239 are appropriate. Deepgroove ball and spherical roller bear-ings have cages and are thus less susceptible to wear when inade-quately lubricated than full comple-ment bearings.

Intermediate gear wheelsAn internal bearing arrangement ismost suitable for intermediate gears as it takes up the least space. Internalbearing arrangements are character-ised by rotating outer rings. Therefore,there is rotating outer ring load andstationary inner ring load. This meansthat the outer rings should have inter-ference fits and the seatings should bevery accurately machined in order tokeep the rotating inaccuracies whichcause increased friction and additionalforces on the bearing cage to a mini-mum.

With opposing meshes the circum-ferential forces are added, so that highradial load carrying capacity is re-quired. The axial forces from the

39

3

Bearing arrange-ment for an outputshaft of a cart-ridge-type unitwith full comple-ment cylindricalroller bearings ofseries NCF 29 V

Bearing arrange-ment for an inter-mediate gearwheel with twocylindrical rollerbearings of the NJ design


the inner ring) oil should be supplied atthe side. To prevent the supplied oilfrom being rejected by the bearing, theseal gap at the supply side should notexceed 1 mm.

Shifting gear wheelsFor reasons of space these gear whe-els are supported internally in a similarmanner to the intermediate gears. Thetorque is transmitted in the engagedcondition so that the bearings are sub-jected to the tooth forces. The innerand outer rings rotate but the relativespeed is zero. Both rings have rotatingload but the rolling elements do notroll. The continuous changes in loadunder these stationary conditionscause micro-sliding to take place at therolling element/raceway contacts. Asthere is no relative rotation of the rings,a washboarding type of wear will beproduced in the raceways. This wearcan be reduced by using highly viscouslubricating oil containing anti-wearadditives.

Where the wheels have helicalteeth, the axial force produces a tiltingmoment and consequently a rotatingtilting motion which leads to axial move-ment in the rolling element/racewaycontacts. This increases wear. Ballbearings, adjusted to zero clearance,behave favourably as the balls can

helical teeth oppose each other andpartially cancel each other producing atilting moment on the bearing whichcan cause misalignment.

Two cylindrical roller bearings of theNJ design provide the requisite highradial load carrying capacity in a re-stricted space as shown in fig . Thedesign of the associated components ofthe arrangement is simple. The bearingarrangement of helical intermediategear wheels must be checked for angu-lar misalignment. An unfavourable com-bination of wheel diameter, pitch anddistance between bearings can produceinadmissible values of misalignment.An extended support width (distancebetween bearing pressure centres)can be achieved using, for example,angular contact ball bearings.

Taper roller bearings in a back-to-back arrangement ( fig ) also inc-rease the support width as well asreducing the influence of the tilitng moment on the misalignment if theyare adjusted to zero clearance, or alight preload.

Straight cut gear wheels may besupported by a single spherical rollerbearing ( fig ). The intermediategear wheels are thus free to align sothat a good mesh is achieved.

In order to be able to use standardbearings (without lubrication holes in

15

14

13

40


Bearing arrange-ment for an inter-mediate gearwheel with twotaper roller bear-ings arrangedback-to-back

Bearing arrange-ment for an inter-mediate gearwheel with a single sphericalroller bearing

also roll in the axial direction and be-cause the movement is reduced by theclearance-free adjustment. Wear isalways load-dependent so that bear-ings under low specific loads wearless. The washboarding effect is alsoless prominent as engagement alwaystakes place at new positions so thatthe wear is evenly spread over theraceway.

For the support of shifting wheels,deep groove ball bearings have provedto give good performance ( fig ).Bearings with increased radial internalclearance (C3) are used. The clear-ance-free adjustment via the inner ringsproduces a contact angle in the bear-ings of approximately 15, so that thesupport width of the bearings is ex-tended. This reduces movement in therelatively stationary bearings underrotating load and thus reduces wear. Inaddition, the clearance-free back-to-back arrangement improves guidanceof the wheel.

Lubrication of the bearings from theoutside is difficult as all components ofthe arrangement shaft, bearings andwheel rotate and because the bear-ings are partly covered e.g. by thecoupling. The most reliable method isto supply oil internally through theshaft.

16

41

3


Bearing arrange-ment for shiftinggear wheel withtwo deep grooveball bearings

each particular bearing position mustbe considered. To make the situationclearer in Tables to , the text hasbeen kept as short as possible.

42

Demands on the bearingsModern spur gears generally have hardened gear wheels with groundteeth. It is then possible to obtain highperformance with relatively little frictionand low noise. A prerequisite for this isthe use of high-performance bearings,which should have the properties listedin Table .

In addition to these general require-ments with respect to ball and rollerbearings for high-performance gear-boxes, other demands deriving fromthe specific operating conditions at

1

42


Demand Required bearing design feature

High load carrying capacity Optimised rolling element size and number.Logarithmic roller/raceway contact.Good lubricant film formation through low friction andlow raceway surface roughness.

High stiffness Optimised rolling element size and number.Logarithmic roller/raceway contact.

High dimensional and running accuracy Particularly the inner ring running accuracy should preferably be to tolerance class P6 or better.

Low friction Low friction in roller end/flange contact for taper and cylindrical roller bearings.Low friction in roller/raceway contact.Lightweight precision cage.Low raceway surface roughness.

Low running noise High precision of all bearing components.

Specific operating conditions Requirements of bearings/steps to guarantee performance

High speed and thus high friction Use low-friction bearings. and high operating temperature Avoid over-dimensioning.

Ensure lubricant supply when starting up cold. Provide good cooling.

Large temperature differential when Check required bearing internal clearance; if necessary starting up (slim input shaft heats up select bearings with C3 clearance.more quickly than the better cooled Ensure axial displacement at non-locating bearing position.solid casing)Vibration from drive; imbalance Use bearings with stable cages, e.g. cylindrical roller bearings forces with steel window-type cages or outer ring centred machined

cages, or spherical roller bearings with steel window-type cages.

Idling under light load Check minimum load. Avoid over-dimensioning.Use bearings with small roller masses where possible.Do not use full complement cylindrical roller bearings.Choose bearing types less susceptible to smearing,e.g. spherical and taper roller bearings.

Demands on rolling bearingsfor spur gears

Demands on inputshaft bearings

Table 1

Table 2

Specific Requirements of bearings/steps operating conditions to guarantee performance

Heavy radial loads Use bearings with high load carrying capacity.

Low to Check lubricant film formation. If necessary increase viscosity or moderate speeds improve cooling. Use lubricants with wear-reducing additives.

Bearing selectionThe following check list will be founduseful when selecting bearings in ordernot to forget any important factors.

Adjusted basic rating life Axial load carrying capacity when

the flanges of cylindrical roller bear-ings are under load

Friction Stiffness Misalignment

Sufficient play to prevent inadmiss-ible clamping when temperature differentials are large

Minimum load Static safety under peak loads

A preliminary choice can be made fromthe bearing series shown in Table .5

Demands on inter-mediate shaft bearings

Demands on output shaft bearings

Bearing selection

43

3


Table 3

Specific Requirements of bearings/steps operating conditions to guarantee performance

Very low speeds When lubricant film formation inadequate, i.e. a viscosity ratio (actual to required viscosity) < 1, use lubricants with suitable EP additives. When < 0,5 bearings with cages (not full complement bearings) must be used.When < 0,1 reduce the specific bearing load; aim for s0 > 10.

Shock loads from power Use robust, self-aligning, spherical roller bearings.take-off;deformations

Operating conditions Bearing series normally usedInput shaft Intermediate Output Intermediate Shifting

shaft shaft gears gears

Light loads 62 63 619 60 618/C363 NJ 2 EC 160 62 619/C3

60

Moderate loads NJ 2 EC NJ 22 EC NCF 29 V NJ 2 EC 160/C3320 X 322 239 CC 320 X 60/C3222 E(CC) 222 E(CC)

Heavy loads 322 NJ 23 EC 230 CC NJ 3 EC 62/C3232 CC NJG 23 VH 303223 E(CC) 223 E(CC) 232 CC

322/DF 223 E(CC)High speeds NU 2 ECMA/C3

QJ 2 N2MA/C4

In addition to the bearing series listed above, a CARB can be used as the non-locating bearing for locating/non-locating bearing arrangements

Table 4

Table 5

Shafts in bevel gearboxesBevel gears are generally speedreduction gears. The high-speed driveshaft is termed the pinion shaft and theslow-speed driven shaft carries the larger bevel gear wheel.

The pinion shaft is driven by themotor via a coupling, a spur gear or abelt drive. The power take-off is eithervia a coupling or with bevel/spur gearsvia a pinion.

Pinion shaftsThe pinion is generally supported in anoverhung arrangement. In a few casesthe pinion is supported between thebearings but it is difficult to design in abearing with sufficiently high load car-rying capacity at the head. The over-hung arrangement offers more space.

Two taper roller bearings in a back-to-back arrangement as shown in fig offer a cost-favourable and axi-ally as well as radially stiff arrangementfor small to medium diameter shafts (d < 90 mm). The bearings are adjustedusing a shim between the shaft shoul-der and the inner ring of the bearing atthe input side. The adjustment is deter-mined to give zero clearance when thebearings are in operation and warm or, if required for stiffness reasons, aslight axial prelod. When determiningthe initial axial clearance the tempera-ture differential between shaft andcasing must be considered as well asthe deformations of shaft and casing.

17

Oil should be supplied between thetwo bearings. A baffle plate ensuresthat both bearings are reliably suppliedwith lubricant. The oil drain at the coverside reduces the amount of lubricantreaching the seal.

Bearing arrange-ment for a bevelpinion shaft withtwo taper rollerbearings arrangedback-to-back

44

3 Design of bearing arrangementsShafts in bevel gearboxes

For larger shafts, the requisite loadcarrying capacity can be achieved usinga locating/non-locating bearing arrange-ment as shown in fig . The locatingarrangement is at the drive side andconsists of two matched single rowtaper roller bearings (DF execution).The intermediate ring which is suppliedwith the bearing pair ensures that aminimum axial clearance remains whenthe bearings are mounted which cancope with temperature differentials be-tween shaft and casing of up to 20 C.For greater temperature differentialssuch as may occur, for example, inoperation when ambient temperaturesare very low, paired bearings with larger axial clearance are required(special execution). In order not todeform the thin-walled intermediatering when tightening the cover screws,the length of the centring flange (spigot)on the cover should be such that a pre-load corresponding to approximately 0,01 mm is obtained.

The matched taper roller bearingsoperate as a double row bearing. Asthe axial load from the pinion domin-ates, one of the two bearings de-pending on the direction of the load is completely unloaded. Experienceshows that this is not a disadvantagewhen there is little vibration.

The non-locating bearing adjacent tothe bevel pinion may be either a spher-ical roller bearing, a cylindrical rollerbearing or a CARB.

18

For one-piece casings, spherical rol-ler bearings offer mounting advantagesand they are also relatively insensitiveto smearing when loads vary consider-ably and there are long periods ofidling. If cylindrical roller bearings areused, the requisite axial displacementcan always take place in the bearingitself so that the outer ring can have an interference fit in the housing, andradial guidance is enhanced. The sameis true of CARB ( fig ). At thisposition the bearing will not only enablethe axial displacements to be easilyaccommodated, it will also accept theangular misalignments caused by theoff-centre point of action of the toothforces with no reduction in life.

Oil should be supplied to the twotaper roller bearings between the outerrings. Experience shows that for smalland medium-sized gears (up to approx-imately d = 150 mm) the non-locatingbearing can be adequately lubricatedby the oil returning from the locatingbearings. For larger gears, however, itis necessary to arrange for a separateoil supply to the non-locating bearing.For spherical roller bearings, the oilshould be supplied via the lubricationgroove and holes in the outer ring forthe best results.

19

Bearing arrange-ment for a bevelpinion shaft withtwo matched single row taperroller bearingsarranged face-to-face (locatingposition) and one spherical roller bearing(non-locatingposition)

Bearing arrange-ment for a bevelpinion shaft withtwo single rowtaper roller bear-ings arrangedback-to-back(locating) and one CARB (non-locating bearing)

45

3


Fig 19

Although the bearing arrangementshown in fig is similar to that in fig

, there are considerable functionaldifferences. All roller rows are alwaysunder load irrespective of the directionof the axial load. If the direction of axialload is from the pinion tip to the driveinput, the taper roller bearing at thecover side with its radially free outerring will be axially loaded, and the op-posing bearing will be radially loaded.If the load direction is reversed thenthe smaller axial load will act on theinboard bearing which is also underradial load. The taper roller bearing atthe cover side will then only be sub-jected to a minimum axial load by thesprings. Because all roller rows arealways under load, this arrangement isless sensitive to vibrations than thatshown in fig .

Mounting is more complex becausethere is no intermediate ring betweenthe taper roller bearings which have tobe adjusted on mounting. The radiallyfree outer ring of the taper roller bear-ing at the cover side is prevented fromturning by an O-ring.

A variant of this bearing arrange-ment incorporates a spherical rollerthrust bearing which has a higher loadcarrying capacity. It replaces the taperroller bearing which only carries axialloads.

With respect to lubrication, the samerecommendations apply as for thearrangement shown in fig .18

18

1820

Output shaftsThe gear wheels are generally ar-ranged between the bearings fordesign reasons. This is also true forthe bevel/spur gearboxes.

For shaft diameters up to approx-imately 90 mm, two taper roller bear-ings mounted back-to-back ( fig )provide a technically advantageousand cost-favourable arrangement. Withlarger dimensions, the casings areoften inadequately stiff with regard tothe axial forces (tooth force + internalaxial force of the bearings). This makesadjustment of the bearings difficult andshaft guidance is generally not suffi-ciently accurate. The bearing arrange-ment with cross location is then notaltogether suitable.

The axial force from the gear wheelalways acts in one direction. As theaxial force from the pinion dominates,it is possible that the direction of theresultant axial force will change. Thismust be taken into consideration whenadjusting the mesh.

When adjusting the taper roller bear-ings, the shim at the gear wheel sidedetermines the position of the wheel inthe gearbox. The shim at the pinionside is used to set the axial clearanceof the taper roller bearings.

Oil from the collecting pockets abovethe bearings runs down at the coverside of each bearing. From there theoil must pass through the bearing andthus lubricate it. Oil retaining plates en-sure that there is an adequate supplyof oil available even when starting up.

21

Bearing arrange-ment for a bevelwheel shaft withtwo taper rollerbearings arrangedface-to-face

Bearing arrange-ment for a bevelpinion shaft withone taper rollerbearing as a thrustbearing and onetaper roller bear-ing as a radialbearing (locatingposition) and onecylindrical rollerbearing (non-locating position)

46


The locating/non-locating bearingarrangement shown in fig has theadvantage, compared with that shownin fig , that no bearing adjustmentis required. The bearings are also in-sensitive to axial deformation of thecasing. This will only be subjected tothe tooth forces and not to the internalbearing forces, so that there will beless deformation.

A double row angular contact ballbearing is used as the locating bearing.Alternatively, single row angular con-tact ball bearings in matched setshaving the same diameters as the double row bearing and being margin-ally wider can be used for higher loadcarrying capacity.

To determine the position of the gear wheel in the gearbox and to adjustthe mesh, a split washer is insertedbetween the bearing outer ring and the retaining ring. When doing this thebearing can remain on the shaft. Acylindrical roller bearing of the NUdesign is used as the non-locatingbearing at the other side where theradial load is heavier.

The locating/non-locating bearingarrangement shown in fig is similarin design and function to that shown infig . At the locating side, two singlerow taper roller bearings are arrangedface-to-face. Compared with the doublerow angular contact ball bearing, thetaper roller bearings provide higherload carrying capacity and greater stiffness.

22

23

21

22Adjustment of the bevel gear wheel

is simplified using a special (hook-shaped) sleeve. In order to prevent thethin-walled intermediate ring of the paired taper roller bearings from beingdeformed as the cover screws are tightened, the length of the spigot inthe cover should be chosen to give apreload corresponding to approxim-ately 0,01 mm.

Oil should be supplied to the taperroller bearings via the lubrication groove and holes in the intermediatering. To allow an even distribution overthe two bearings, an oil drain shouldbe provided at the cover side.

47

3

Bearing arrange-ment for a bevelwheel shaft withtwo matched single row taperroller bearings(locating position)and one cylindricalroller bearing(non-locatingposition)

Bearing arrange-ment for a bevelwheel shaft with a double rowangular contactball bearing (locating position)and a cylindricalroller bearing(non-locatingposition)


Demands on the bearingsModern bevel gearboxes usually havehardened gear wheels with groundhelical teeth. This enables high powertransmission to be achieved with littlefriction and little noise generation. Aprerequisite for this good performanceis the use of high-performance ball androller bearings which should have theproperties listed in Table .

In addition to these general require-ments for bearings for high-perform-ance gearboxes, there are additionalrequirements which are specific to the actual bearing position.

6

Bearings for the pinion shaftHigh radial and axial forces act simul-taneously on the pinion shaft. There-fore high radial load carrying capacityis required of the non-locating bearingand high axial load carrying capacity of the locating bearing. Because of thehigh speed, bearings having low frictionshould be used. These two require-ments are in part contradictory.

Experience shows that pinion bear-ings do not fail from fatigue but are en-dangered by other influences. Fromthis it is possible to derive the require-ments and actions listed in Table .7

Demands on rolling bearingsfor bevel gears

Demands on bevelpinion shaft bearings

48


Demand Required bearing design feature

High load carrying capacity Optimised rolling element size and number.Logarithmic roller/raceway contact.Good lubricant film formation through low friction andlow raceway surface roughness.

High stiffness Optimised rolling element size and number.Logarithmic roller/raceway contact.

High dimensional and running accuracy Particularly the inner ring running accuracy should preferably be to tolerance class P6 or better.

Low friction Low friction in roller end/flange contact for taper roller bearings.Low friction in roller/raceway contact.Low raceway surface roughness.

Low running noise High precision of all bearing components.

Most frequent reason for How to alleviate problem/demandspinion bearing damage on bearings

Lubrication breakdown Guarantee lubrication when starting up in the cold state.

Overloading because of too When selecting bearing size, check the temperature differential heavy a preload between shaft and casing. C3 internal clearance often required.

Inadequate lubricant film generation be- Use low friction bearings.cause of too high operating temperatures Avoid over-dimensioning.

Improve cooling.

Smearing on rollers and Avoid over-dimensioning.raceways caused by roller Spherical roller bearings are more favourable than cylindrical slip or sliding roller bearings in larger size range (d > 150 mm).

When using cylindrical roller bearings aim for small roller diameters; use a full complement bearing.

Wear caused by contaminants Avoid contaminating the gearbox during production, assembly and in operation.

Table 6

Table 7

To obtain good meshing it is neces-sary among other things to have abearing arrangement with high radialand axial stiffness. The locating bear-ing should therefore have a large contact angle and as small an axialclearance as possible.

Bearings for the output shaftThese bearings are predominantlyradially loaded so that high radial loadcarrying capacity is also required ofthe locating bearing. Because of theslow speeds the risks in respect ofthermal behaviour and over-dimension-ing compared with the pinion shaft arenegligible. The requirements for axialand radial stiffness, for minimum axialclearance and for bearing accuracycorrespond to those for the pinionshaft bearings.

Bearing selectionWhen selecting the bearings it is use-ful to refer to the cheklist given below.

Adjusted basic rating life Permissible speed Axial and radial stiffness Sufficient bearing clearance in the

mounted but cold state to avoid inad-missible preload under conditions ofmaximum temperature differentials

Minimum load

A preliminary selection can be madeusing the overview of the bearing series commonly used; see Table .8

Bearing selection

49

3


Bearing Bearing series normally usedarrangement

Bevel pinion shaft Bevel gear wheel/Bevel/spur gear wheelInput side Pinion side Gear wheel side Opposite side or

spur pinion side

Cross location 72 BE 72 BE 72 BE 72 BE73 BE 73 BE 73 BE 73 BE313 323 B 322 322323 B 323 B 332 332

303 303323 323

Locating bearing(s) (2) 72 BECB 33(2) 73 BECB (2) 72 BECB313/DF (2) 73 BECB322 + 293 E 320 X/DF

322/DF303 + 294

Non-locating NU 22 EC(/C3) NU 2 ECbearing NU 23 EC(/C3) NU 22 EC

232 CC(/C3) NU 3 EC223 CC(/C3) NU 23 EC

223 EC

In addition to the bearing series listed above, a CARB can be used as the non-locating bearing for locating/non-locating bearing arrangements

Table 8

Shafts in worm gearboxesGenerally worm gearboxes are used to reduce speed. There are two maintypes: one for mounting on the machinebase and a cartridge type for mount-ing on the input (drive) shaft of themachine.

The drive from the prime mover iseither via a coupling or a belt drive.The power take-off is via a coupling ora quill (hollow) shaft connection.

Worm shaftsThe heaviest axial loads act on theworm shaft at the same time as speedsare high. Where there is a belt drive,the radial loads will also be heavy.

ment ( fig ) offer a cost-favour-able, low friction bearing arrangementwith low noise for moderate perform-ance and where diameters are small(bearing bore diameter d 50 mm).The angular contact ball bearings aresuitable for high speeds and becauseof the large contact angle they are alsoappropriate for predominantly axialloads. The two bearings are adjustedagainst each other via the cover so thatthey will have a slight preload whenrunning at the operating temperature.When determining the degree of adjust-ment it is necessary to consider thetemperature differential between shaftand casing, but also casing deforma-tion.

24

The temperature differences and theassociated thermal expansion in theradial and axial directions are also largein worm gearboxes. Only small massesand surfaces of the worm shafts areavailable to remove heat. Therefore,there are large temperature gradientsfrom the shaft to the casing and thesemust be considered when adjustingthe bearings.

The distance between bearings isdictated by the casing and togetherwith the small torques this often leadsto the use of slim shafts. If there is abelt drive, then shaft bending shouldbe calculated so that inadmissiblebearing misalignment can be avoided.

Two single row angular contact ballbearings in a cross-located arrange-

The same type of arrangement butusing two steep-angled taper rollerbearings ( fig ) can carry heavierloads than that with the angular con-tact ball bearings for the same shaftdiameter. Therefore, taper roller bear-ings are preferred for higher perform-ance gearboxes and for medium tolarge diameters. When determining the degree of adjustment, it must beremembered that taper roller bearingsare axially stiffer than angular contactball bearings and are therefore moresensitive to excessive preload. It isthus advisable to aim at zero clear-ance when the bearings are running atthe operating temperature. When start-ing up (worm already warm, casing still cold) a slight preload will be pre-

25

Bearing arrange-ment for a wormshaft with twotaper roller bear-ings arrangedface-to-face

Bearing arrange-ment for a wormshaft with twoangular contactball bearings in a cross-locatedarrangement

50

3 Design of bearing arrangementsShafts in worm gearboxes

sent which experience shows can betolerated when lubrication is good.

The locating/non-locating bearingarrangement ( fig ) is more costlyfrom a design point of view and be-cause a third bearing is involved but ithas the following advantages:

higher load carrying capacity (e.g.for belt tension forces);

if paired angular contact ball bear-ings are used, no individual adjust-ment is required;

axial displacement at the non-locat-ing bearing position is guaranteed.

A further performance increase can beobtained by replacing the pair of angu-lar contact ball bearings by two match-

26

ed single row taper roller bearings (DF execution) as shown in fig .The intermediate ring which is sup-plied with the bearing pair ensures thatthere is a minimum axial clearance inthe mounted condition, which is suffi-cient for temperature differentials be-tween shaft and casing of up to 20 C.In order not to deform the thin-walledintermediate ring when the cover scre-ws are tightened, the spigot (centringshoulder) in the cover should have alength such that a preload correspon-ding to approximately 0,01 mm can beobtained.

27

Bearing arrange-ment for a wormshaft with twomatched angularcontact ball bear-ings (locatingposition) and onecylindrical rollerbearing (non-locating position)

Bearing arrange-ment for a wormshaft with twomatched taper roller bearings(locating position)and one cylindricalroller bearing(non-locatingposition)

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3


The bearing arrangement shown infig is particularly suitable when theaxial load in one direction predomin-ates, as for example in lifting gear. Thespherical roller thrust bearing takes thedominant axial load as well as the axialforce produced in the taper roller bear-ing, which in this case is only subjectedto radial load. If the axial load changesdirection, then the taper roller bearingtakes the radial as well as the axialload, while the spherical roller bearingis spring loaded to give the minimumaxial load required for the correct mo-tion of the rollers. Both bearings areadjusted via the cover. When determin-ing the axial clearance it is necessaryto consider the temperature differentialbetween shaft and casing.

28The advantages of this bearing

arrangement are the very high loadcarrying capacity in the one directionand also that all three bearings arealways under load. Bearing noise isthen particularly low and the bearingsare less sensitive to vibration.

Fig shows a bearing arrange-ment for maximum loads and shock-type operation as encountered, forexample, in rolling mills when the rollsare set. The radial forces are taken upby two radial spherical roller bearingsmounted as non-locating bearings,whilst the axial forces act on the spher-ical roller thrust bearings which haveradial freedom in the casing. The axialclearance of the spherical roller thrustbearings is obtained by adjusting the

29

Bearing arrange-ment for a wormshaft with a taperroller bearing asthe radial bearingand a sphericalroller thrust bear-ing as the thrustbearing (locatingposition) and acylindrical rollerbearing (non-locating position)

Bearing arrange-ment for a wormshaft for maximumloads with tworadial sphericalroller bearingsand two sphericalroller thrust bearings

52


Fig 30: Bearingarrangement for aworm wheel shaftwith two deepgroove ball bear-ings in a cross-located arrange-ment

Fig 32: Bearingarrangement for a worm wheelshaft with two cylindrical rollerbearings

Fig 31: Bearingarrangement for aworm wheel shaftwith two taper roller bearingsarranged face-to-face

width of the spacer sleeve. The springsensure that the requisite minimum loadis applied to the bearing which is re-lieved of axial load.

Worm wheel shaftsThe high torques on the worm wheelshafts require large shaft diameters.As speeds are slow, the load carryingcapacity of low cross section bearings(light series) is adequate.

Because of the low speeds, lubrica-tion of the worm wheel bearings by oilspray is usually not sufficient and spe-cial arrangements must be made forlubricant supply. An oil wiper on theworm wheel or separate grease lubri-cation of the bearings have been foundto give good results.

In most cases the worm wheel has agloboid form and requires accurateaxial guidance, but it must also be pos-sible for the axial position of the meshto be changed.

Two deep groove ball bearings inthe cross-located arrangement shownin fig generally have adequateload carrying capacity and are verycost-favourable. The adjustment of themesh and the bearings is made via the covers. The mesh should prefer-ably be adjusted via the one cover first and then the bearing clearance via the other cover. The temperature ofthe slowly rotating worm wheel shaft is usually low, so the bearings can beadjusted to almost zero clearance. Tokeep the oil level down and still pro-

30

vide adequate lubrication for the bear-ings they are greased and a gap-typeseal is provided on the inboard side.

The arrangement shown in figwith two taper roller bearings is in-tended for heavier loads than thatshown in fig but is otherwise similar. It should be remembered whenusing taper roller bearings that incontrast to deep groove ball bearings the axial adjustment of the bearingswill influence the radial guidance of the worm wheel. Therefore, the casingmust be sufficiently stiff so that it willnot be deformed (beaten out) underload. This would otherwise lead to too large a bearing clearance and inadmissible alterations to the mesh.

30

31

When two cylindrical roller bearings( fig ) of the NJ or NCF (full com-plement) design are used, the radialguidance of the worm wheel is notinfluenced by any axial adjustment, sothat sett

skf rolling bearings information

Documents

spherical roller bearings

cylindrical roller bearings

carb roller bearings

taper roller bearings

theuse of rolling bearings

spur gearboxes

bevel gearboxes

worm gearboxes