ball screws 2

8/2/2019 Ball Screws 2

1/5

Printed in U.S.A., Copyright 1999. Penton Media,

Inc. All rights reserved. Machine Design (ISSN 0024-9114) is published by Penton Media, Inc., 1100 Superior Ave., Cleveland, OH 44114-2543.

Editorial content is indexed in the Applied Science Technology Index, the Engineering Index, SciSearch andResearch Alert. Microfilm copies available fromUniversity Microfilms, Ann Arbor, MI 48106. Fax (313)665-5022.

Permission to photocopy is granted for users registered with the Copyright Clearance Center (CCC) Inc. tophotocopy any article, with the exception of those for which separate ownership is indicated on the first page of

the article, provided that the base fee of $1.25 per copy of the article, plus $.60 per page is paid to CCC, 222Rosewood Dr., Danvers, MA 01923 (Code No. 0024-9114/99 $1.25 + .60).

Permission to reprint: Barbara LoSchiavo; Purchasedreprints: Phyllis Dixon (216-696-7000, ext. 9607); Advertising Materials: Advertising Dept., M ACHINE DESIGN ,1100 Superior Ave., Cleveland, OH 44114-2543, 216-696-7000. Inserts should be sent to M ACHINE DESIGN , PentonPress, 680 N. Rocky River Dr., Berea, OH 44017-1691.

Penton Media, Inc.


2/5

Whether the goal is high

precision orlow cost, ball

screws areoften the best option for linearactuation.JOHN DEGENOVA Thomson Saginaw Ball Screw Co.Saginaw, Mich.

The next time youre faced with a designproject that calls for linear actua-tion and youre consider-

ing old standbyslike hydraulic orpneumatic cylin-ders, think a littlelonger. Ask yourself practical questions like, Do I really want todeal with pneumatic noise? Or, If a hydraulicline breaks how much time and money will themessy cleanup demand? If the answers tothese questions lead to second thoughts aboutyour plans, think about ball screws as well.

Ball screws should be the method of choice inlinear-actuation applications. Ball screws con-

vert rotary input to linear motion and offer sev-eral advantages over other actuators, such as

Acme screws, hydraulic or pneumatic systems,and belt, cable, or chain drives. For example,ball screws are up to three times more efficientthan Acme screws. This lowers system powerrequirements and allows using smaller gears,clutches, and motors. Ball screws cost less thanhydraulic or pneumatic systems, operate morequietly, and dont require pumps, hoses, fluids,or shop air. And although belt, cable, or chaindrives are often less expensive than ball screws,they also are less precise and stretch as they

MACHINE DESIGNhttp://www.machinedesign.com JULY 22, 1999

MECHANICALMECHANICAL Edited by Todd ZaludMECHANICALMECHANICALMECHANICALMECHANICAL

Miniature ball screws are an efficient low-cost option for

linear actuation. Themechanisms are used in avariety of automotive

applications and come in sizesas small as 0.375-in. diameter

and 0.125-in. lead with 90% efficiency.

(Acmescrews are

typically just 30 to 60% efficient.)

Single-nut screws have170-lb dynamic load

capacity and 1,600-lb static

load capacity. Loadcapacities double when usingtwo nuts.

WHAT YOU OUGHT TO

KNOW ABOUT BALLSCREWS

wear, which leads to inaccurate positioning.

BALL-SCREW BASICSBall screws are force and motion-transfer de-

vi ce s in th e fa mi ly of po we r- tr an sm is si onscrews. They operate like conventional powerscrews but the rolling friction of bearing ballsreplaces sliding friction. Ball screws consist of ascrew, nut, and balls that operate similarly tobearing components.

The screw has a precision ground or rolled he-lical groove acting as the inner race. The nut hasinternal grooves that act as the outer race. Cir-cuits of precision steel balls recirculate in thegrooves between the screw and nut. Either thescrew or nut turns while the other moves in alinear direction. This converts torque to thrust.

A simple calculation determines the torque re-quired to drive a ball screw: T = L P /5.65, whereT = torque (lb-in.), L = screw lead (in.), and P =axial load, (lb).


3/5

For example, consider a ball screwwith a 1.875-in. lead moving a 535-lbload. The torque required is:

T = L P /5.65= (1.875 535)/5.65= 177.54 lb-in.

Af te r selec ting a s crew-and-nutcombination, other ball-screw compo-

nents are needed, such as ball returnsand wipers. Ball returns either inter-nally or externally carry balls fromthe end of their path back to the be-ginning to complete their circuit. Thetype of ball return often depends onspace constraints and the number of redundant circuits.

Wipers keep contaminants out of critical internal ball-screw compo-nents and keep lubricants applied tothem. In many applications wipersextend ball-screw life and enhancemachine reliability. Wipers are eitherinternally or externally mounted.

58 MACHINE DESIGN JULY 22, 1999 http://www.m

FIGHTING BACKLASHBacklash is free axial motion of ball nuts alongscrew threads. Designers who need to avoidbacklash use preloaded nuts instead of standardnuts. Preloading bearing balls loads them in thedirection opposite the working load. This, in turn,applies opposing pressure to shaft threads andstiffens operation.

MECHANICALMECHANICAL

Semiconductor wafer elevators use ball screws totransport wafers for processing and inspection.The elevators also connect to robotic wafer arms ormove wafers into furnaces.

End-flange single nuts are compact and offer easymounting.

Double-nut vernier preload is adjustable to let usersoptimize performance.

High-helix single nut arrangements achieve high travelrates with minimum rpms.

High-helix double nuts provide additional capacity andload redundancy compared to their single-nutcounterparts.

High-helix single nuts are also available for precision-rolled ball screws providing high lead accuracy at alower cost than high-helix double nuts.


4/5

heavily in commercial and militaryaircraft. Commercial planes use ballscrews in mechanisms such as enginethrust reversers and propeller pitchcontrols. On many commercial air-craft several ball screws position mul-tiple wing flaps using a single drive.Military aircraft use ball screws in

A RANGE OF APPLICATIONSBall screws are used in a variety of

jo bs . T he ir hi gh ef fi ci en cy ma ke sthem useful in high and low-load ap-plications. Off-the-shelf models haveload limits ranging from 170 to200,000 lb.

High-precision ball screws are used

horizontal stabilizers, main landinggears, and variable engine inlet andexhaust-nozzle actuators.

Though aircraft applications repre-sent some of the most advanced tech-nology, ball screws are also used inslightly less demanding industrialand automotive applications. Indus-

MACHINE DESIGNhttp://www.machinedesign.com JULY 22, 1999


Feathering shows concavearcs in thread forms.

Give old ball screws new life Spalling appears as small cavitiesat the base of screw threads.

Chipped or broken lands arebits of material missing fromthe thread corners.

Brinnelling resembles linear dashesat the base of screw threads.

Although ball screws inevitably wear over time, worn screws can oftenbe reconditioned several times throughout their life. Reconditioningcosts up to 50% less than new equipment.

Lube-for-life ball screws use a self-lubricating element mounted on each endof the nut. The element continuouslydispenses a thin film of lubricant on themoving balls and screw threads. This savesmoney and hassle by eliminating the need

for oil lubrication.

Rotation

Ball nut

Ball screw

Axis

Four-point contact

Skid

Scuff

Two-point contact

Rotation

Ball nut

Ball screw

Axis

Although many preloaded ball nuts contact rolling balls at four points, designs with two-point ball contact haveseveral advantages. Four-point contact may be acceptablein applications carrying low loads, but otherwise it cancause erratic ball rotation and skidding. This can lead to

excessive heat generation and high torque requirements.Thomson Saginaw designs, on the other hand, use two-

point contact to nearly eliminate skidding. This lowerswear, increases positioning accuracy, and extends travellife.


5/5

Circle 196 60 JULY 22, 1999 http://www.machined


trial ball-screw applications includemilling-machine tables, robotics, andsemiconductor wafer transport sys-tems. Miniature ball screws in ABSautomotive brakes rapidly open and

close valves that apply and releasefluid pressure to brake pads. Thiscreates a pumping effect to help pre-

vent brake lock-up.

DESIGN FOR RELIABILITYManufacturers use various de-

sign features when applications re-quire high reliability. Aside fromusing wipers to prevent contami-nant ingression, another way man-ufacturers increase reliability iswith redundant load paths. An ex-ample of a load-path redundancy is

the use of two or more circuits of balls within a ball nut. This in-creases reliability because if onecircuit malfunctions the ball screwwill continue to operate.

Screw threads with multiple startsalso provide redundancy. Double-start screws have two threads con-centric with each other allowing in-dependent circuits of balls to operatein each path. When multiple circuitsare combined with double-start ar-chitecture, many redundancies arepossible. For instance, two circuits in

each of two starts produce four inde-pendent load paths. Aviation and aerospace appl ica-

tions often require ball screws withseveral redundant load paths. Al-though a ball screws projected lifedrops with each lost circuit, struc-tural integrity and function are pre-served. A ball screw with four cir-cuits in normal operation, forexample, might be designed to oper-ate for 209,000 cycles. The screw canstill function for 2,600 cycles with

just one circuit operating.Sometimes standard ball screws

wont provide the right solution for aparticular application. In many casesa variation of conventional ballscrews can solve multiple problems.Hollow ball screws, for instance, con-

vert torque to thrust while allowing apath for coolant flow or wiring.Coolants running through the screwshaft can either help maintain toler-ances in precision ball-screw opera-tions or lubricate cutting tools. The

hollow screws also have lower rota-tional inertia than solid screws,which can dramatically increasestarting and stopping speeds. Cross-sectional properties of hollow ball

screws also allow higher operatingspeeds than those of similar solidscrews.

Although solid ball screws reachlengths exceeding 70 ft, sometimesspace constraints limit their lengths.In applications requiring long travelin tight spaces, telescoping ballscrews are a handy solution. A hol-low outer screw in these mechanismssurrounds a solid or hollow innerscrew. The inner screw extends fromthe outer screw, increasing thescrews reach. Three or more screws

can be combined this way for evenlonger travel. Telescoping ballscrews are widely used in machineryand are required on aircraft to actu-ate leading-edge wing flaps.

Another space-saving techniquecombines ball screws with electricmotors. Performance Pak Actuatorshave a motor output shaft connected,through a gear transmission, to aball screw. The screw is enclosed in asealed tube with a mounting bracketthat connects to the load and extendsfrom the outer tube. Actuating the

motor extends or retracts the screwto push or pull a load. The devicesare used in positioning applications,such as commercial satellite dishesor hospital beds, in material han-dling, and in applications requiringsimple lifting, opening, or closing op-erations. The actuators can also holda load stationary without consumingpower, or in power-off situations. s

WE WANT YOUR FEEDBACK. Did you find this articleinteresting? Circle 817

Do you want more informationon this topic? Circle 818

Comment via e-mail [email protected]

What related topics would you like to seecovered? What additional information on

this topic would you find useful?

ball screws 2

Documents