toleranc e fits

1MENG 499A. El-Assal1

Tolerances and Fits

The term tolerances refers to the permissible deviation of a dimension from the specified basic size.

The proper performance of a machinecan depend on the tolerances specified for its parts, particularly those that must fit together for location or for suitable relative motion.


The term fit usually refers to the clearances that are permissible between mating parts in a mechanical device that must assemble easily and that must often move relative to each other during normal operation of the device.

Such fits are usually called running or sliding fits. Fit can also refer to the amount of interference that

exists when the inside part should be larger than the outside part.

Interference fits are used to ensure that mating parts do not move relative to each other.

Tolerances and Fits, Contd.



where:d=D ... basic sizeDmax , Dmin ... limits of size for the hole dmax , dmin ... limits of size for the shaftES ... hole upper deviation EI ... hole lower deviationes ... shaft upper deviation ei ... shaft lower deviation



Depending on the mutual position of tolerance zones of the coupled parts, 3 types of fit can be distinguished: Clearance fit, A Transition fit, B Interference fit, C


System of Fit

Although there can be generally coupled parts without any tolerance zones, only two methods of coupling of holes and shafts are recommended due to constructional, technological and economic reasons.

Hole basis systemThe desired clearances and interferences in the fit are achievedby combinations of various shaft tolerance zones with the hole tolerance zone "H". In this system of tolerances and fits, the lower deviation of the hole is always equal to zero.

Shaft basis systemThe desired clearances and interferences in the fit are achievedby combinations of various hole tolerance zones with the shaft tolerance zone "h". In this system of tolerances and fits, the upper deviation of the hole is always equal to zero.


Type of Fit

Depending on the mutual position of tolerance zones of the coupled parts, 3 types of fit can be distinguished:

Clearance fitIt is a fit that always enables a clearance between the hole and shaft in the coupling. The lower limit size of the hole is greater or at least equal to the upper limit size of the shaft.

Transition fitIt is a fit where (depending on the actual sizes of the hole and shaft) both clearance and interference may occur in the coupling. Tolerance zones of the hole and shaft partly or completely interfere.

Interference fitIt is a fit always ensuring some interference between the hole and shaft in the coupling. The upper limit size of the hole is smaller or at least equal to the lower limit size of theshaft.


Type of Fit, Contd.

where:d=D ... basic size//// ... hole tolerance zone\\\\ ... shaft tolerance zone


Recommended FitsClearance fits: H11/a11, H11/c11, H11/c9, H11/d11, A11/h11, C11/h11, D11/h11

Fits with great clearances with parts having great tolerances.Use: Pivots, latches, fits of parts exposed to corrosive effects, contamination with dust and thermal or mechanical deformations.

H9/C9, H9/d10, H9/d9, H8/d9, H8/d8, D10/h9, D9/h9, D9/h8Running fits with greater clearances without any special requirements for accuracy of guiding shafts.Use: Multiple fits of shafts of production and piston machines, parts rotating very rarely or only swinging.

H9/e9, H8/e8, H7/e7, E9/h9, E8/h8, E8/h7Running fits with greater clearances without any special requirements for fit accuracy.Use: Fits of long shafts, e.g. in agricultural machines, bearings of pumps, fans and piston machines.

H9/f8, H8/f8, H8/f7, H7/f7, F8/h7, F8/h6Running fits with smaller clearances with general requirements for fit accuracy.Use: Main fits of machine tools. General fits of shafts, regulator bearings, machine tool spindles, sliding rods.


Recommended FitsClearance fits: H8/g7, H7/g6, G7/h6

Running fits with very small clearances for accurate guiding of shafts. Without any noticeable clearance after assembly.Use: Parts of machine tools, sliding gears and clutch disks, crankshaft journals, pistons of hydraulic machines, rods sliding in bearings, grinding machine spindles.

H11/h11, H11/h9Slipping fits of parts with great tolerances. The parts can easily be slid one into the other and turn.Use: Easily demountable parts, distance rings, parts of machines fixed to shafts using pins, bolts, rivets or welds.

H8/h9, H8/h8, H8/h7, H7/h6Sliding fits with very small clearances for precise guiding and centringof parts. Mounting by sliding on without use of any great force, after lubrication the parts can be turned and slid by hand.Use: Precise guiding of machines and preparations, exchangeable wheels, roller guides.


Recommended Fits, Contd.Transition fits: H8/j7, H7/js6, H7/j6, J7/h6

Tight fits with small clearances or negligible interference. The parts can be assembled or disassembled manually.Use: Easily dismountable fits of hubs of gears, pulleys and bushings, retaining rings, frequently removed bearing bushings.

H8/k7, H7/k6, K8/h7, K7/h6Similar fits with small clearances or small interferences. The parts can be assembled or disassembled without great force using a rubber mallet.Use: Demountable fits of hubs of gears and pulleys, manual wheels, clutches, brake disks.

H8/p7, H8/m7, H8/n7, H7/m6, H7/n6, M8/h6, N8/h7, N7/h6Fixed fits with negligible clearances or small interferences. Mounting of fits using pressing and light force.Use: Fixed plugs, driven bushings, armatures of electric motors on shafts, gear rims, flushed bolts.


Recommended Fits, Contd.

Interference fits: H8/r7, H7/p6, H7/r6, P7/h6, R7/h6

Pressed fits with guaranteed interference. Assembly of the parts can be carried out using cold pressing.Use: Hubs of clutch disks, bearing bushings.

H8/s7, H8/t7, H7/s6, H7/t6, S7/h6, T7/h6Pressed fits with medium interference. Assembly of parts using hot pressing. Assembly using cold pressing only with use of large forces.Use: Permanent coupling of gears with shafts, bearing bushings.

H8/u8, H8/u7, H8/x8, H7/u6, U8/h7, U7/h6Pressed fits with big interferences. Assembly using pressing and great forces under different temperatures of the parts.Use: permanent couplings of gears with shafts, flanges.


EXAMPLE LIMITS AND FITS USING HOLE BASIS

Description Hole Shaft

Loose Running H11 c11

Free Running H9 d9

Loose Running H11 c11

Easy Running - Good quality easy to do- H8 f8

Sliding H7 g6

Close Clearance - Spigots and locations H8 f7

Location/Clearance H7 h6

Location- slight interference H7 k6

Location/Transition H7 n6Location/Interference- Press fit which can be separated H7 p6

Medium Drive H7 s6Force H7 u6


Factors Affecting Tolerances and Fits

The option of the system for the specified type of product or production is always influenced by the following factors:

Constructional design of the product and the method of assembly.

Production procedure and costs for machining the part. Type of semi-product and consumption of material. Costs for purchase, maintenance and storage of gauges

and production tools. Machine holding of the plant. Options in use of standardized parts.

Hint: Although both systems are equivalent in the view of functional properties, the hole basis system is used preferably.



Consider the plain surface bearings. A critical part of the design is the specification of the diametrical clearance between the journal and the bearing.

The typical value is just a few thousandths of an inch.

But some variation must be allowed on both the journal outside diameter and the bearing inside diameter for reasons of economy of manufacture.



There will be variation of the actual clearance in production devices, depending on where the individual mating components fall within their own tolerance bands.

Too small a clearance could cause seizing. Conversely, too large a clearance would reduce the

precision of the machine and adversely affect the lubrication.


Tolerances, Production Processes, and Costs

A unilateral tolerance deviates in only one direction from the basic size.

A bilateral tolerance deviates both above and below the basic size.

The total tolerance is the difference between the maximum and minimum permissible dimensions.



The term allowance refers to an intentionaldifference between the maximum material limits of mating parts.

For example, a positive allowance for a hole/shaft pair would define the minimum clearance between mating parts from the largest shaft mating with the smallest hole.

A negative allowance would result in the shaft being larger than the hole (interference).



The term fit refers to the relative looseness (clearance fit) or tightness (interference fit) of mating parts, especially as it affects the motion of the parts or the force between them after assembly.

Specifying the degree of clearance or interference is one of the tasks of the designer.



It is costly to produce components with very small tolerances on dimensions.

It is the designers responsibility to set the tolerances at the highest possible level that results in satisfactory operation of the machine.

The production of part features with small tolerances usually involves finer surface finishes.


Machining Costs Versus Surface Finish Specified

This shows the general relationship between the surface finish and the relative cost of producing a part.The typical total tolerance produced by the processes described is included in the figure.The increase in cost is dramatic for the small tolerances and fine finishes.


Machining Costs Versus Surface Finish Specified

The term tolerance grade refers to a set of tolerances that can be produced with an approximately equal production capability.

The actual total tolerance allowed within each grade depends on the nominal size of the dimension.

Smaller tolerances can be achieved for smaller dimensions, and vice versa.


Tolerance Grades4 5 6 7 8 9 10 11

Lapping & HonningCylindrical GrindingSurface GrindingDiamond TurninigDiamond BoringBroachingPowder Metal-SizesReamingTurningPowder Metal SinteredBoringMillingPlanning & ShapingDrillingPunchingDie Casting


Tolerances of Different Processes


Cost of Tolerances


Preferred Basic Sizes

The first step in specifying a dimension for a part is to decide on the basic size, that dimension to which the tolerances are applied.

The analysis for strength, deflection, or performance of the part determines the nominal or minimum size required.

Unless special conditions exist, the basic size should be chosen from the lists of preferred basic sizes in Table for fractional-inch sizes, decimal-inch sizes, and metric sizes from the SI.


The ISO system of tolerances and fits (ISO 286-1)The designation of an ISO standard tolerance would be, respectively to the tolerance grade, IT01, IT3, and IT12 etc. (i.e. increasing tolerance. )


Geometrical Tolerances

Geometrical Tolerances: specified for an individual feature by means of a feature control frame (a rectangle) divided into compartments containing the GTOL symbol followed by the tolerance value.

Where applicable, it also follows the tolerance with a material condition symbol.

Matl Cond, Tertiary

Matl Cond, Secondary Matl Cond, Primary

Matl Cond, Tolerance

Value

Type D. R. Primary

D. R. SecondaryD. R. Tertiary

Matl Cond, Primary, Compound

Matl Cond, Primary, Basic

Datum Ref, Primary, Basic

Datum Ref, Primary, Compound


Geometrical Tolerances

Geometric tolerances specify the maximum variation that is allowed in form or position from true geometry.

The geometric tolerance is, in essence, the width or diameter of tolerance zone within which a surface or axis of hole or cylinder can lie, which results in resulting feature being acceptable for proper function and interchangeability.


GTOL Symbols


Tolerance Frame Variations

The tolerance frame can be divided into two or more compartments. These compartments include from left to right

The symbol for the feature to be toleranced

The tolerance value. If the tolerance zone is circular or cylindrical it is preceded with a

Letters for datum when the toleranced feature is specified in relation to one, or more datum.

If more than one datum is specified then additional partitions are provided


Maximum Material Indication in Tolerance Frame

The maximum material condition, when used, is indicated by a symbol placed after the tolerance value, after the datum letter, or both.


Additional Frames- Notes

If a single frame cannot convey sufficient information it is acceptable to stack additional frames and/or provide additional notes..


Surface Texture (Finish)

The quality of machined surface is characterized by the accuracy of its manufacture with respect to the dimensions specified by the designer.

Every machining operation leaves characteristic evidence on the machined surface. This evidence in the form of finely spaced micro irregularities left by the cutting tool.

Each type of cutting tool leaves its own individual pattern which therefore can be identified.

This pattern is known as surface finish or surface roughness.


Surface Texture , Contd.

1. Roughness :Roughness consists of surface irregularities which result from the various machining process. 2. Roughness Height :It is the height of the irregularities with respect to a reference line.

2. Roughness Height:It is the height of the irregularities with respect to a reference line.

3. Roughness Width :The roughness width is the distance parallel to the nominal surface between successive peaks or ridges which constitute the predominate pattern of the roughness.

4. Roughness Width Cut Off :Roughness width cut off is the greatest spacing of respective surface irregularities to be included in the measurement of the average roughness height.

5. Lay :Lay represents the direction of predominant surface pattern produced and it reflects the machining operation used to produce it.



6. Waviness :This refers to the irregularities which are outside the roughness width cut off values. Waviness is the widely spaced component of the surface texture.

7. Waviness Width :Waviness height is the peak to valley distance of the surface profile.

8. Arithmetic Average (AA):A close approximation of the arithmetic average roughness-height can be calculated from the profile chart of the surface. If X is themeasured value from the profilometer, then the AA value can be calculated as shown below.

9. Root Mean Square (rms)The rms value can be calculated as shown below. Its numerical value is about 11% higher than that of AA.


Surface Texture, Contd.



Surface texture Ra is a measurement of the average distance between the median line of the surface profile and its peaks and troughs

The relevant English standard is BS1134- Assessment of surface texture. (measurement units mm (micro.m))

ANSI B46.1 is used in USA (measurement units minch(micro.inch) ) The ISO standard is DIN ISO 1302 which uses N numbers . Rules of Thumb for surface finish

(Rough turned with visible toolmarks....N10 (12.5mm (micro.m)) Smooth machined surface....N8(3.2mm (micro.m)) Static mating surfaces (or datums).... N7(1.6mm(micro.m)) Bearing Surfaces.... N6(0.8mm (micro.m)) Fine lapped surfaces.... N1(0.025mm (micro.m))

(mm)(micro.m) 0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50 (minch)(micro.inch) 1 2 4 8 16 32 63 125 250 500 1000 2000 N-Grade N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 N11 N12

Finish Ground Finishes Smooth Turned

Medium Turned

Rough Machined


Roughness Average

Process 50 (2000)25

(1000)12.5 (500)

6.3 (250)

3.2 (125)

1.6 (63)

0.80 (32)

0.40 (16)

0.20 (8)

0.10 (4)

0.05 (2)

Flame Cutting

Snagging

Sawing

Planing, Shaping

Drilling

Chemical Milling Elect. Discharge

Mach.

Milling

Broaching

Reaming

Electron Beam

Laser

Electro-Chemical

Boring, Turning Barrel Finishing

Electrolytic grinding

I

n

M

i

c

r

o

m

e

t

e

r

s

m

(

M

i

c

r

o

i

n

c

h

e

s

i

n

.

)


Process 50 (2000)25

(1000)12.5 (500)

6.3 (250)

3.2 (125)

1.6 (63)

0.80 (32)

0.40 (16)

0.20 (8)

0.10 (4)

0.05 (2)

0.025 (1)

Roller Burnishing

Grinding Honing

Electro-Polish Polishing Lapping

Superfinishing Sand Casting

Hot Rolling Forging

Perm Mold Casting

Investment Casting

Extruding

ld Rolling, Drawing Die Casting

Roughness Average I

n

M

i

c

r

o

m

e

t

e

r

s

m

(

M

i

c

r

o

i

n

c

h

e

s

i

n

.

)

Tolerances and FitsTolerances and Fits, Contd.Tolerances and Fits, Contd.Tolerances and Fits, Contd.System of Fit Type of Fit Type of Fit, Contd.Recommended FitsRecommended FitsRecommended Fits, Contd.Recommended Fits, Contd.Factors Affecting Tolerances and FitsFactors Affecting Tolerances and FitsFactors Affecting Tolerances and FitsTolerances, Production Processes, and CostsTolerances, Production Processes, and CostsTolerances, Production Processes, and CostsTolerances, Production Processes, and CostsMachining Costs Versus Surface Finish SpecifiedMachining Costs Versus Surface Finish SpecifiedTolerance GradesTolerances of Different ProcessesPreferred Basic SizesThe ISO system of tolerances and fits (ISO 286-1)Geometrical TolerancesGeometrical TolerancesGTOL SymbolsTolerance Frame VariationsMaximum Material Indication in Tolerance FrameAdditional Frames- NotesSurface Texture (Finish)Surface Texture , Contd.Surface Texture , Contd.Surface Texture, Contd.Surface Texture , Contd.Roughness Average Roughness Average

toleranc e fits

Documents