bearing preload

10. Bearing Preload

Bearing is usually selected to have a smallclearance during normal operation, but some bea-rings are selected to have a negative clearance,when mounted, to generate the internal stress, sothat various effects can be achieved.

This is so-called preload method, which can beapplied only to the rolling bearings, not sliding ones.

10-1 Purpose of PreloadThe objectives and application examples of

preloading are shown in the Table 10-1.

10-2 Methods and Characteristics ofPreload

There are two main types of preload, namely, aposition preload and a constant pressure preload.

Position preload can be further divided intoseveral sub-groups, namely, a method tightly fittinga pair of preloaded bearings, a method adjustingthe dimensions of a spacer or seam to obtain theproper preload without using a matched pair ofbearings, and a method employing the direct cont-

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10. Bearing Preload

Applications

Precision bearings for position controlling, used for mainshaft bearing of machining tools or precision measuringinstruments.

Pinion bearings of main shaft bearing of machining toolsor automotive differentials.

Bearings for small motors of home appliances and others.

Used where vibration is strong. Bearings for motors requ-ired to stop frequently and kingpin thrust ball bearings ofautomotive vehicles.

Angular contact ball bearings for high frequency motorsor cylindrical roller bearings for jet engines

Ball bearings with contact angles or roller bearings ofhigh speed rotation

For a thrust ball bearing or thrust self-aligning roller bea-ring used on the side shaft, or to prevent skidding due toring’s own weight load when stopped.

Preload Purpose

To precisely determine the position of a shaft in radialand axial directions, and to increase its rotating precisionat the same time.

To increase the bearing rigidity

To prevent vibration or abnormal noise generated bytrembling shaft.

To prevent false brinelling

To restrict the sliding revolution and sliding rotation ofrolling element.

To restrict the gyration sliding of rolling element

For exact position control of rolling element against therings.

Table 10-1 Preload Purposes and Application Examples

rol of proper degree of fastening force to apply theappropriate amount of preload by measuring thestarting friction moment without using spacer orseam.

These kinds of position preload allow a bearingto keep the constant relative position regardless ofits operation status.

The constant pressure preload is a method thatuses any of coil spring, plate spring, or board springto apply a proper amount of preload to bearing.Because the rigidity of preload springs is generallyand sufficiently smaller than that of bearing, thepreloads are kept almost constant although bea-ring’s relative positions vary during operation.

The comparisons between position preload andconstant pressure preload are listed below.

- Influence on the increase of bearing rigidity :Constant pressure preload < Position preload

- Variation of bearing rigidity by bearing load :Constant pressure preload > Position preload

- Variation of preload by temperature and load :Constant pressure preload < Position preload

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Table 10-2 Methods and Characteristics of Preload

Preloading Example Drawing Method of Application ApplicationMethod Preload Application Examples

Angular contact ballbearings

Tapered rollerbearings Thrust ball bearingsAngular contact ballbearings

Angular contact ballbearings Deep groove ballbearings Tapered rollerbearings

Thrust ball bearingsThrust sphericalroller bearingsThrust cylindricalroller bearings

Position Preloading

Constant Pressure Preload

ID and OD widthvariation or a smallamount of specifiedpreload is applied.

The amount ofpreload is adjustedby controlling thefastening of screws, and the amount ofpreload isdetermined bymeasuring thestarting friction torque of a bearing.

Preload is appliedby using coil orspring

Preload is appliedby using coil orspring

GrinderLatheMeasuring instrument

LathePrinterAutomotive pinionAutomotive wheel

MotorWinder spindleGrinder

Rolling millExtruder



10-3 Preload and Rigidity of Bearing

It is necessary to know the correlations betweenthe applied load and displacement of bearing to findout correlations between preload and rigidity, and totheoretically determine the proper amount ofpreload.

The correlations between load and displacement,when only axial load is applied to bearing, is easyto analyze, because all rolling elements receivesame amount of load. But, when the radial or com-bined load is applied, it’s difficult because of var-ying load distribution.

Axial displacement against axial load can becalculated as follows.

For ball bearings, the axial displacement, δa is

cδa = ———(Q2/Da)1/3 (Equation 10-1)

sin α

Where,δa : Axial displacement [mm]c : Constant(Refer to Table 10-3)α : Contact angleQ : Weight of rolling element [kgf ]Da : Ball diameter [mm]

For tapered roller bearings, the axial displa-cement, δa is

0.0006 Q 0.9

δa = ————·——— (Equation 10-2)sin α la

0.8

Where,la : Effective contact length of roller [mm]

And, load of roller, Q is

FaQ= ———— (Equation 10-3)Zsin α

Where,Fa : Axial load [kgf]

Z : Number of rolling elements

In case of tapered roller bearings, because theircontact angles do not change regardless of theaxial loads, the same nominal contact angles asdetermined in the design can be used. But for ballbearings, the following Equation has to be used toobtain effective contact angles, because their con-tact angles change depending on the axial loads.

cosαo c——— = 1 + ——— (Q/Da

2)2/3 (Equation 10-4)cosα fo + fi − 1

In the above Equation, fo and fi represent theratios of raceway radius of outer and inner rings toball diameter, Da, and in case of ball bearings, theirinitial contact angle, αo , can be obtained by usingthe inside residual clearance, ∆r, as follows.

∆rcosαo = 1 + ———— (Equation 10-5)2(fo + fi − 1)Da

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10. Bearing Preload

Table 10-3 Corelations between f and c

f 0.51 0.515 0.5175 0.52 0.525 0.53 0.54

c×105 176 194 201 207 218 227 242

(“ f ”is the ratio of radius of raceway groove to balldiameter.)



10-4 Evaluation of PreloadAs mentioned earlier, various effects can be

achieved by applying the preload appropriately, butapplication of excessive preload can become thecauses for excessive heat generation, increasedfriction moment, and/or reduction of bearing life,etc.

Therefore the amount of preload should bedecided after careful analysis of bearing operatingconditions and the purpose of preload, and others.

For example, the main purpose of preload for thebearings of main shaft of machining tools is toincrease its rigidity, so the amount of preload canbe calculated by using the elastic modulus requiredfor bearing in the shaft system. But, in case ofmachining tools, RPM range of main shaft isgenerally very wide, which means that good resultcan be obtained when heavy cutting job is carriedout at low speeds, while the light cutting job at highspeeds may generate excessive heat.

Also, in case the main purpose is to prevent falsebrinelling, the exact amount of preload needs becalculated just enough to prohibit the creation ofclearance by vibration load, so as to prevent rollingelement from being vibrated by outside vibrationwhen shaft is not rotating.

However, for electric motors, it is essential toreview whether the heat generation and shorteningof bearing life, caused by preload, has some effecton the performances or system life of the electricmotor or not.

Therefore, the appropriate amount of preloadshould be decided only after comprehensive ana-lysis of theoretically calculated values as well asthe empirical/experimental data.

10-5 Controlling of PreloadVarious preload control methods are shown

below.(1) Control by measuring the starting friction

moment of bearing

This method uses the starting friction moment,which is measured by using the co-relationsbetween itself and axial load, so as to control the

preload. This method is widely used for taperedroller bearings when they are applied with thepreload.

(2) Control by measuring the spring

displacements

This method is used for constant pressure preload.By using the findings of corelations between theload of preload spring and its displacement, preloadcan be controlled in accordance with the springdisplacements.

(3) Control by measuring the axial displacement ofbearing

By using the findings of co-relations between theaxial load and axial displacements, preload can becontrolled in accordance with its axial displac-ements.

(4) Control by measuring the torque(fasteningforce) of nut

In case that the preload is applied by using thefastening nut on a matched pair of bearings with-out using a spacer or seam, if the nut has beensufficiently smoothened and fastened by applyingsufficiently strong torque, the fastening force, inother word, the preload, can be applied within a co-mparatively minor fluctuation, which makes itpossible to control the preload. This method iswidely used for tapered roller bearings in the auto-motive vehicles.

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bearing preload

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