Bearing Preload Bearing Preload Depending on the application

Bearing Preload

Bearing Preload Depending on the application it is necessary for there to be a positive or a negative operational clearance in a bearing arrangement. Bearing internal clearance (fig 4) is defined as the total distance through which one bearing can be moved relative to the other in the radial direction (radial internal clearance) or in the axial direction (axial internal clearance). before mounting.

Bearing Internal Clearance

Bearing Preload Operational clearance is the internal clearance in a mounted bearing which has reached its operating temperature. In the majority of applications, the operational clearance should be positive, i. e. when in operation, bearing should have a residual clearance, however slight. However, there are many cases where normally a negative operational clearance, is desirable. Negative clearance inside a bearing is called Preload.

Bearing Preloading is done in order to : Ø Ø Enhance stiffness of the bearing arrangement Increase running accuracy. Examples : Ø Ø Work spindle bearings of machine tools Pinion bearings in automotive axle drives Bearing arrangements of small electric motors Bearing arrangements for oscillatory movement

Bearing Preload The application of a preload, e. g. by springs, is also recommended where bearings are to operate without load or under very light load and at high speeds.

Bearing Preload In such cases, the preload serves to guarantee a minimum load on the bearing and thus to prevent bearing damage resulting from sliding movements of rolling elements over the races.

Types of Preload Ø Radial Preload Ø Axial Preload

Types of Preload Ø Radial Preload Cylindrical roller bearings, because of their design, can only be radially preloaded Ø Axial Preload Thrust ball bearings can only be axially preloaded.

Types of Preload Ø Axial Preload Single row angular contact ball bearings and taper roller bearings, which are normally subjected to axial preload, are generally mounted together with a second bearing of the same type in a back-to-back or face-to-face arrangement

Types of Preload

Types of Preload Deep groove ball bearings are also generally preloaded axially, although for this, the bearings should have a greater radial internal clearance than Normal (e. g. C 3) so that, as with angular contact ball bearings, a contact angle which is greater than zero will be produced.

Types of Preload The distance between the pressure centres of two bearings (angular contact, ball or taper roller bearings) is longer when the bearings are arranged back-to-back, and shorter when they are arranged face-to-face, than the distance between the bearing centres.

Types of Preload This means that the bearings arranged back-to-back can accommodate large tilting moments even if the distance between bearing centres is relatively short. The radial forces resulting from the moment load and the deformation caused by these in the bearings are smaller than when the bearings are arranged face-to-face.

Types of Preload If the shaft becomes hotter in operation than the housing, the preload which was adjusted (set) at ambient temperature during mounting will increase, the increase being greater for face-to-face than for back-to-back arrangements.

Types of Preload In both cases , thermal expansion in the radial direction serves to reduce clearance or increase preload. This tendency is increased by thermal expansion in the axial direction when the bearings are face-to-face, but is reduced for the back-to-back arrangement.

Types of Preload For a given distance between bearings and when the coefficient of thermal expansion is the same for the bearings and associated components, the radial and axial thermal expansions will cancel each other out so that the preload will not change. This applies only to back-to-back arrangements.

Effects of Preload The main effects of bearing preload are : Ø To enhance stiffness Ø To reduce running noise Ø To enhance the accuracy of shaft guidance Ø To compensate for wear in operation Ø To give a long service life.

Types of Preload High stiffness : Bearing stiffness (in N/µm) is defined as the ratio of the force acting on the bearing to the elastic deformation in the bearing. The elastic deformations caused by load in preloaded bearings are smaller for a given load range than in bearings which are not preloaded.

Types of Preload Quiet running: The smaller the operational clearance in a bearing, the better the guidance of the rolling elements in the unloaded zone and the quieter the bearing in operation.

Types of Preload Accurate shaft guidance: The support of a shaft in preloaded bearings gives more accurate guidance to the shaft because the ability of the shaft to deflect under load is restricted by the preload. The more accurate guidance and the increased stiffness afforded by preloaded pinion and differential bearings means that the mesh will be kept accurate and remain constant, and that the additional dynamic forces will be kept small. As a result, operation will be quiet and the gear mesh will have a long life.

Types of Preload Compensation for wear and settling: Wear and settling processes in a bearing arrangement during operation increase the clearance but this is compensated for by the preload

Types of Preload Long service life: Preloaded bearing arrangements in certain applications mean enhanced operational reliability and long service lives. A properly dimensioned preload has a favourable influence on the load distribution in the bearings and therefore on life.

Determination of Preload may be expressed as: Ø A force Ø As a path (distance) Ø As a friction torque

Determination of Preload Empirical values for the optimum preload can be obtained from proven designs and can be applied to similar designs. For new designs it is recommended that the preload force be calculated and checked by testing. As generally not all influencing factors of the actual operation are accurately known, corrections may be necessary in practice.

Determination of Preload The reliability of the calculation depends above all on how well the assumptions made regarding : Ø The temperature conditions in operation Ø The elastic behaviour of the associated components (most importantly housing ) coincide with the actual conditions.

Determination of Preload When determining the preload, the preload force required to give an optimum combination of stiffness, bearing life and operational reliability is first calculated. Then the preload force to be used when adjusting the bearings during mounting is calculated; the bearings are in the cold state and not subjected to the operating load.

Determination of Preload The appropriate preload at the operating temperature depends on the bearing load. An angular contact bearing can accommodate radial and axial forces simultaneously. Under radial load, a force acting in the axial direction will be produced in the bearing, and this must generally be taken up by a second bearing which faces in the opposite direction to the first.

Determination of Preload Purely radial displacement of one bearing in relation to the other will mean that half the bearing circumference (i. e. half the rolling elements) are under load and the axial force produced in the bearing will be : Fa = e Fr (for angular contact ball bearings) or Fa = 0, 5 Fr/Y (for taper roller bearings) where Fr is the radial bearing load (fig 24).

Determination of Preload

Determination of Preload The values of e for angular contact ball bearings and of the axial factor Y for taper roller bearings will be found in the product tables.

Determination of Preload For a single bearing subjected to a radial load Fr, therefore, an external axial force Fa of the above magnitude must be applied if the prerequisite for the basic load ratings (half the bearing circumference under load) is to be fulfilled. If the applied external force is smaller, the number of rolling elements supporting the load will be smaller and the load carrying capacity of the bearing will be correspondingly reduced.

Determination of Preload In a bearing arrangement comprising two single row angular contact bearings either back -to-back or face-to-face, the two bearings of the bearing arrangement each accommodate the axial forces from the other. When the two bearings are the same, the radial load acts centrally between the bearings and if the bearing arrangement is adjusted to zero clearance, the load distribution where half the rolling elements are under load will be automatically achieved.

Determination of Preload In other load cases, particularly where there is an external axial load, it may be necessary to preload the bearings to compensate for the play produced as a result of the elastic deformation of the bearing taking the axial load and to achieve a more favourable load distribution in the other bearing which is unloaded axially.

Determination of Preloading also increases the stiffness of the bearing arrangement. When considering stiffness it should be remembered that it is not only influenced by the resilience of the bearings but also by the elasticity of the shaft and housing, the fits with which the rings are mounted and the elastic deformation of all other components in the force field including the abutments. These all make a considerable contribution to the total resilience.

Determination of Preload The axial and radial resilience of a bearing depend on its internal design, i. e. on the contact conditions (point or line contact), the number and diameter of the rolling elements and the contact angle. Greater the contact angle, the greater the stiffness of the bearing in the axial direction.

Determination of Preload If, as a first approximation, a linear dependence of the resilience on the load is assumed, i. e. a constant spring ratio, then a comparison shows that axial displacement in a bearing arrangement under preload is smaller than for a bearing arrangement without preload for the same external axial force Ka.

Determination of Preload

Determination of Preload A pinion bearing arrangement, for example, comprises two taper roller bearings A and B of different sizes having the spring constants CA and CB and subjected to a preload force F 0. If the axial force Ka acts on bearing A, bearing B will be unloaded, and the additional load acting on bearing A and the axial displacement da will be smaller than for a bearing without preload.

Determination of Preload However, if the external axial force exceeds the value : Ka = F 0 [1 + (CA / CB)] then bearing B will be relieved of the axial preload force and the axial displacement under additional load will be as for a bearing arrangement without preload, i. e. determined solely by the spring constant of bearing A.

Determination of Preload To prevent complete unloading of bearing B when bearing A is subjected to load Ka, the following preload force will thus be required F 0 = Ka CB /(CA + CB)

Determination of Preload The forces and elastic displacements in a preloaded bearing arrangement as well as the effects of a change in preload force are most easily recognised from a preload force/preload path diagram in the next slide.

Determination of Preload

Determination of Preload This consists of the spring curves of components which are adjusted against each other to preload and enables the following relationships to be read off : Ø Preload force Vs Preload path within the preloaded bearing arrangement Ø Relationship between an externally applied axial force Ka and the bearing load for a preloaded bearing arrangement, as well as the elastic deformation produced by the external force.

Determination of Preload In the diagram, all the components subjected to additional loads by the operational forces are represented by the curves which increase from left to right, and all the unloaded components by the curves which increase from right to left. Curves 1, 2 and 3 are for different preload forces (F 01, F 02 < F 01 and F 03 = 0). The broken lines refer to the bearings themselves whereas the unbroken lines are for the bearing position in total (bearing with associated components).

Determination of Preload Using the diagram it is possible to explain the relationships, for example, for a pinion bearing arrangement (next slide) where bearing A is adjusted against bearing B via shaft and housing to give preload. The external axial force Ka (axial component of tooth forces) is superimposed on the preload force F 01 (curve 1) in such a way that bearing A is subjected to additional load whilst bearing B is unloaded.

Determination of Preload

Determination of Preload The load at bearing position A is designated Fa. A, that at bearing position B, Fa. B. Under influence of force Ka, the pinion shaft is axially displaced by the amount da 1. The smaller preload force F 02 (curve 2) has been chosen so that bearing B is just unloaded by the axial force Ka, i. e. Fa. B = 0 and Fa. A = Ka. The pinion shaft is displaced in this case by the amount da 2 > da 1. When the arrangement is not preloaded (curve 3) the axial displacement of the pinion shaft is at its greatest (da 3 > da 2).

Procedure for Adjusting Preload The radial preload is usually used for: Ø Cylindrical Roller Bearings Ø Double Row Angular Contact Ball Bearings Ø Deep Groove Ball Bearings This is achieved by using a sufficient degree of interference for one or both of the rings to reduce the initial internal clearance of the bearing to zero so that in operation there will be a negative clearance, i. e. preload.

Procedure for Adjusting Preload Bearings with a tapered bore are particularly suitable for radial preloading since, by driving the bearing up on : Ø Tapered journal Ø Adapter Sleeve Ø Withdrawal sleeve a preload can be applied within narrow limits.

Procedure for Adjusting Preload The force required to produce axial preload in Ø Single Row Angular Contact Ball Bearings Ø Taper Roller Bearings Ø Deep Groove Ball Bearings is created by displacing one of the bearings axially in relation to the other by an amount corresponding to the desired preload force.

Procedure for Adjusting Preload

Procedure for Adjusting Preload There are two main groups of adjustment methods based on different working principles: Ø Individual Adjustment Ø Collective Adjustment.

Individual Adjustment With individual adjustment, each bearing arrangement is adjusted separately using nuts, shims, spacer sleeves, deformable sleeves etc. Measuring and inspection procedures guarantee that the established nominal preload force is obtained with the least possible deviation.

Individual Adjustment There are different methods depending on the quantity which is measured: Ø Adjustment using preload path Ø Adjustment using friction torque Ø Adjustment using direct force measurement.

Individual Adjustment Individual adjustment has the advantage that the individual components can be produced to normal tolerances and the desired preload can be achieved with a reasonably high degree of accuracy.

Individual Adjustment using preload path : This method of adjustment is frequently used when the components of a bearing arrangement can be pre-assembled. The preload is achieved, for example, for a pinion bearing arrangement by : Ø Fitting intermediate rings between the outer and inner rings of the two bearings

Individual Adjustment

Individual Adjustment using preload path : The preload is achieved, for example, for a pinion bearing arrangement by : Ø Inserting shims between a housing shoulder and a bearing outer ring or between the casing and the housing , the housing being in this case the flanged angled insert;

Individual Adjustment

Individual Adjustment using preload path : The preload is achieved, for example, for a pinion bearing arrangement by : Ø Fitting a spacer ring between a shaft shoulder and one of the bearing inner rings or between the inner rings of the two bearings.

Individual Adjustment

Individual Adjustment The width of the shims, intermediate rings or spacer rings is determined by : Ø Distance between shaft and housing shoulders Ø Total width of both bearings Ø Preload path (axial displacement) corresponding to the desired preload force Ø A correction factor for the preload path to take account of thermal expansion in operation Ø Manufacturing tolerances of all components Ø Correction factor to take account of some loss of preload force after some time.

Individual Adjustment This method of adjustment is based on the relationship between the preload force and the elastic deformations within the preloaded system. The requisite preload can be determined from a preload force/preload path diagram.

Individual Adjustment

Adjustment using Friction Torque This method is popular in series production because of the little time required and because considerable automation is possible. Since there is a definite relationship between bearing load and friction torque in the bearing, it is possible to stop adjustment when a friction torque corresponding to the desired preload has been reached if the friction torque is continuously monitored. However, it should be remembered that the friction torque can vary from bearing to bearing, and that it also depends on the preservative used, or on the lubrication conditions and the speed.

Adjustment using Direct Force Measurement As the purpose of bearing adjustment is to produce a given preload force in the bearings, it would seem sensible to use a method either to produce force directly, or to measure the force directly. However, in practice the indirect methods of adjustment by preload path or friction torque are preferred as they are simple and can be achieved more easily and at less cost.

Collective Adjustment With this method of adjustment, which may also be termed “random statistical adjustment", the bearings, shaft and housing, spacer rings or sleeves etc. are produced in normal quantities and randomly assembled, the components being fully interchangeable.

Collective Adjustment Where taper roller bearings are concerned, this interchangeability also extends to the outer rings and inner ring assemblies. In order not to have to resort to the uneconomic production of very accurate bearings and associated components, it is assumed that the limiting values of tolerances – statistically – seldom occur together.

Collective Adjustment If, however, the preload force is to be obtained with as little scatter as possible, manufacturing tolerances must be reduced. The advantage of collective adjustment is that no inspection is required and no extra equipment needed when mounting the bearings.

Preloading by springs By preloading bearings in small electric motors and similar applications it is possible to reduce operating noise. The bearing arrangement in this case comprises a single deep groove ball bearing at each end of the shaft. The simple method of applying preload is by a spring or spring "package” (fig 31).

Preloading by springs The spring acts on the outer ring of one of the two bearings; this outer ring must be able to be axially displaced. The preload force remains practically constant even when there is axial displacement of the bearing as a result of thermal expansion. The requisite preload force can be estimated from F=kd Where F = Preload force, N k = Preload factor d = bearing bore diameter, mm

Preloading by springs Depending on the design of the electric motor, values of between 5 and 10 are used for the factor k. If preload is primarily to protect the bearing from vibration damage when stationary, then greater preload is required and k = 20 should be used.

Preloading by springs Spring loading is also a common method of applying preload to the angular contact ball bearings of high-speed grinding spindles. The method is not suitable, however, for bearing applications where high stiffness is required, where the direction of load changes, or where undefined shock loads can occur.

Maintaining the correct preload When selecting the preload force for a bearing arrangement it should be recalled that the stiffness is only marginally increased when the preload exceeds a given optimum value, whereas friction and consequently heat generation increase and there is a sharp decrease in bearing life as a result of the additional, constantly acting load.

Maintaining the correct preload Diagram below indicates relationship between bearing life and preload / clearance.

Maintaining the correct preload When adjusting bearings to preload in an arrangement, it is important to ensure that the established value of the preload force, determined either by calculation or by experience, is achieved with the least possible scatter.

Maintaining the correct preload This means, for example, for bearing arrangements with taper roller bearings, that the bearings should be turned several times during adjustment so that the rollers do not skew and so that the roller ends are in correct contact with the guide flange of the inner ring. If this is not the case, the results obtained during inspection or by measurements will be false and the final preload can be much smaller than the requisite value.

Bearings for preloaded bearing arrangements For certain applications, SKF supplies single bearings or matched bearing sets which are specially made to enable simple and reliable adjustment, or which are matched during manufacture so that after mounting, a predetermined value of the preload will be obtained.

Bearings for preloaded bearing arrangements These include : Ø Taper roller bearings to the CL 7 A and CL 7 C specifications for automotive pinion and differential bearing arrangements Ø Single row angular contact ball bearings for universal pairing Ø Paired single row taper roller bearings for industrial gearboxes Ø Special bearings in various designs, for example, taper roller bearing hub units