The Design of Rolling Bearing Mountings

Automotive differentialsDesignSpiral bevel-gear drives – with or without intersectingaxes – are now almost always used for front and rearaxle drives. Very high axial loads arise which, withnon-intersecting axes, may be several times the tangentialload at the pinion. Due to the limited space andthe elevated torque values, the pinion bearings are veryheavily loaded. The pinion bearings should provide foreven meshing of pinion and crown wheel under load;therefore, the pinion bearing arrangement should be asrigid as possible. The pinion is either an overhung or astraddled arrangement. The overhung arrangement isusually fitted with two tapered roller bearings adjustedagainst one another. Compact bearing arrangements(double-row tapered roller bearings with an unsplitcup or a cup with a flange) are common.The crown wheel is mounted in common with the differential.The meshing accuracy of the teeth shouldvary as little as possible and mounting should, therefore,be provided with sufficient rigidity. The rigidityrequirements are easier to meet than with the pinionsince more mounting space is available for this applicationand the axial loads are generally lower.Bearing adjustmentRigid pinion and crown wheel guidance is achieved byadjusting the bearings against each other with a preload.With grey-cast iron housings, thermal expansionof the shaft increases the preload in nearly all cases afteroperating temperature is reached; the preload must,however, never be such as to exceed the elastic limit ofthe bearing material.The opposite applies to aluminium housings, whichare being used more and more because of their lightness.So, the preload has to be selected such as toachieve the required rigidity, but the additional bearingloading must not significantly reduce the bearinglife. This is the case if the axial preload does not exceedabout half the external axial force F a applied.LubricationDifferentials rely exclusively on oil lubrication. Bearingsand gears are lubricated with the same oil. Sincethe lubricant is subjected to severe stressing in the spiralgearing, hypoid oils with EP additives are used.While the splash oil sufficiently lubricates the crownwheel shaft bearings, which have to accommodate lowerloads, inlets and outlets must be provided for the oilfor the pinion shaft particularly for the bearing on theflange side. Attention should be paid to the oil flow directionwhich is always from the small end to the largeend of the tapered rollers. The oil ducts have to be arrangedand dimensioned such as to ensure that oil circulatesin every speed range.The pinion shaft is normally sealed by means of radialshaft seals, in some cases in combination with a flingersheet.Bearing dimensioningFatigue life analysis of the bearings mounted in differentialsis based on maximum torque and correspondingspeed as is the case with automotive gearboxes. Thepercentage times at the individual speeds are based onexperience. This information is then used to determinethe mean index of dynamic stressing. The rolling bearingsmounted in cars should have an average f Lm valueof 1...1.3.Wear of these bearings should be minimal since differentialdrives require a high guiding accuracy and asquiet running as possible. With today's bearing dimensioningthe service life of differential bearings is eitherterminated by fatigue or wear.A detailed calculation of the attainable life is usuallynot necessary as these bearings have proved their worthsufficiently in the automotive sector. Bearing dimensioningbased on a comparison calculation with theindex of dynamic stressing f L is sufficient.FAG 52