BW78 diff gear backlash

[slewinson]

If you put your engineering hat on, the bearings will indeed take all of the direct load and transmit it to the housing. No structure is totally rigid, and large transient loads will cause some deflection of the bearing, the pinion shaft or the diff carrier. If these loads are large enough then they could cause plastic deformation of the material. This could manifest itself as a minute stretching of the pinion shaft or a slight deepening of the bearing pocket in the carrier.

This phenomenon is cumulative and non reversible and accounts for the decrease of preload and then appearance of end float in the pinion shaft. I seriously doubt that there would be enough wear in the pinion bearings to create end play without the bearings being visibly damaged and subsequently failing in a short period of time.

The addition of a rigid spacer would stiffen up the pinion shaft significantly, reducing possible deflection and maintaining preload.

Hope this helps to explain why things loosen and eventually break, not only in the differential, but everywhere that there are significant load reversals (connecting rods are another good example).

Simon

[Charlescrown]

OK the crush tube is there to ensure the correct setting of preload. The bearings are both supported by the housing. The only plastic deformation that can occur is stretching of the pinion or compression of the housing so what part of the tube can now play any part in end float?

[slewinson]

OK the crush tube is there to ensure the correct setting of preload. The bearings are both supported by the housing. The only plastic deformation that can occur is stretching of the pinion or compression of the housing so what part of the tube can now play any part in end float?

Bullseye!

Typically, when you assemble a differential there are a few considerations on how it all falls into place. #1 is the pinion depth, set by the combined inner bearing and shim stack thickness. Get this wrong and the gears will whine louder than a woman whose credit card has been forcibly removed. #2 is the preload, set by the difference in length between the spacer and the internal gap between the bearing inner races. This is easiest to set by measuring the torque to turn the pinion. Its purpose is to prevent hammering of the roller faces by load reversals. #3 is the pinion to crown wheel backlash and finally #4 is the carrier bearing preload.

A hypoid gear set generates significant axial load on the pinion, and also not insignificant radial load on the pinion bearing. If you look at the geometry, this radial load will translate to axial load as the roller tries to slide up the outer race. All of this results in continuous axial load reversals on the pinion shaft and its bearings.

Replacing the crush spacer with a solid hunk of steel has more merit than just "not crushing". It will allow for a realistic torque to be applied to the pinion nut. This makes the bearing inner races more a part of the structure and stiffens up the pinion shaft to inner race interface, better maintaining correct alignment of the rollers and thus improving their ability to withstand larger loads that we all want to create.

Although the crush spacer has been maligned, I feel that bad assembly practices are at the root of some failures; the outer races need to be scrupulously clean as does the recess in the carrier. If this is not so, the load will rapidly force the outer race into its "correct" and fully seated position with resulting end float on the pinion and bearing destruction. Ditto for the cleanliness of the inner races, pinion shaft, shims, spacer and pinion flange. Lack of correct seating here, eg. dirt between the inner race and the pinion gear can result in too much preload and the bearings burning up. Most shops that I have seen assembling differentials do not get close to the level of cleanliness that I feel is necessary to have a 100% trouble free bearing fitment.

Sorry to blather on but I have seen so many equipment failures that are blamed on the incorrect cause. Feel free to fire off any questions and I will answer them if I can.

Simon

[vlad01]

I agree on the cleanliness.

When I removed the axles and installed new ones with new bearing and also removed the diff cover on mine. I made sure it was surgical clean.

[TdracerTd]

Some great info in this thread.

[slewinson]

Some great info in this thread.

Thanks. Great to hear that it is useful.

Simon

[slewinson]

I agree on the cleanliness.

When I removed the axles and installed new ones with new bearing and also removed the diff cover on mine. I made sure it was surgical clean.

It is quite common for everybody to stress how clean things need to be for an automatic but there seems to be a perception that manual gearboxes or diffs seem to be immune to dirt.

Simon

[vlad01]

absolutely a misconception. they would probably be worse as autos have a filter and heaps of fluid, diffs and manuals have none and the low amount of oils unsure dirt and wear particles recirculate much more often doing progressive damage.

[slewinson]

absolutely a misconception. they would probably be worse as autos have a filter and heaps of fluid, diffs and manuals have none and the low amount of oils unsure dirt and wear particles recirculate much more often doing progressive damage.

Agree totally

[VX L67 Getrag]

O.K. only had a QUICK skim over but I think there needs to be a STICKIED thread about how to setup a diff correctly!!!

So many I have bought setup & mesh is no where near correct & can play with noise with the accelerator & thought there's NO WAY I will ever try & rebuild a diff mysef!