Pinion Bearing, Crush Tube Questions (1 Viewer)

[Rusty Marlin]

I know what the book and the interweb says about the crush tube, it's to set bearing preload...I'm having a hard time with this explanation, to the point of tossing the Bull S#!t card.
The inner and outer bearing races are fully seated into the machined cast iron seats in the diff housing. They are at a fixed location and spacing, there is NOTHING flexible about this arrangement.
The tapered roller bearings must bear on these races or there will be radial and end slop, the preload (IN MY MIND) should be set off the torque on the pinion nut holding the tapered roller bearings into the outer races, EXACTLY like setting up preload on wheel bearings.

So what does the crush tube actually do? It can't possibly be setting the preload in the bearings, that's coming from the rolling elements in the bearings contacting the races. If the crush tube was setting the preload, then the bearings might be floating in space and there would be slop, because you might stop tightening before the rollers came into contact with the races.

The only thing that makes sense in my mind is the crush tube is providing enough tightening resistance to the pinion nut to cause proper thread load (80% of yield) to hold the nut in proper load to resist backing off the pinion shaft threads (even though its mechanically locked with a deformed shoulder and loc-tite).

I thought it might be acting as a "shock absorber" and taking some of the load off the cast seats in the housing, but on reflection this doesn't make sense either as the tube is pushing against the inner races which are pinched between the pinion gear shim and the pinion flange and it would just rotate with the assembly. The need to compress the roller bearings to the outer races is still setting the preload which means the diff housing is still seeing the same initial compression load with or without the crush tube.
I suppose it may be acting as a secondary support to the diff housing such that if any additional load were applied it would resist that load along with the housing.

Any Automotive Drive Train Engineers here that can answer this question other than by quoting the book or internet.
I have put this question to 4 other ME's here at work along with the drawings and diagrams and we are all shrugging our shoulders.

 

[80t0ylc]

You need to talk with Zuk (@gearinstalls.com) He's considered the guru of differentials by many here on mud. His business is in Chandler, AZ. But, according to a search, he was last here (IH8MUD) in Feb., I guess because he's retired. So you might try his business. Best to email him at gearinstalls@gmail.com

Edit: BTW he's not retired - just taking a year long break...lol!

 

[fourtrax]

it is all machined surfaces but. not perfactly consistanct. in my mind i comes down to production cost and how to handle the machining variances between all the parts. the proper thing to do is to put it all together and measure the slop, calculate the thickness required to address it and install a shim to set the clearances. but to do this you have to take the time to assemble, measure , add correction remeasure. it was much more economical to create a crush sleeve that would distort under torque to create the proper preload. only real drawback is that if something hits the flange then the sleeve can be further distorted ( washing out your preload). chances are your probabbly not going to hit anything with your axle flange driving down the road so, the crush sleeve is a pretty roboust but economical solution. if your going to be playing off-road/ the rocks it would be a good idea to actually replace the crush sleeve with a solid spacer (shim).

 

[Bambusiero]

It's not at all true - what you said - that because the bearing races are seated into machined cast iron, so there cannot be any movement. Not true.
Everything moves under pressure - everything, no matter how thick and strong, whether a tiny little bit - or more.
Even the hardened bearings move and flex, under pressure, and they right spring back when the pressure is removed (or moves over to a different place as it rolls).
But it's not something you can easily design for - to change from no load at all, to some much larger maximum load, than it is to change from load "A" to load "B", if both are below the point of permanent plastic deformation.
Like seat belts - you want them tight to start with (your pre-load), then you can take quite an impact. That same impact all at once breaks stuff.
They also sell non-crush able re-usable solid bearing spacers, where the pre-load is set by putting in just the right shims, and torquing down on that.
They say the preload is then repeatable after disassembly.
The crush sleeve is getting crushed, and is presenting some resistance (the pre-load), along a smooth sort of force/deformation curve. But it's a one time deal - you can't let off the pressure and start over with those.

 

[BILT4ME]

Also, the crush sleeve is pressing against the INNER race of the inner and outer bearing to "keep them apart" and setting tolerances between them that also changes with heat and the equipment is driven. Yes, the outer races are "fixed" in the cast iron case, but the bearings "float" between them and require a certain amount of clearance for oil and thermal expansion.