12 replies to this topic

[colinu]

QUESTION: Is 12thou end float on a diff's output shaft too much?

 

I've just finished rebuilding my gearbox, then turned my attention to the diff as I noticed there was too much play in the output shafts for my liking. Play was sideways wobble on the cage-side output shaft, and in/out float on both output shafts. This is a MiniSpares X-Pin diff that I fitted new 17k miles ago. Sideways wobble will be cured by new bush in the end of the cage (and half an hour in the machine shop), but I'm having problems eliminating the in/out float...

 

I've replaced the mica washers with the thicker BTA102 variant (measured at 41thou, compared to the 35thou of the old ones I removed). That solved the issue at the crown wheel end, but I still had a good 12thou of in/out float at the cage-end output shaft. Since then I've also fitted new planets, planet bushes, and output shafts. No change... still 12thou of float.

 

I'm at a loss to understand where that extra/unwanted float is coming from.

 

The only things I haven't changed are the X-Pins/block, and the cage itself. There isn't any noticeable wear on the thrust face of the diff cage (you'd be able to see/feel 12thou of wear very easily), and likewise there's no wear on pin surfaces where the planets run (pins measure 0.563" along their full length).

 

Any suggestions of what I should try next?

 

Adding 10thou to the mica shims (if that were even possible) runs the risk of the output shaft hitting/binding the X-pin center block. My only thought is I need thicker planet thrusts (which aren't available?)... or to fab up additional planet shims (from 10thou bronze sheet) and slip them between the cage and the current planet thrusts... but that seems like putting a bandaid on a problem rather than underfunding/addressing the root cause. And the really frustrating thing is this diff was tight as a gnat's chuff when I fitted it 17k miles ago!

 

... or am I missing something really obvious?

 

 

   Cheers,

   Colin.

Edited by colinu, 08 February 2019 - 06:30 AM.

[Spider]

Hi Colin,

 

As long as the thrusts are in good order, I wouldn't loose any sleep over any amount of end float.

 

In use, the Output Shafts are thrust (quite heavily !) outwards.

 

Further, there was a period in manufacture when the centre 'block' wasn't fitted. These had up to 1/4" end float.

[colinu]

Maybe so, but still doesn't seem right to me... and I'd certainly (out of curiosity if nothing else) like to no where 12thou of end float appeared from over the last 17k miles.

[DeadSquare]

Maybe so, but still doesn't seem right to me... and I'd certainly (out of curiosity if nothing else) like to no where 12thou of end float appeared from over the last 17k miles.

 

Isn't it called "Bedding in" as the faces polish up?

[colinu]

Bedding in... I doubt that's the explanation as all faces that could have "polished up" (with the exception of the pins/block which show/measure as unworn) have been replaced with brand new items (planets, their shims, output shafts and their mica washers).

[Spider]

Well, the added end float can only come from the Output Shaft Thrust Faces wearing, which is unlikely, the 'pip' in the center of the Output Shaft wearing (considering it don't contact anything in use, I doubt this), the Thrusts being a different thickness, that you've checked, the Diff cage wearing on it's thrust face, or the Center Block in the diff, wearing, though, again, there's nothing that contacts this in use.

[colinu]

Yes I'm still bamboozled too as there doesn't seem anywhere where this extra end float has come from.

On the plus side... a friend just dropped by with an mildy used (<20K miles?) X-Pin diff and swapping parts around in a compare and contrast manner I got no improvement (and his showed similar mount of end float)... so I guess I may be worrying about nothing. Although the engineer in me still would like to understand the root cause.

[Spider]

One other 'cause' could be that the original thrust(s) may have has a small wave in it, so, in effect, it's fitted thickness was more than it's measured thickness.

[tmsmini]

And I got a quick demo/lesson on the assembly and disassembly of a cross pin diff...

[colinu]

Maybe not a masterclass, but thanks again for dropping by with your diff Terry.

Edited by colinu, 08 February 2019 - 06:54 AM.

[colinu]

SOLVED... or so I hope.

 

I fabed up some backing shims for the planet thrust washers out of 8thou phosphor bronze sheet today (bottom-left of photo). Punched them between a planet and thrust to give them the correct dome shape. They fit behind the thrusts (i.e. the planets don't run against them). I also fitted a new output shaft bush to the diff cage and machined it to size, and sanded down the mica washer (crown wheel side) to 30thou. Now it's reassembled there is no noticeable float or wobble in the output shafts 

 

P.S.

A friend was texting with Guessworks earlier this week... and sounds like John's rule of thumb is that if the movement can't be eliminated with a bit of grease then it's too much.  JOHN feel free to correct me if that's wrong.

Attached Files

•  8thou_diff_shim.jpg   34.39K   0 downloads

Edited by colinu, 10 February 2019 - 02:11 AM.

[Spider]

While that may have been the reason for the change in in float, it not good that when initially assembled the gears were so deeply meshed. Only 'precision' gears (like for measuring instruments) are designed to operate in this way and it leads to rapid wear and breakage of the gears. Power Transmission Gears, the likes of which are in a gearbox and a diff all need to have back-lash.

[colinu]

Moke - I agree that zero back-lash in a gearbox, diff, etc is a bad idea. What I've been trying to achieve is to eliminate what in my opinion was excessive back-lash (wobble/float) in the output shafts... excessive compared to when the diff was new.  Those shims I fabed will bed in a little more and give a thou or two extra float. Only time will tell if I've taken this exercise too far...