This afternoon, I was helping a friend assemble his rear axle, using some new parts. We got the drive shaft together and moved on to the ring gear setting. Our first lash reading was maybe 0.030", so we added a ring gear shim and after reassemble the pinion was stuck and wouldn't move. So it all came apart. We checked the ring gear to be flat and there were no burrs holding things up, the ring gear seemed to fit on the diff case OK, but I tried a 0.002" feeler and could fit it between the ring gear and the diff case flange. Seemed odd. So we reassembled the diff case and ring gear without any shims and the assembly would rotate easily. I next checked the lash on every other tooth. Started out with 0.020" and as we moved around the gear the lash increased to 0.030" and then down to 0.015". Seemed like for the most part the majority of the lash measurements were between 0.015 and 0.020", but a section of about 8 teeth the lash was closer to 0.030". So, apart it came again. We then tried to measure the run out on the diff case flange, where the ring gear mates. We first noticed that between the hole there was a dip of a few thousands and there was one section of the flange that was at least 0.030" low. Our conclusion was that the diff case flange is bent. My question is, can this be machined out? How and where would the case be chucked up? Would I have make a jig to hold the case? Or... should I just make wall art out of this case and get a different one?
You should be able to machine out the run-out. I think using ring gear shims is a bad practice. The ring gear drives chiefly on the friction of the ring gear to the carrier. Any shims greatly reduces this friction causing the torque to be driven through the bolts. A better way is to shim the axle thrust washers.
The ring gear bolts maintain the contact force. The shim is just part of the sandwich. Friction coefficients between the shim and gear and shim and flange are about the same as between the gear and the flange. If the shim was made of greased lightning, then there would be a significant loss of friction.
Ted, not true. Every shim diminishes friction. This can be dangerous if you are using stock brakes. I have seen rear ends with shims that sheared some of the bolts and one that sheared ALL of the bolts in a panic stop situation. Please do it right, shim the washers. There is no down side to shimming the washers.
I respectfully disagree.
You say: "Friction coefficients between the shim and gear and shim and flange are about the same as between the gear and the flange."
What makes you think this is true? Is this based on some study or do you just believe it is true?
I think it is not true because in all serial friction systems each layer reduces friction. An example is the three washers in a T transmission. This is not true of a parallel friction system, such as the T clutch.
Do you think my real world observations of sheared ring gear bolts are faulty?
What is the down side to shimming the washers?
Ted, with all respect I think you are advocating a dangerous situation based on your gut feelings.
You are right Tom.
You can put shims behind many things, but ring gears and the like are not one of them.
It would be like putting a shim behind a loose Pinion gear.
The pinion gear has to be tight to metal behind it. With the flexing and stress on the gear, it will eat into anything between the Gear, and carriage.
That is also not the way to adjust clearance anyway.
Well.... figuring that I had nothing to lose, we chucked the diff case in a lathe and indicated on the flange. We toke some time to make sure everything was straight and we got about the same readings as we did with the test indicator in the axle housing. So about 0.030" was cut from the flange. The ring gear was mounted onto the case and we checked the pinion lash. This time the lash was consistant on every tooth. A shim had to be used to lower the lash to about 0.012". Should have this unit back in the car soon and we will see how she works.
I respectfully disagree. If the bolts failed then they were likely loose and weakened by fatigue. If the bolts are properly tightened there is no reason that a shim between the ring gear and the flange should be a problem. This is my educated opinion, I am entitled to mine. You folks are entitled to yours.
Ted, with all due respect, of course you have a right to your opinion, but in my opinion what you said was not stated as opinion, but rather as fact. Also, in my opinion what you are saying is bad advice and dangerous. There is no downside to shimming the washers. (In my opinion.)
Ted, you say that a shim doesn't significantly reduce the carrier to gear friction. In your educated opinion how much do you think it would reduce the friction (if any)? In your opinion would it be OK to use two shims between the ring gear and the carrier? How about 10 shims?
I stand by my statement. I am done, carry on.
Ran out of educated opinions?
Mike, you didn't say whether you used a shim between the ring gear and carrier or behind the steel thrust washers. I'm with Tom on this. The best place to shim the assembly is in the thrust washer stack.
Allan from down under.
I don't see how you could easily make a shim to correct a non - concentric misalignment problem. Cutting the flange true was the easy way to fix this.
If I did the math right and unless you are building a race car, at 35 MPH the engine is turning at about 1300 RPM, with a standard rear end that means you are turning the ring gear at about 365 RPM. Unless the ring gear is bent and it being made of tuffer stuff then the housing it bolts to, I would think when bolting together the gear would pull the case flat.
I would find a different case and then remember you are working with 100+ year old technology trying to apply today's standards. There is an amount of "looseness" built in that has allowed the car to last for all these years. MHO
My experience with a bent flange was the gear did not pull the flange flat. A few well placed blows with a sledge hammer did pull it in where it was usable.
Ya, I fix my spark plug gaps like that!
Since that time I have acquired a lathe and a press and use tools with more precision.
Sometimes you use what you have at hand and just get the job done.
I think if you set your spark plug gaps with your pocket knife you will have better luck.
I don't see anything wrong with doing a little "body and fender" work to help bring things closer to true, although I'd finish up by lathe if at all possible.
I used a shim between the diff flange and the ring gear. Maybe not perfect but it will do for me. This car will see normal duty and should serve well. I've done it both ways.