NP205 Idler shaft setup question

[Tetanus]

I'm rebuilding a GM np205 and trying to setup the idler shaft tolerance. My haynes manual says to check the setup the tolerance to between 0.000 - 0.002" and then torque the idler shaft nut to 200 ft/lbs. I tried this once and the tolerance got too tight as I started tightening the nut (the idler shaft is pulled toward the nut and the idler gear did not spin freely).

So, my question to those that have done this is should I make sure that the tolerance is correct after the nut is torqued (this seems to make more sense to me) vs. before the nut is tightened...like I'm reading in the book?

 

[Hogback Fabrication]

I believe there should be .000-.002" of clearance with the nut installed and torqued.

 

[454k30]

I used the shims that came out when I dismantled it and torqued the hell out of it. Spins great and no abnormal vibrations.

 

[Leadfoot]

I'm rebuilding a GM np205 and trying to setup the idler shaft tolerance. My haynes manual says to check the setup the tolerance to between 0.000 - 0.002" and then torque the idler shaft nut to 200 ft/lbs. I tried this once and the tolerance got too tight as I started tightening the nut (the idler shaft is pulled toward the nut and the idler gear did not spin freely).

So, my question to those that have done this is should I make sure that the tolerance is correct after the nut is torqued (this seems to make more sense to me) vs. before the nut is tightened...like I'm reading in the book?

It's almost counter intuitive (more shims equal more slop and less shims equal less slop....the opposite of most applications).

You need more shims (or thicker ones). The shims hold the opposed tapered roller bearings apart from each other. Without the shims and tightening the nut, you force the tapered roller bearings against their races and lose your .000-.002 tolerance. As you add shims, you keep the bearings apart and therefore they cannot be forced against their races. Once you put too many shims in you get too much tolerance (i.e. slop). I generally try to keep it no less than .0005 (ie. slight needle movement) and no more than .002 as trying to get .000 and knowing you aren't overloading your bearing is tough.

FWIW.

 

[4X4HIGH]

It's almost counter intuitive (more shims equal more slop and less shims equal less slop....the opposite of most applications).

You need more shims (or thicker ones). The shims hold the opposed tapered roller bearings apart from each other. Without the shims and tightening the nut, you force the tapered roller bearings against their races and lose your .000-.002 tolerance. As you add shims, you keep the bearings apart and therefore they cannot be forced against their races. Once you put too many shims in you get too much tolerance (i.e. slop). I generally try to keep it no less than .0005 (ie. slight needle movement) and no more than .002 as trying to get .000 and knowing you aren't overloading your bearing is tough.

FWIW.

I swear the last NP205 i did had the shims between the case and bearing NOT between the bearings.

 

[Leadfoot]

I swear the last NP205 i did had the shims between the case and bearing NOT between the bearings.

Then somebody most likely was in there before you.

Here is a link to a previous thread on this topic

http://www.coloradok5.com/forums/showthread.php?t=50216&highlight=shim

There is a link to an exploded view of the NP205. Other than a poor representation of tapered roller bearings, it does show the "spacer" and shims between the two bearings.

I will admit that my first build on a 205, I put the shims in the wrong locations and in order to get my .001-.002 I just left the idler shaft looser than was recommended . If I tightened it to the recommended torque the tolerance went to zero, so I just left it loose enough to get my .001". That was not a good idea on my part but I was young and dumb (now I'm just older).

Ask JEK as I believe he's done writeups on the 205 as well. He should be able to confirm or deny this...

 

[Tetanus]

I swear the last NP205 i did had the shims between the case and bearing NOT between the bearings.

When I took this 205 apart, the shims were between the case and the bearing too. Humm... either people are putting them together wrong or the diagram isn't correct. Anybody know of a way to be sure??

 

[Tetanus]

It's almost counter intuitive (more shims equal more slop and less shims equal less slop....the opposite of most applications).

You need more shims (or thicker ones). The shims hold the opposed tapered roller bearings apart from each other. Without the shims and tightening the nut, you force the tapered roller bearings against their races and lose your .000-.002 tolerance. As you add shims, you keep the bearings apart and therefore they cannot be forced against their races. Once you put too many shims in you get too much tolerance (i.e. slop). I generally try to keep it no less than .0005 (ie. slight needle movement) and no more than .002 as trying to get .000 and knowing you aren't overloading your bearing is tough.

FWIW.

Well, I slept on it and have thought about it more. I'm thinking that you're right Leadfoot. The diagram shows the shims between the bearings, and my book diagram does too (I just always like to put things back together the way they came apart and make sure to note the dissassembly order and it's hard to go against that). The book also says that you can bench set up the tolerance then install the idler gear into the case and recheck. This would be impossible to do if the shims are between the case and bearing! So I'm going to try putting it together with the shims in the location according to that diagram.

I totally understand what you were saying about tightening the nut and losing the tolerance...when I tightened the nut, the gear would bind. I wasn't sure if it was the bearings binding or the gear was binding against the case and needed shims to space it away. Now I know it was the bearings!

Thanks for all the help!