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Pinion Depth Shimming

Pinion Depth Shimming

Pinion Depth Shimming

Hi, I'm a mechanic learning about setting up differentials of the typical/basic type; open differentials with ring and pinion gear.  Nothing fancy.

I am not sure I understand why there are pinion depth shims on the pinion shaft between the cone bearing and the pinion gear.  But because there is a different amount of shimming on each differential I've seen I am guessing that there is a variance in the manufacturing process that is being compensated for.

I had thought maybe the pinion gear itself was the variable, but after extensive reading I've found a couple sources that say it is the housing that is the cause.

So my questions are:

Q1: what is the manufacturing process for the housing?  

(ie cast from ductile iron, line bored for carrier bearings, bored for pinion bearings?)

Q2: What are the shims compensating for exactly?

Q3:  Is there manufacturing variation in the depth of the pinion gear, ie the toothed portion?

Q4:  Is there variation in the total depth of the pinion gear and shims?

I know that there are shims correcting a variation and that the correct pinion depth is important for proper gear contact but I have no idea which dimension is variable and which ones should be constant.

I'm sort of mind boggled as to whether the shimming stays with the housing or with the gears.  It's often marked on the pinion gear but some sources claim it is specific to the housing, and the usual recommendation seems to be to transfer the old shimming to the new gears when doing a replacement.

Thanks for your help, apologies for any ambiguities about my inquiry and my long winded explanation.


RE: Pinion Depth Shimming

Are you may be referring to shim that goes on the pinion shaft behind the first bearing?  

It's obvious you know the The pinion gear is made to function in a certain 3 dimensional relationship to the ring gear.

One of them is the dimension on this image called the "crown to crossing point"

relative to the ring gear centerline it is controlled by how housing is machined, and the combined tolerances of the cone and cup of the first "Timken" bearing.  Assuming all housings or all bearings were, or are "the same" will get us in trouble.

RE: Pinion Depth Shimming


Those are some excellent questions.

Q1- the manufacturing process for an automotive hypoid R&P housing can vary.  Some are cast iron, some are cast aluminum, and some are even steel weldments.  However, all have the bearing bores final machined for both pitch center and axial shoulder locations.

Q2- the typical pinion shims are used to adjust the heel/toe location of the mesh contact.

Q3- it is much easier to manufacture accurate R&P gear sets in mass production than producing accurate housings.  That's why R&P gears come in matched sets.  Pinion shims are the most expedient method of adjusting the R&P mesh contact location.  The R&P gear teeth do typically have numerous subtle geometry mods to ensure that the mesh contact is optimized under loaded conditions.

Q4- I don't know if I understand this question.  By definition, there will usually be some dimensional variation when using shimming.

Hope that helps.

RE: Pinion Depth Shimming

Don't forget that, if adjustment is only provided for the pinion, then the most important consideration above all else is ensuring that the gear set has the required amount of backlash before setting the contact.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

RE: Pinion Depth Shimming

It is much cheaper to take a little too much from the pinion gear housing and shim the pinion than to get it 'just right' and not be able to use a given pinion gear set. Now where did I leave that tube of prussian blue?

RE: Pinion Depth Shimming

Nothing is manufactured perfect. In all types of machines where there are surfaces that act as load bearing, and that require a certain tolerance range for operating clearances, it has to be adjustable somehow. Also there is a consideration for pinion deflection under load, that makes it desireable not to have a perfect tooth contact pattern, under no load conditions.

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