Contact Patterns on Ring Gears

[Bruce2]

Since reading the contact pattern is somewhat subjective, I thought I'd show a recent contact pattern I achieved and ask opinions.
Maybe this topic could be a place to show patterns you have if you're having trouble setting the R&P.

[doc]

Interesting!

Let's leave thread as is and see what response is. You might consider changing title to something like "Trans gear contact pattern" or similar just for clarity and to make easy to find. If it gets traction, we can make it a sticky.

doc

[Bruce2]

Subject changed.

Here's the current gearbox pattern:



I didn't take pics of the pattern prior to this one where it had slightly more backlash. The previous pattern looked better to me, so I think I will go back to it. Then I'll post pics.

I was able to rig up my dial indicator to measure the backlash directly. This contact pattern had 0.25mm of backlash.

[helowrench]

Bruce,
To me that looks good.
DISCLAIMER: zero experience on these transaxles, but in the aviation world, we are cautioned to keep the contact pattern exactly like the top RH picture shows.
off the heel, off the toe, and not all the way off the top. You would be amazed how that pattern spreads out under load.

[Pablo2]

I looked everywhere for that tutorial on setting up K R&Ps, but couldn't find it.
I think your drive pattern will improve @ .006" backlash.
If a sticky, both Ring and Pinion patterns should probably be shown. JMO

[Bruce2]

Here's the pattern I had with 0.3mm of backlash:

http://images.thesamba.com/vw/gallery/pix/1236263.jpg

(Shoptalk won't accept the link)

It looks virtually no different from the previous. I'll give .15mm a try, Paul.

Pinion tooth patterns: In the past I used to look at both, but I found I was chasing a ghost since the pinion gear tooth is not centered to the ring gear teeth. There's no way to get the pattern centered on both at the same time. The most recent book I found on American car rear ends did not mention pinion tooth contact patterns. All the talk and photos was showing ring gear patterns only. I took this to mean: ignore the pinion.

[raygreenwood]

Fantastic thread!

A question for the experts here please.

Im just starting a tear down and basic re-bearing, re-seal and adjustment of a 004 four speed in about 3 weeks (if the suspension work gets finished in time).

In short order I will be doing the same to my two spare boxes so its going to be solid work.
All of these boxes are smooth and quiet gear wise and have pretty standard bearing pre-load on the differntial and pinions. No slop.
So as I have done in the past....I will be pre-measuring all positions so I can for sure arrive at hte same pinion to ring gear lash on reassembly....and simply have the bearing pre-loads to set.

The question is this.....the ring and pinion on this box meshs in a hypoid configuration. The pini ok n shaft crosses across the ring gear between 4 and 7 oclock meshing at 4 o'clock position on the bellhousing end of the casd.
I am planning to make gear tooth impressions just to check everything. Is there a difference in the mating pattern with a hypoid gear?
Thanks in advance. Ray

[helowrench]

Ray,
If you are making before/after comparisons for gear mesh patterns, I have a tip.

use a very light thin spray paint (we used silver colored) brush it onto the gear teeth, paint the entire tooth..
after it dries, run the gears through. Then use a heavyweight cellophane clear tape. rub the tape on with a qtip.
when the tape is pulled gently from the tooth, the paint will lift. Affix the tape to a nice white sheet of paper.

This gives the ability to measure the pattern much better, and also gives a valid reference for comparison purposes.

[Pablo2]

Hypoid coast & drive flank photos embedded, 6.17K.

Ray, here is a link to a Samba photo of another hypoid ring gear drive flank. http://images.thesamba.com/vw/gallery/pix/1178901.jpg I suppose one advantage to hypoid is that you can see a pretty distinct burnish pattern too (the single wiped clean tooth). This is a new R&P. Used will likely have a somewhat wider pattern.

The reason I use bluing is that I can apply to the pinion teeth first, get a positive imprint on a few ring gear teeth (which tells me if something's radically wrong), then commence with rolling bluing onto all ring gear teeth for a final full pattern check.

[Pablo2]

This is the drive flank of a well-used 915 pinion (8:35K). I failed to get a good photo of the coast flank, but you can barely see a small, centered pattern on the center tooth (Porsche has smaller coast contact area than VW). It took me forever to get this pattern .. a Type II VW is relatively simple.

[raygreenwood]

Exellent post thank you!
In the past I have always printed the teeth to get a pattern before putting in new bearings so I would have a decent mental picture of what it should look like reassembled correctly.
The problem was I was never sure wether as a hypoid it should look the same as a skew bevel type gear. I was lucky enough that last time I did this.....I actually had white lead to work with.

Also this transmission like the one you had pictured...is very inaccessable to viww at the point of gear contact. I will post a few pics when I get to that stage.

Thanks for the suggestion Helowrench. Not to jack the thread....but as white lead for this purpose is largely unobtaniun.....and being that I work with liquid and fluid rheology for a living....I started looking at metal suspensions and pastes a few years back to see what I could do to easily duplicate white lead paste.

Learned a lot in the last 7 years with conductive inks and coatings. The white lead paste is in many cases about 75% lead powder or more with a hammered or tunbled flake, a small portion of 3-5% of one of more types of talc (out of the 120 or so commercially available and all with differnt attributes)...and a small amount of oil.
At least thats one recipe I found. Regardless of recipe....it creates a paste that has no normal shear based/thixotropic characteristic.....and is porous enough to be non hydraulic so it does not pump out e easily. The closest attribute I have found is that its rheopectic....or shear thickening.

I found a high metal content conductive ink at work last year that would work ideal for this. Sadly....its 50/50 silver and silver chloride. Runs about $1.25 a gram....and is corrosive to metal. Still looking!

[Henryhoehandle]

I am going to make this a sticky..nice work, guys.

[helowrench]

I found a high metal content conductive ink at work last year that would work ideal for this. Sadly....its 50/50 silver and silver chloride. Runs about $1.25 a gram....and is corrosive to metal. Still looking!

the rattlecan that we used was actually a high silver content paint, that ran $100 ish per.
although, I suspect that silver wheel paint with AL in it would work well also.

[Bruce2]

Paul, I reduced the backlash to .1mm like you suggested. This caused a huge shift in the pattern towards the toe. The side of the contact pattern was right on the end of the tooth, so no good. Sorry, didn't have the camera to record it. Leaving the diff shims where they were, I reduced the pinion brg shim by .05mm to put the pinion deeper into the ring gear. Better, but not much. I decided to increase the backlash to 0.2mm. The pattern looked better than any before. What do you all think?

http://images.thesamba.com/vw/gallery/pix/1236660.jpg

(again, too big for shoptalk)

[Pablo2]

I actually suggested .006" (.15mm), which I consider a minimum for a street car. But .008" (.2mm) looks pretty darn good in this case. VW suggests .15-.25 range, but .15-.18 is usually the sweet spot.

I use the VW 385 bar for setting specified pinion depth, then set backlash to .006" (.15mm) and start checking with bluing from there.

At some point, we should probably bring Ro, Po, and some other measurements into the discussion. A while back, I started keeping notes of easy-to-do measurements, and can now pick what pinion shim(s) to use in advance of measuring with the 385 bar (for each specific factory R&P ratio), using only "r" and "A". I was prompted to begin taking notes when VW went through a period of NOT etching "r" on factory installed R&Ps, and when they switched to using Po instead of Ro.

This diagram was for early Porsche R&Ps, but it applies to VW as well. "r" for VW is of course found on the ring gear, not the pinion head as stated in the diagram notes.

Edited with "R" drawn in.