Hints and Tips - Crown wheel and pinion meshing

From: Peter Jacobs
Sent: 19 August 1999
To: british-cars-pre-war
Subject: Crown wheel and pinion meshing

Can anyone explain why a new spiral bevel crown wheel and pinion set I have fitted to my car runs quietly in the drive direction but makes a noise on the over-run? I have fitted new bearings to the pinion shaft and to the diff carrier and set both to zero end float. The clearance (backlash)between CW and P is set to 0.006in. Marking with engineers blue is textbook centre tooth, although this required the pinion to be moved rearward beyond the point where the pinion engagement covered the crown wheel tooth width. I have spent hours on this and explored the full range of settings available but cannot get it to run quietly. Can anyone give me the benefit of their experience in such matters?

Peter Jacobs, Almondsbury, Bristol.


From: Peter Jacobs
Sent: 20 August 1999
To: British-cars-pre-war
Subject: Crownwheel and pinion meshing

Thank you all for your prompt response regarding my buzzing axle. I have moved the pinion out of mesh by just one notch on the adjuster and it has got a little quieter, so the 0.006" backlash may well have been too tight as Geraint suggests. I look forward to seeing the factory instructions from the "Hornets Nest", and learning some more from the article Geraint has read. With an understanding of the mechanism that generates the noise in one direction and not the other, I should be able to make a reasoned adjustment instead of resorting to trial and error.

I had set the gears in dry condition and thought that when I filled the axle with the correct SAE140 oil it would damp out the tooth contact noise. Obviously I had not considered the effect of the much greater pressure on the gearing afforded by the car on the over-run than you get when turning the assembly by hand. This turns the whirring into a howl.

From another source I have been told to keep pressure on the throttle at all times!

Peter Jacobs.


From: Lawrence J Alexander
Sent: 21 August 1999
To: Peter Jacobs
cc: British-cars-pre-war
Subject: Re: Crownwheel and pinion meshing

Apparently, Geraint's answer to you was private, yet this is a subject that probably interests a few more of us. Would you be so kind as to post anything further that you find helpful. I, for one, have never understood what causes rear axle gears to whine and have often been plagued with this noise. Indeed, two of the vehicles I currently own (one, a 1983 Dodge van!) are so blessed...........'twould be nice not to have to play the radio quite so loud!

Lawrie Alexander
British Sportscar Center


From: C Knight
Sent: 23 August 1999
To: Peter Jacobs
cc: british-cars-pre-war
Subject: Re: Crown wheel and pinion meshing

Peter Jacobs wrote:
>
> Can anyone explain why a new spiral bevel crown wheel and pinion set I have
> fitted to my car runs quietly in the drive direction but makes a noise on
> the over-run?

Peter

You don't say what the differential is from. On most diffs the pinion assembly should be set to a specified position by shims and the crown wheel/differential assembly adjusted sideways by slackening one side and tightening the other until the correct clearance is obtained. Only then should teeth be blued and the rolling contact checked. The fine setting of the tooth contact height is done by adding or subtracting a VERY limited quantity of shim from under the pinion. As an example on the Morris/MGT Type diff if more than 0.004" is required then the gears are worn beyond the recommended tolerance (or incorrectly machined). The toe and heel contact along the tooth can then be centralized by re-adjusting the differential cage either by slackening one side and tightening the other on Morris units, or by adding/removing shims on other makes. If you have made the adjustments correctly I would suspect the accuracy of the gear machining.

Best of luck

Cliff Knight


From: Peter Jacobs
Sent: 23 August 1999
To: C Knight
cc: british-cars-pre-war
Subject: Re: Crown wheel and pinion meshing

Cliff,

The problem crown wheel and pinion is fitted to my 1927 Delage 6-cylinder car. The engineering design is massive and the pinion can be moved fore and aft via a threaded and castellated nut with access from the axle nosepiece. The crown wheel and differential assembly are located in large ball races with outboard thrust races located against threaded locking rings. This arrangement provides an infinite range of adjustment. Although I have a lot of Delage info, there is very little workshop data to be found, so reverse engineering is the order of the day. One would have thought that the heel of the pinion should align with the outside diameter of the crown wheel for the nominal position, but this does not give the expected marking, nor does it give silence on the overrun. I too am wondering whether the fault is in the machining of the gears. The gear set is not Delage manufacture, has no marking other than 3XX, and I know nothing of its origins except that it came from the spares holding of a Delagiste long since deceased. When I receive the treatises on crown wheel and pinions mentioned in earlier messages perhaps all will be revealed. I will let you know what transpires.


From: TA TERRY
Sent: 24 August 1999
To: Cliff Knight; Peter Jacobs
cc: british-cars-pre-war
Subject: Re: Crown wheel and pinion meshing

You folks might want to take a look at this URL:

http://www.differentials.com/install.htm


From: Peter Jacobs
Sent: 15 September 1999
To: British-cars-pre-war
Subject: Crown wheel and pinion meshing

Thanks for all your advice. I removed the axle once more (no mean task as it has a massive torque tube and the petrol tank and springs have to come off as well). Wheels and axle shafts were removed. The car had done about 200 miles on the previous cwp setting, and the contacting faces of the teeth were lightly burnished and just beginning to erase the machining marks. All traces of oil were then washed off.

Ignoring the previous setting I repositioned the heel or widest point of the pinion to coincide with the outer edge of the crown wheel, just were you would expect it to live, and set the backlash at 0.006" to 0.010". I coated four teeth of the crown wheel with engineers blue and rotated the wheel several time in both directions to mark the other teeth. This demonstrated the cause of my difficulties. It was incredibly difficult to see what was actually going on because the thin blue afforded such poor contrast to the colour of the heat treated gear. It also picked up in the light machining marks. It was obvious that I had misinterpreteted these markings first time around, swayed by information from someone with experience of fitting this size assembly that I should expect correct meshing futher in towards the centre of the crown wheel.

The solution was to clean off all the blue, and paint four teeth with white cellulose touch up paint instead. The result was simple and obvious marking, and was seen to be almost in the correct place straight away ie mid height, and towards the toe on both sides of the teeth. By a series of adjustments to pinion depth accompanied with a corresponding lateral adjustment to the crownwheel to maintain the backlash it was possible to establish a setting where there was silence in both directions of rotation. Further experimentation showed that only a relatively small change of pinion depth either side of this setting would produce gear noise in one direction of rotation or the other.

After all this careful engineering, and having replaced the axle etc, I jacked the car off its stands and lowered it to the ground. That crunching noise was the jack I had forgotten about pushing its way through the silencer box!

Ah well, it was in need of replacement anyway.

Peter


Home | Hints & Tips | Suppliers | Engineering Data | Links | Events | For Sale | Gallery | Books | Videos | About Us