Using Prussian Blue on Bevel Gears

[Duccout]

Has anyone done the above task? I have been trying to set up a used pair of Mototrans crank bevels and have wasted hours going back-and-forth. Mototrans lower bevels do not have grind marks, so I have no idea how the mesh can be set up correctly, so I thought that it would be a simple task to check with Prussian blue, but however I set them the result was inconclusive - one tooth would be contacting at the back, but another tooth would be contacting at the front.

In the end, after almost tearing my hair out, I gave up and just set them in the position that they had already been running in, according to the polished teeth. It is hit-and-miss as I can't be sure that they are in exactly the same position, but l have no other answer. How did Mototrans set them up without grind marks? Perhaps they had some sort of assembly jig....



Cheers,


Colin

[themoudie]

Aye Colin,

I think that some quiet reading time is required, rather than thrashing about, with shims, gears and Prussian Blue.

The illustrations and descriptions used in the links to KHK Gear Manufacturer (Japanese) website should help explain what you are trying to achieve.

KHK_Stock_Gears_webpage_link

KHK_Stock_Gears_Bevel_Gears_link

KHK_Gears_Q&A_about_gears

Your are not alone with this problem, as I too have pairs of MotoTrans helical bevel gears, with no ground flats to use in re-assembly and I shall be assembling them into a different head, different cam set up, so,the original gear alignment shimming is unlikely to be optimal. As for the lower pair of helical bevels, they are again without a ground flat.

IF YOU HAVE A MOTOTRANS ENGINE AND INTEND TO DIS-ASSEMBLE IT; BEFORE DIS-ASSEMBLING THE UPPER AND LOWER BEVELS TO THE CAMSHAFT AND IF THEY SHOW NO SIGNS OF HEAVY WEAR, GRIND A FLAT ON THEM. IT MIGHT NOT BE DEFINITIVE, BUT WILL AT LEAST GIVE A GOOD STARTING POINT FOR SHIMMING WHEN RE-ASSEMBLING.

I hope this helps.

Good health, Bill

[Duccout]

Hi Bill,

I did look up articles and You Tube videos of how to do the job (it appears to be the same technique as used for setting-up vehicle differential gears) and it looks simple, but in practice, it is not. For instance one tooth will show perfect engagement, but the next three teeth will leave no discernible print in the blue, then another tooth will show engagement at one side of the tooth and another the other side, so there is no uniformity to allow adjustments to be made.

The videos of differentials being adjusted show a yellow paint being used on the gears, so maybe something thicker than blue would have worked, or maybe the problem is that the gears are worn, or maybe my gears are not a matched pair anyway. How anyone could make a living out of working on these engines is beyond me, it takes me hours to set them up!

Cheers,

Colin

[Rocla]

Hello Duccout, I did this task, 10 years ago, thanks to support from Motoscrubs.com members, including you, if I remenber well. The blue must be regular on all the teeths length. First time, before using the blue, there were not enough play and the engine was whining. But the second time, the play was just the right one. Maybe you have a problem of bevel gear axis geometry?

[Duccout]

Hi Rocla,

Yes, the gears that I am using I bought a while ago and they may have worn unevenly. Sometimes it just needs someone who knows how to do it to explain the correct way.

Cheers,

Colin

[blethermaskite]

Colin, I have been where you are quite a few times and have had exactly the same problem......frankly I believe engineers blue is too thin (I should probably duck now ) I use a thin artists paintbrush stroke of red oil based gloss paint, I think a major problem with the 'mesh' is the degree of runout on the crankshaft (I know you are working on a rebuilt crank by Nigel so I would be surprised if he has not got runout to the absolute minimum) however if the bevels had previously been working with a less than perfect crankshaft they will have worn in a 'bit off' which you are now fruitlessly trying to correct. So all you can do is try to average out the degree of mesh lengthwise along the teeth whilst making absolutely sure you have no tight spots (which will trash them) I aim to get about 1.5 thou backlash at the tightest spot with the best length contact and live with whatever the loose clearance is. Others may disagree with my method, I'm sure Nigel will suggest his method on this. Cheers, George

[Duccout]

Hi George,

I think that you are probably spot-on; paint is used on car differentials, so maybe blue is not suitable. I was reluctant to try it as the bottom end is assembled and I was worried about getting it off the vertical gear afterwards, especially if it did not work!

Again I reckon you are right about wear - I have little run-out, but the bevels I am using will have worn to the other engine's crank run-out, so that will not be in the same place as mine. There is also a problem with the wear on the bevels not allowing adjustment, because if I try to bring the horizontal bevel in to where I would like it to be it binds on the unworn outer section of the teeth in one spot, badly, so that it almost seizes in that spot. This leads me to think that the bevels must go back in the same position that they have been running in, because the teeth have worn to mesh with each other.

Finally, Nigel recently mentioned on here that narrow-case bevels can be ground to a smaller diameter to allow them to fit wide-case engines; does anyone know if this could be done on a bench grinder, say by using the shaft to scribe an accurate line around the circumference to grind to? I doubt it, as I think that the bevel would have to be held rigidly to get an accurate finish.

Cheers,

Colin

[blethermaskite]

Do you think this 'pair' of gears you have are an actual match? to get the paint off just put a lint free rag in the timing case and wash the gear with acetone or celly thinners, in a rough and ready way..... sure you could reduce the bevel dia. by hand using various types of grinders......but best if you have (or can get access to) a lathe and use a tool post grinder, that's a proper way to do it.
Cheers,
George

[Duccout]

Hi George,

I bought the gears as a pair, but when spares are used and are 50 years old, anything is possible! I might give the paint a try....

Cheers,

Colin

[Paradiso]

This article might help.