[TR] How do they manufacture gears?

[MMoore8425 at aol.com]

In a message dated 2/11/2009 8:48:27 A.M. Pacific Standard Time,  
tr3driver at ca.rr.com writes:

I've  been wondering about that.  Some simple experiments with  original
Triumph gears seem to indicate that they are not case  hardened.  In fact,
they aren't very hard at all.  Compared to  say, a cheap socket, they cut
quite easily.  So I would guess that  they are only heat treated and left at
a moderate  temper.

Randall



I have designed and manufactured gears for gearboxes and final drives in  
industrial equipment, (sometimes more successfully than others!) 
 
There is a big decision point issue regarding surface hardening or not  as 
this is a big cost driver.
When I can, I prefer to through harden to 38 Rc if possible, and not  surface 
harden. Bad things can happen when you surface harden (cracking,  
insufficient case depth, distortion etc.) You're also doing this because  something is 
stressed highly enough that wear is an issue.
 
I once watched some Harley Davidson gears being cut through hardened to 58C  
using a Japanese $2M gear cutter. These were very high precision high speed  
gears. 
 
 If you surface harden, you usually need to worry about surface  accuracy and 
I have had to have gears ground to retain the accuracy. Some  methods distort 
more than others, some methods promote cracking more than  others.
 
Through hardening to too high a level can also cause the material to become  
brittle. I once made a gearset of 440C stainless steel (55 Rc through)  and  
they broke up due to shock loading, even though we never approached the yield  
strength. We later referred to them as the 'glass gears"! 
 
 
Cost is a big issue, and the designer has to look at all the alternatives,  
such as material choice, design, methods of manufacture, tooling etc. etc.
 
I once saw some gears in San Diego being cut out of a stack of plywood  glued 
together! I have no idea what his application was, but he could have had  
steel or aluminum for almostthe same price as he was paying for machine  time.
 
Mike Moore