What's the Diff?
Making a Wood Bros. Differential Gear Out of a Free Pattern
The pinion placed on the differential and the casting machined with the spider gears, or beveled gears, back inside the windows on the spider of the differential. Notice the difference between the oil tube and the completed casting. Mike Murphy added 1/8-inch pipe, bent it to fit the contour of the casting and put a grease fitting in the end of the pipe.
In a Steam Traction, March/April 2006
article on casting pinion gears, I talked about triumph, trials and
tragedies. This article is no different: It's all about turning
tragedies and trials into triumphs. The title is somewhat
misleading, although after you read it, you might say the same
thing to yourself - What's the Diff?
Well the "Diff," in my case is the differential. The Saxon gear
on my 22 HP Wood Bros. engine is made out of two pieces, the outer
ring gear and the center body, called the spider. The spider has
four ears and sits inside the outer ring gear. On either side of
these ears are springs. These springs are to cushion any start-up
under a heavy load and cushion any transmission of power to the
drive train. I believe Case has the same in its differentials.
The differential is one of many places on a steam engine that
will show wear. They are mostly lubricated by oil, by gravity feed.
The problem is that you have to stop the engine in a certain
position in order to oil the spider gear shafts, and the oil that
runs in will run out when you advance to oil the rest of the
shafts. This is just one reason it is a good idea to inspect the
differential when you have the chance.
The wear that showed on my Wood Bros. was in the spider itself.
My dad had an Illinois engine that showed wear in the same place.
As I remember, the beveled gears were bored and new shafts were
made for his engine.
My differential showed wear at a little different area. The wear
was in the ends of the beveled gear shafts, but not so much in the
shaft, as it was in the main casting or spider. I would have to say
this was maybe more from neglect than it was from lack of
I think what may have happened is that the pins that held in the
shafts broke or came out causing the shafts to move around in the
casting, and move around they did!
At one time the differential was taken apart and square headed
nails were used for pins that must have been missing, but it looked
like the damage had already been done. I thought if these areas
were machined and trued there wouldn't be enough of the main
casting left and I would lose strength. So my decision was to make
a free pattern that I knew would be a low production pattern. I am
no pattern maker, but I have seen it done once or twice.
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