What's the Diff?

Making a Wood Bros. Differential Gear Out of a Free Pattern


| June 2007



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 lubrication.

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!