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

Saxon gear on 22 HP Wood Brothers engine needed repair

| Summer 2007

In Steam Traction, March/April 2006 article, I talked about triumph, trials and tragedies. Well this article is no different. It’s all about turning tragedies and trials into triumphs. My original title for this piece was somewhat misleading: "What's the Diff?" But 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 their differentials.

The differential is one of many places on a steam engine that will show wear. They are mostly lubricated by oil and 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 can 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 the 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! 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 I knew that would be a low production pattern. I am no pattern maker, but I have seen it done once or twice.