Eccentrics on the Drive Shaft

What's an eccentric?

I usually think of it as a person with an unconventional approach to life.

You know the type. The person who wears odd clothing or espouses ideas that don't fit the community norm.

But that definition doesn't fit here. We're talking about a Shay geared locomotive, after all.

You could, of course, say that the Shay was the eccentric locomotive of its day. An oddball -- unconventional in the way it functioned and looked.

Last Saturday, I took a few minutes to talk to Sam Thompson, El Dorado Western Railway's lead machinist.

I asked Sam what he was working on. You see, as a novice around locomotives, I have to continually ask, "What's that slug of steel on the lathe going to become?"

"It's cast iron," Sam corrected.

Sam explained: He's machining the cast iron billet into the middle eccentric for the Shay's drive shaft.

"The old one is laying over by the band saw (pictured above right)."

The middle eccentric is split in half to install in onto the crank shaft (pictured above left), Sam explained.

Unlike the two end eccentrics, the middle eccentric is warn beyond machining. Dave Stohl was able to machine the other eccentrics.

"We'll need somebody to push this thing through the band saw -- that's going to be cute," Sam said.

So, what's an eccentric?

It's "a mechanical device with an off-center axis of revolution that converts the rotary motion of one component of a mechanism to reciprocating motion in another."

Much like the valve lifters in an internal combustion engine, the Shay's eccentrics lift the valves that let steam into the cylinder.

Now that I think of it, railroaders fit the definition. Aren't all steam railroaders a little off-center?

The cast iron billet that Sam is using to build an eccentric to replace the original.

The end eccentric installed on the crank shaft.