Crankin’ out Crankpins

One of the driving wheels, sans crankpin.

One of the driving wheels, sans crankpin.

The Sierra #3 has three sets of driving wheels. Each wheel has a large pin that sticks out of the side of it. The side rods of the locomotive attach to these pins, and when they move back and forth and push the pistons, causing the iconic “chug chug” sound. The old crankpins were slightly out of quarter, and worn. In order to quarter them (the process of making sure they are exactly 90 degrees opposite), we’d have to remove too much of the material. If the crankpins are too thin, they can’t transmit the power as effectively.
The old pins were cut off flush to the surface, then drilled out. This method caused less stress than pressing them out.
Newly machined crankpin, alongside the old one.
Newly machined crankpin, alongside the old one.

Now we are in the process of machining new crank pins on the lathe in the shop. Each pin takes about 5 hours of machining. When they are complete, we will shrink the pins by freezing with liquid nitrogen, which will cause them to shrink just enough that we’ll be able to slip them into the holes. As they warm up, they’ll expand, and a tight fit will be achieved.

The lathe in action.
The lathe in action.
Advertisements