The Sierra #3 has three sets of large driving wheels. These wheels each have a large, horizontal nub on which the locomotive’s rods are mounted. As we’ve shown in previous posts, these nubs are called the crankpins, and are a very important piece of the machinery that makes the locomotive go. It is very important that they are positioned correctly, and are of sufficient mass and strength to tolerate the constant pounding forces against them.
These pins are made of hardened steel, but the wheels themselves are made of cast iron. Due to the limited archival documentation available on this locomotive, we can’t confirm that the wheels are the originals from 1891, but it is likely. The original crankpins were most likely pressed into place, but old cast iron is brittle and succeptible to damage. To reduce the stress on the old wheels, and ensure an even, tight fit (called an “interference fit”), it was decided to freeze the pins for insertion. By soaking the pins in liguid nitrogen, it was possible to shrink the steel enough to slide it into the holes. By speaking with others who have experience with this type of procedure, and running a practice run ahead of time (measuring the steel at both temperatures), we determined we would have somewhere between 30 seconds to 3 minutes to insert and clamp the pin. However, the heat transfer to the wheels (at an ambient temperature of 100 degrees farenheit) added some unpredictability.
Norm Comer demonstrates how he plans to insert the frozen pin.
Worst case scenarios were discussed. If the pin stuck during insertion, the only way to remove it would be to ship it back to the Sacramento shops for reboring and quartering again. Another fear was that previously repaired cracks to the wheels could open up, possibly requiring casting of a new wheel. Either of these possibilities could result in increased costs and project delays.
The six crankpins were each machined in the Railtown Tri-Dam shop by machinist Bob West. Each pin measured within a hair’s width to ensure an exact fit to its hole. The two pins on the center (flangeless) driving wheels are larger than the other four, because they each have two rods attached to them. We never actually weighed the pins, but the largest are estimated to weigh around 50 lbs each.
One of the larger crankpins soaking in liquid nitrogen ( -321 degrees Farenheit).
Dennis "DJ" Ponder, wearing double-gloves, stands by.
Norm Comer and Dennis "DJ" Ponder grapple with the pin. . .
. . . as they move it into position. .
. . . and into the hole. . .
. . . a final push sets it in place, and George Sapp tightens the clamp (to hold it in place while it cools to fit).
- And then the process starts over again.
While the staff at Railtown were completing the final three pins, closing this phase of the project, the staff at the Sacramento shops of the California State Railroad Museum were waiting for the delivery of the chassis which had been loaded earlier in the day. It was a long day all around.
This officially marked the beginning of a new phase in the project. Over the next two months, the new boiler will be attached to the frame, at the Sacramento shops, while the Railtown crew completes the restoration of the tender. By early November, the boiler and frame will be back in the Railtown shops for the remainder of the assembly. Stay tuned!