The crossheads and guides are mechanisms that ensure that the piston rod only moves in the same direction of travel as the piston. Since the rods are connected to the crank pins on the driving wheels, without the crossheads and guides, there would be excessive transverse force, which would cause wear on the piston, and create a great deal of friction. This process is much easier to understand in action (click the image to activate animation).
Much work has been required to refurbish the crosshead guides on the Sierra No. 3, including machining 4 new guides (the long rectangular rods the crossheads slide back and forth on). Bob West is currently measuring to machine the new brass (yes, another bearing surface!) for the guides.
We have been very fortunate this past couple of months. Mike “Tony” Stroud and Ted Bray , two Columbia area machinists, have been volunteering in the shop. That has sped up the work and helped keep the project on schedule.
Citation for animationThe Sierra #3 is equipped with Stephenson-type valve gear, rather than the Walschaerts displayed. We’ll explore these interesting adaptations in a future entry.