Crossheads and Guides

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).

from wikipedia http://en.wikipedia.org/wiki/File:Walschaerts_motion.gif#filelinks

The movement of the crossheads and guides can clearly be seen in this animation. Notice the two long rods to the right of the driving wheels (crosshead guides), and the mechanism that slides along them (crosshead). This particular image depicts a different type of mechanism that can be found on the Sierra #3, but the idea is simliar.

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.

Tony and George with guides

Volunteer Machinist, Tony works with George on the guide pins.

Tony's handiwork

Tony's handiwork. Two new pins adjacent to the original.

Bob West and Rob Buchanan review numbers for crosshead guide measurements for a final time.

Bob West and Rob Buchanan review numbers for crosshead guide measurements for a final time, in preparation for machining the brasses.

Checking the measurements.

Checking the measurements.

Checking measurements against the guides themselves.

Checking measurements against the guides themselves.

Bob discusses how measurements are taken of the piston rod diameters.

Bob discusses how measurements are taken of the piston rod diameters.

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.

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