So this is a bit different, but I've been scratching my head over it lately. How did builders figure out the size of the components that went into the valve gear on a locomotive? I'd imagine there must be some sort of set of formulas that would help identify how large the individual components in the motion had to be...

Yes, this was done by the mechanical engineers at the time. Before there were standard practices in engineering, there was a lot of guess work and copying of other design concepts. A lot of things broke or didn't work well so the designs morphed and changed over time. A lot of mechanical engineering has its foundation in the industrial revolution.

OK - let's say you know the area of the valve face (piston valve) and the operating pressure - can you figure out how much force it will take to shove the valve against it? If not, this is going to be over your head I'm afraid...….. if you can, then you know how much force you need to design into the components.

Formulas to tell how strong a part had to be to resist bending have been around for about 200 years. For most metals they actually put test pieces in a machine and push on them until they bend. Or they use a different machine and pull on the test piece until it pulls apart (tensile strength) Knowing the force and the thickness you can figure out the basic strength of that kind of metal. It varies of course, steel is stronger than iron, etc.

After that is was some amount of experimentation, often times a railroad would order one copy of a new locomotive design and test it "on the road". Parts that were not strong enough were found and made thicker and stiffer. Then they would order a whole group of engines.

In the case of one class of new NYCRR Hudson locomotives they found out that the higher boiler pressure (275 psi ?) was bending the main rods !!!. So they dropped the boiler pressure by 10 psi and the problem went away.

Cheers, Kevin.

You might find the book "Apex of the Atlantics" by Frederick Westing interesting, if you can find a copy that is. It contains some interesting information on how the Pennsylvania refined and lightened their Walchearts valve gear components as much as possible by testing on their stationary plant.

Pat Hosford
Volunteer
BC Forest Discovery Centre