A question for the steam experts.

Model Railroader General Discussion (Model Railroader)
1

In the animation at this link:
http://travel.howstuffworks.com/steam1.htm
(scroll down to the locomotive animation)
It shows a steam loco in motion. The piston is moving in and out
and the loco proceeds forward. I get this. My questions is how does
the engineer make the loco go in reverse. This may sound dumb
on my part, but it seems to my that in reverse the piston would
still be moving in and out. What determines which direction the
wheels turn?

I have looked at this animation and my head hurts. Someone please
help! [:D] Dave

2

This page will provide some insight:

http://home.new.rr.com/trumpetb/loco/rodsr.html

Yes, the piston just goes back and forth–it is a question of how the gearing attached to the piston is set. The animation you looked at does not feature reversing gear…

3

The valving is more complicated than shown, Picture the high pressure steam and the exhaust reversed, pushing the wheel in the opposite direction.

4

Jetrock,
Thanks, that provided a lot of insight. I knew there was a trick to it. Makes some
sense now. Dave

5

And for some GREAT explanations of the different kinds of valve gear, and how they all work - check out this site: http://www.tcsn.net/charlied/ Not everything in his download package was used on locomotives, but all the various locomotive types are there. The animations are great, you can see exactly how the linkage and valves work.

–Randy

6

A steam engine is double acting. That means each 1/2 stroke is a power stroke. You reverse a steam engine by reversing the order in which the valves allow steam admission. That’s the function of the valve gear.
Here’s an animation of Walschaert valve gear that shows forward and reverse motion. http://home.new.rr.com/trumpetb/loco/rodsr.html

Andre

7

That is correct. The radius arm and elevating link work with the Johnson bar to move the cylinder valve forward or backward in its chamber. This could be done while the locomotive was in motion in some cases to effect braking (before friction brakes were added to locomotives and rolling stock). While stationary, the engineer simply moves the lever in the cab back or forward and gets the motion suggested by the position of the lever. As stated, by physically relocating the valve in its sleeve, you cover one inlet port and expose the other, thereby forcing the steam to exert pressure on the opposite side of the piston from what it had been previously. Piston is therefore forced to move in the other direction, but this now causes the wheels to rotate in the opposite direction as before.

8

You said It selector, you said it!