Mike E asked:- > Is there a maximum wheel rpm used to calculate small scale steam boiler > capacity. I seem to remember something around 350 RPM for prototypes.
The old British "rule of thumb" used to be "one inch of driver diameter for each mile per hour of maximum designed speed", which works out at about 365 rpm. The speed of full sized steamers tends to be ultimately restricted by the rate at which steam can be forced through the steam passages to the cylinders. As the scale is reduced, the volume of steam is reduced by a cube law, whilst the area of the steam passages reduces by a square law, which is why it's so difficult to make our small locos go slowly without designing in some form of wire drawing, either on the inlet or exhaust side. Mike C.
