Slide Valve Design
There are a number of sources on the
web of programs to help design and display steam engine valve gears.
However, these all seem to assume you have the dimensions for the slide
valve (or piston valve) you will use. What follows is a very good
explanation of how to design a valve for a steam engine. These are
pages from the "Engineers and Mechanics Guide" by Theodore Audel
published before 1920. Copies are available on EBay, but at rather high
First are the pages explaining the nomenclature of the steam cylinder
and slide valve.
Basic nomenclature for a steam cylinder
Top of a slide valve a steam cylinder
Valve in the
neutral position - centered on the seat. Slide valves are almost always
used as shown with boiler pressure on top of the valve. This is called
used on full size locomotives are almost always inside admission, the
steam entering between the inside edges of the valve piston. When Erich
Thomsen added a piston valve hidden inside the steam chest of his
locomotives, it it was of outside admission.
"Lap" is the
overlap of the edges of the valve with the ports in the valve seat.
"Steam lap" is also called "Outside lap" OR "Admission lap" and "Exhaust Lap" is called
"Inside lap". "Admission" and "Exhaust" are preferred since an inside
admission valve (piston valve) has the steam and exhaust reversed.
engines almost always have positive exhaust lap. There are cases where
negative exhaust lap is necessary because of the way the valve gear
(notable Stephenson as used on locomotives) operates.
beginning of the stroke, the valve must be moved to begin admitting
steam. Moving the valve the distance of the Admission lap would just
match the edges. The lead is the extra distance to allow steam to flow.
Definition of linear and angular advance.
Port opening versus port width.
we get to the real topic of this page. The Allen Valve topic is a
bonus. Allen valves were often used in locomotives when they were still using
with the general engine parameters. A 5" scale locomotive with 12.5 in
drivers goes 322 RPM at 10 MPH. I would use 300 RPM as a start. The
Redwood Valley locomotives mostly use 4.25 by 6.5 cylinders - another
very good starting point.
It is well
worth your time to master the Bilgram diagram. It is the easiest way to
design a valve. I use AutoCAD rather than paper and any CAD program
will lead to faster and more accurate results than pencil and paper. I
will expand on this below.
Port opening versus port width again.
The completed valve.
Here are the separate steps to creating a Bilgram using a CAD program.
spreadsheet to calculate parameters for a steam locomotive. In this
case a hypothetical 5" scale Heisler. Cells D10-D14 are based on
suggested rules by Henry Greenly, British designer of many model
locomotives, including those of the Romney, Hythe, and Dymchurch.