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 prices. First are the pages explaining the nomenclature of the steam cylinder and slide valve.

 Steam Cylinder 1

Basic nomenclature for a steam cylinder

 Steam Cylinder 2

Top of a slide valve a steam cylinder

 Steam Cylinder 3

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 "Outside Admission".

 Piston valve

Piston valves 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.

 Steam Cylinder 4

"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.


 Steam Cylinder 5

Slide valve 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.

 Steam Cylinder 6

At the 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.

 Steam Cylinder 7

Port opening versus port width.

Now 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 slide valves.

Slide valve design 1
Design start 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.

Slide valve design 2
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.

Slide valve design 3
Port opening versus port width again.

Slide valve design 4
The completed valve.

Here are the separate steps to creating a Bilgram using a CAD program.

Bilgram diagram 1
My 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.

Here .

Bilgram diagram 2

Here .

Bilgram diagram 3

Here .

Bilgram diagram 4

Here .

Bilgram diagram 5

Here .

Bilgram diagram 6

Here .

Bilgram diagram 7

Here .

Bilgram diagram 8

Here .

Bilgram diagram 9