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.

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 "Outside Admission".

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.

"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.

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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.

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.

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.

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.

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.

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.

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