Due to the efforts of Dr. Allan Wallace of Adelaide, Austrailia, Steam in the Garden Online is able to make available simulations of various types of popular valve gear. Dr. Wallace has created a PC-DOS based simulator that allows for the design of valve gears. Based on that simulator, the animations that you see here were captured and formated so that you can enjoy them via the Web.

To view these simulations you must use Netscape Navigator 2.0 or later. Other browsers may work, let me know what you experience. The simulation will be very slow until the entire file is loaded and will then progress quite smoothly.

A valve gear is a mechanical linkage used on steam engines, particularly railway locomotives.

Such steam engines are based on a piston being pushed back and forth in a cylinder by steam which is controlled by a valve. The task of the valve gear is to operate the valve.

The valve gear primarily needs to ensure correct timing of steam admission and exhaust from each end of the cylinder. However, it also needs to provide means for the engine driver to reverse the locomotive, and to control the quantity of steam admitted at each stroke of the piston.

The nineteenth century engineers who designed valve gear mechanisms used such techniques as models, graphical construction, intuition, trial-and-error and empiricism. Valve gears became the source of more argument, misunderstanding, mysticism, patent activity and lawsuits than any other aspect of the steam engine.

This package is a new chapter in this ongoing (and some would say obsolete) debate. Today's robot technology has provided mathematical techniques and hardware for analysing the kinematics of mechanisms at the computer console.

These programs enable you to analyse the kinematics of some common form of valve gear.

Strictly, it is not a design package, although it can be used as a design tool. There are a number of references available which explain how to design a motion. This software then allows you to check your design before cutting metal. It is also a useful diagnostic tool for studying existing motions and experimenting with modifications.

Its great feature is that it shows you EXACTLY what is happening to the valve, because it calculates on the basis of linkage geometry, not by assuming sinusoidal motion between extremes of amplitude. The effects of rod angularity, for instance, are taken into account.

Essentially, you enter the key dimensions of the mechanism. You can then have the motion plotted on screen for any chosen crank angle, showing in detail the position of the valve relative to the ports.

The program can repeat this calculation automatically for one complete rotation of the wheels, and present results in the form of a plot of valve motion vs piston displacement (oval diagram), or as an indicator diagram.

The facilities exist for you to "notch up", and to check the sensitivity of the motion to displacements of the driving wheels due to suspension movement or worn axleboxes.

Finally, the change in timing of valve events as the expansion gear can be plotted, so that you can see how even the beats will be.

THE VALVE SIMULATOR IS A SHAREWARE APPLICATION
The author released this package into the Public Domain on a shareware basis as a means of judging user interest and response.

The shareware concept is one in which the user can try- before-you-buy, and the programmer receives (perhaps) the encouragement to enhance the product.

Anyone is free to copy and distribute the software, so long as it is retained complete and unmodified. The author accepts no responsibility for any claim arising from the use of the program.

If you find the package useful and would like to support its further development, you are urged to register by sending $AUS20 ($US15) to the author. Please quote the version of your package. You will be sent a copy of the latest version, and your name will be placed on a mailing list to receive advice of new releases or versions.

In the interests of supporting the model engineering fraternity, any correspondence is welcomed. If sufficient feedback occurs, further development of the package will occur, and others may be released also.

Address for correspondence:

Dr A K Wallace
31A Birkinshaw Ave
TRANMERE 5073 South Australia

Ph (08) 340 1666 Fax (08) 340 2568
A.H. Ph (08) 332 8183
Email: awallace@adelaide.on.net