Valves

by Charles A. Purinton

The Live Steamer, March-April 1950



Valves and valve gears can be the making or the breaking of the performance of any engine. There are other things that enter the picture as well, but, if the valves and valve gear are right, the other conditions can be dealt with and usually corrected without too much trouble.

It could be that an explanation and a skwtch or two might be welcome. I realize that I am treading on dangerous ground here, as a complete answer to valve and valve gear problems takes a lifetime of study to master and a lot of technical study, in addition.

When talking about this subject, one will hear some of the terms mentioned, such as lap, lead, valve travel, steam ports, exhaust port, bridges and line in line. To a good many of the older (I do not mean in years) and more experienced Live-Steamers, these terms are clear and well understood. To the man building his first engine, and some of the more experienced ones as well, they are about as clear as mud and not too well understood.

Lap is the amount the steam edges of the valve overlap the steam ports when the valve is central. Its purpose is to allow the steam to be used expansively. This means less fuel and water is used.

Lead is the amount the valve has opened the steam port for admission when the piston is on a dead center. It is given for the purpose of getting the steam into the cylinder so that it will be there ready to push the piston as it passes the dead centers.

Valve Travel is the total distance that the valve moves in one direction and measured with the reverse lever in either full gear position. it should be equal to one lap plus one steam port times two. For example, if one lap is 1/8 inch and one steam port is 1/8 inch, then the total full gear travel should be one-half inch. It is important to know the total full gear valve travel so that the valve gear itself can be designed to give this full gear valve travel.

The steam ports, the exhaust port and the bridges are best explained by Sketch #1 and the Port Face. The bridges should be equal to the steam ports. The exhaust port should be at least twice the size of the steam ports, measured fore and aft.

Line and Line is a somewhat ambiguous term, as it is used in two different ways. Sometimes it is applied to a valve that has no lap. In this case, the steam edge of the valve and the steam port are line and line. Such a valve would permit steam to follow the piston its whole stroke and would be extremely wasteful.

It is my belief that this term is more commonly used in connection with the exhaust edges of the valve and the exhaust edge of the steam ports. This is the condition shown on Sketch #1. The valve cavity should always be this way, if it is a trifle more, it will help the steam to get out easily, and this is just as important as getting it in.

Square
From The Live Steamer, March-April 1950

What is meant by the term Square or Out of Square applying to a locomotive?

When an engine is Square it means that the valve setting is such that the four exhaust blasts, which occur with each revoltuion of drive wheels, are evenly spaced and of equal intensity. being Out of Square is the reverse of this condition and indicates some trouble with the valve gear (or valve setting).