Inside Clearance. By the expression inside clearance is meant the amount the steam port is uncovered by the exhaust cavity of the valve when the valve is in its central position. Formerly it was customary to have an inside lap of about 1/16 of an inch but in recent years in the development of engines which require a free exhaust at high speeds, the inside lap was reduced until now there is in some cases from 1/8 to 3/16 inches inside clearance. The effect of changing a valve from inside lap to inside clearance, other things remaining unchanged, is to hasten release and delay compression and hence to increase the interval in which the exhaust port remains open. It also permits a greater extent of exhaust port opening. As a consequence, the exhaust is freer and the back pressure is reduced, giving an advantage in the operation of the engine, which is desired at high speeds. Experiments have shown that an increase in inside clearance for high speeds will bring about an increase in the power of the locomotive, but an increase in inside clearance at slow speeds entails a loss of power and a decrease in efficiency. The loss in power at low speeds, due to inside clearance, is greater at short cut-offs and diminishes as the cutoff is increased. Tests have shown that at moderate speeds, say, 40 to 50 miles per hour, all disadvantages are overcome.
Requirements. The valve motion of a locomotive engine must meet the following regulations:
- It must be so constructed as to impart a motion to the valve which will permit the engine to be operated in either direction.
- It must be operative when the engine is running at a high or low speed and when starting a heavy load.
- It should be simple in construction and easily kept in order.
A number of valve gears have been developed which fulfill these requirements more or less satisfactorily, such as the Stephenson, the Walschaert, the Joy, and the fixed link, the Stephenson gear being the one most commonly used in the United States. A study will be made of the Stephenson and Walschaert gears, the latter resembling in some respects the Joy valve gear. The Walschaert gear has been extensively used in Europe for many years and of late years has become quite common in America. There are a few modifications of the Stephenson gear which have been made to meet structural requirements but the great majority of American engines are fitted with a device as illustrated in Fig. 64. The action of this device is fully explained in the article on "Valve Gears."
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