All About Engines

i3o All About Engines beyond that desired, the bell-crank lever is turned so that the lower arm slides off or misses completely the end of the lever which opens the valve. The duration of the opening, then, is under the control of the governor, because the governor determines the time during which the bell-crank depresses the lever. Such an arrangement is called a “ trip-gear.” It is quick, delicate, and reliable in its action, and regulates to a nicety the amount of steam which enters the cylinder at each stroke ; while the power required to drive it is much less than that needed for a flat valve, which is pressed upon the ports with the full force of the steam. There are endless forms of drop valves and trip gear, and they are wonderful to watch, especially the “ hit or miss ” action by which the governor exer- cises its control. They are fitted on most large hori- zontal engines, but not on vertical engines, because the valves work most satisfactorily in a vertical position, and are, therefore, not easy to adapt to vertical cylinders. In some engines—those made by Galloway, Limited, of Manchester—the exhaust valve is a slide valve, because with low pressures the chief disadvantage of that form of valve vanishes ; and there are other modifications, of which we need describe only one, called the “ Uniflow ” engine. In the ordinary slide or piston-valve engine the steam enters and leaves by the same passages, and even in the Corliss and drop-valve engines it follows the piston up and then comes back again. The result is that steam ports, cylinder covers, and portions of