Page:The Kinematics of Machinery.djvu/501

 ENGINES AS REVERSED RATCHET-TRAINS.

479

Hydraulic and steam engines (in their usual forms) have in general the same arrangement as that of piston and valve pumps or fluid ratchet-trains, and are made both single and double-acting. "When in motion, the difference between them is simply that in the former the pressure -organ is no longer driven, but is itself the driving link of the mechanism. The valves therefore can no longer be self-acting but must move with chain-closure. The

KG. 354.

Fro. 355.

general relation of their motion to that of the piston is, however, the same as in the case of the pump-valves. In order that this motion may be brought about without any great expenditure of energy, fast-clicks are used, that is, balanced lifting-valves, or (especially in the steam-engine) slides, to which the required motion is given by means of suitable valve-gear. We may there- fore say: the common steam-engines, hydraulic engines, etc., are reversed fluid ratchet-trains.* We shall return to this question further on.

posing the fluid to move in the opposite direction to that shown by the arrows, the valves being moved at the right time by some separate mechanisms. In the corresponding train of Fig. 352, the same thing would occur if the bar a moved down instead of up. It will be noticed that it does not follow in either case that the element a should be the driver.
 * This reversion of the action can easily be seen in Fig. 353, forinstance, by sup-