XIV. PISTONS.
A piston is generally a cylindrical piece which slides backwards and forwards inside a hollow cylinder. The piston may be moved by the action of fluid pressure upon it as in a steam-engine, or it may be used to give motion to a fluid as in a pump.
A piston is usually attached to a rod, called a piston rod, which passes through the end of the cylinder inside which the piston works, and which serves to transmit the motion of the piston to some piece outside the cylinder, or vice versâ.
Fig. 46.
A plunger is a piston made in one piece with its piston rod, the piston and the rod being of the same diameter.
A piston which is provided with one or more valves which allow the fluid to pass through it from one side to the other is called a bucket.
Simple Piston.—The simplest form of piston is a plain cylinder fitting accurately another, inside which it moves. Such a piston works with very little friction, but as there is no adjustment for wear, such a piston is not suitable for a high fluid pressure if it has to work constantly. This simple form of piston is used in the steam-engine indicator, and also in pumps.
Fig. 46 shows the piston of the circulation pump of a marine engine. A is the cast-iron casing or barrel of the pump; B is a brass liner fitting tightly into the former at its ends, and secured by eight screwed Muntz metal pins C, four at each end; D is the piston, which is made of brass, and is attached to a Muntz metal piston rod E. The liner is bored out smooth and true from end to end, and the piston is turned so as to be a sliding fit to the liner. The wear in this form of piston is diminished by making the rubbing surface large.