Hydraulic Engine

a machine that converts the energy in a stream of fluid into the mechanical energy of a driven member, such as a shaft or rod. A distinction is made between hydraulic engines, in which the driven member moves because of a change in the angular momentum of the fluid (hydroturbine and waterwheel), and positive-displacement hydraulic engines, which are actuated by hydrostatic pressure as the fluid fills the operating chambers and moves the plungers. (The plunger is an operating member that performs work directly because of the action of the fluid pressure on it; it may be a piston, plate, pinion tooth, and so on.)

In the first type of hydraulic engine the driven member performs only rotary motion. In the positive-displacement type, the driven member may perform either a restricted reciprocating or reciprocating-rotary motion (hydraulic cylinders) or an unrestricted rotary motion (hydraulic motors). Hydraulic cylinders are subdivided into force and torque types. In the force hydraulic cylinder, a rod connected to a piston performs rectilinear reciprocating motion relative to the cylinder; in the torque hydraulic cylinder, also called a quadrant, a shaft performs reciproeating-rotary motion relative to the housing through an angle of less than 360°.

Hydraulic motors are subdivided into piston types, in which the operating chambers are fixed and the plunger performs only reciprocating motion, and rotor types. In the latter the operating chambers move and the plungers perform a rotary motion that can be combined with reciprocating motion (link hydraulic motors). Link hydraulic motors are subdivided into plate and rotor-piston (radial and axial) types depending on the form of the plunger. The most common is the axial rotor-piston type, in which the pressure of the working fluid on the piston develops a reactive force on an inclined washer, thus causing the shaft to rotate. Positive-displacement hydraulic engines are used as hydraulic drives for machines. The operating pressure of the fluid may be as high as 35 meganewtons per sq m, or 350 kilograms-force per sq cm. Hydraulic motors are made with power up to 3,000 kilowatts.