HYDRAULIC CYLINDERS HYDRAULIC TERMS
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Open Center System - A hydraulic system in which the control valves are open to continuous oil flow, even in neutral. Pressure in this system is varied, but flow remains constant. Open Center Valve - a valve, in which the inlet and outlet ports are open in the neutral position, allowing a continuous flow of oil from pump. Open loop circuit - A system where the oil is drawn from a reservoir at atmospheric pressure, circulated by a pump under pressure, through valves, into an actuator and then returned to the reservoir at near atmospheric pressure. Most hydraulic systems are of this design. Operating Pressure - The pressure at which a system is normally operated. Orifice - A restricted passage in a hydraulic circuit. Usually a small drilled hole to limit flow or to create a pressure differential in a circuit. O-Ring - A static and/or dynamic seal for curved or circular mating surfaces. O-Ring Boss (SAE) Port - This is a port that has straight or parallel threads as well as an O-Ring to seal the threads.
P Packing Kit - All the necessary seals to rebuild a hydraulic cylinder or component.
Parallel Circuit - The most commonly used in mobile equipment. Oil is available to all ports. However, if two or more spools are fully shifted at the same time, the oil will follow the path of least resistance and the cylinder or motor with the lightest load will begin to function first. The oil can be divided so that it will flow to two or more functions by metering the spools. Pascals Law - states that “pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio (initial difference) remains the same. Force applied to a confined fluid acts with equal pressure in all directions. Pilot-Operated Solenoid Valves - The spool is shifted by fluid pressure tapped from an inlet port or an outside pilot line. The solenoid opens and closes miniature orifices directing fluid to the end of the spool. The pilot valve usually sits atop the slave valve. Pilot Pressure - Auxiliary pressure used to actuate or control a component. Pilot Valve - A valve used to operate another valve or control. Pipe Port (NPT) - Pipe threads are tapered threads that count on interference between the male and female threads to do the sealing. Piston - A cylindrical part which moves or reciprocates in a cylinder and transmits or receives motion to do work. The disc-shaped element within a cylinder connected to the rod. The surface area of the piston dictates the cylinder’s force capabilities. The hydraulic fluid acts on the surface of the piston opposite the rod. Seals are used on the piston to prevent leakage. Poppet Valve - A valve design in which the seating element pops open to obtain free flow in one direction and immediately reseats when flow reverses. Port - The open end of a fluid passage. May be within or at the surface of a component. Positive Seal - A positive seal is a term that refers to a seal that has the capability to seal with no leakage. Power Beyond - An optional power port on open center valves for directing flow to a downstream valve. Proportional Flow Dividers - These valves will divide the flow of one pump into two equal flows regardless of variations in load. Most of these valves are 50/50 ratio; however, other ratios can be ordered from the manufacturer. Pressure - Force of a fluid per unit area, usually expressed in pounds per square inch (psi). Pressure Compensated Flow Control Valves - These valves are designed to control the speed of hydraulic cylinders or motors, eliminating the variations in speed caused by changes in load. These valves have an inlet and a “controlled flow” (CF) port as well as an “excess flow” (EF) port. Adjustable flow control valves allow the operator to adjust the flow of the “CF” by varying the size of the orifice of that port. Once the “CF” is set, it will remain nearly constant with variations in pressure on either the “CF” or the “EF” port. Any remaining flow is bypassed to the excess flow port which can be used to supply another circuit or can be directed to tank.
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