Glossary of Hydraulic Terms
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PUMPS
The building block of any hydraulic system is the pump . The four most common designs are the vane, gear, gerotor and piston. All are well suited to common hydraulic uses with the piston design best suited for higher pressures. The variable displacement type is particularly well suited in circuits using hydraulic motors where variable speeds and the ability to reverse is needed.
OUTPUT
The Output of a hydraulic pump (gallons per minute, or GPM) is related directly to it's operating speed. The pressure of a pump is determined by it's manufactured capablilities.
HORSEPOWER
The horsepower required to drive a pump is dependent on both pressure and output in GPM. The higher the pressure, or greater the volume, the more Horsepower(HP) required. As a rule of thumb, a 1000 PSI(pounds per square inch) pump will require 1 Horsepower(HP), for the first gallon per minute and 3/4 HP each gallon per minute after that. Doubling the pressure or output volume will require 4 time the input HP.
The math: Input HP= GPM x PSI / 1714
CONTROL VALVES
The ways you can harness the power of a hydraulic system is through the use of control valves . The three basic types of control valves are the tandem center type (see below), the open center (motoring spool) type, and the closed center type.Both the tandem center and the closed center types are available in a three way or four way valve. BY opening or closing valves, you can control how much or in what direction a hydraulic piston moves. Valves can also control many pistons working with each other at the same time.
TANDEM CENTER VALVES
Tandem center valves when in a neutral position by passes the flow of hydraulic oil to the return line. This is used to hold the cylinder/piston in position with no load on the pumps. When this system is the pump is running constantly to keep a ready supply of hydraulic oil, but as long as the cylinder/piston is not in operation the pump is working under no pressure or load. This system keeps wear and tear on the pump down to a minimum.
OPEN CENTER VALVES
Open center valves are the same as the tandem center, except that in the neutral position all lines are connected back to the reservoir. The primary use of this system is to prevent "shock" loading when the valve is placed in neutral. This takes pressure off the motor. This system is used in situations where the operating device needs to be moved by hand.
CLOSED CENTER VALVES
This type of valve is used in a hydraulic system where the valve blocks the flow of oil from the pump into an accumulator. The accumulator (see below) is used to store the oil under pressure. This valve takes the pressure off the pump and in neutral locks the cylinder in place with no load on the pump.
PRESSURE RELIEF VALVE
A pressure relief valve is a safety device and is required on all hydraulic systems. Once adjusted, the pressure relief valve opens whenever the pressure goes beyond the value set and allows oil to flow back to the reservoir.
UNLOADING VALVE
The Unloading valve is installed in systems using accumulators (below). The function of this valve is to "unload" the pump of pressure until such time as a device on the system, such as a cylinder, actually begins operation. In this way, the pump is allowed to operate without load (pressure) until it is needed. The unloading valve is not intended to replace the pressure relief valve. In fact, the pressure setting of the unloading valve is much lower than the setting on the pressure relief valve.
ACCUMULATORS
An accumulator is used to store hydraulic oil, under pressure, to pressurize the system while the pump is unloaded. This oil is also used to supplement the power pump output during times of heavy use or for limited operations when the pump is not working. Accumulators also dampen surges within the hydraulic system.
HYDRAULIC CYLINDERS
Hydraulic cylinders transform the pressure and oil flow in a hydraulic system into work or mechanical force. They are used where linear motion is required to move something. Cylinders are ususally double-acting, that is, oil under pressure can be applied to either side of the piston to provide movement in either direction. Single acting cylinders are sometimes used where the weight of the load is used to return the cylinder to the closed position.
CYLINDER THRUST
The push of a cylinder in pounds:
Thrust = Piston Area (in.) x Pump Pressure (PSI) or
Thrust = bore diameter squared x .78 x PSI
HYDRAULIC MOTORS
Hydraulic motors are another important piece of the hydraulic system. However, instead of a cylinder (force moving linear) the motor uses hydraulic pressure to rotate. In terms of how it's built, a motor is like al pump. But, when it's operated oil enters the motor and turns the shaft. The speed of a hydraulic motor is dependent on the amount of oil supplied by the pump and the torque is dependent on the amount of pressure supplied.
RESERVOIRS
The size of a reservoir will depend on the capacity of the hydraulic system, as well as what is required by the system. The reservoir should contain a large volume of oil and should provide ample oil to the pump.
PIPING
When you are connecting up a hydraulic system, use the tubing or pipe that is capable of handling heavy pressures and loads required by the hydraulic system. Pipes should have a minimum number of bends and fittings, should be securely fastened and and clean. Iron pipes are not recommended because they have particles that will flake off and contaminate a system.
CLEANLINESS
The most important part of a hydraulic system is how clean it should be. Any extreme pressure system is extremely vulnerable to dirt, particles and other matter that contaminate the close tolerances necessary for any hydraulic system. All pipes, fittings, and other components must be extremely clean before use.
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