Ag Power Web Enhanced Course Materials

If you experience any problems with the site, please contact Pete Hoffman immediately so corrections can be made. Pete can be reached on campus, via email at phoffman@swtc.edu or by phone at 1.800.362.3322 ext 2727.

Definition of Terms - Hydraulics

A	B	C	D	E	F	G	H	I	J	K	L	M	N	O	P	Q	R	S	T	U	V	W	X	Y	Z

A

Accumulator - A container which stores fluids under pressure as a source of hydraulic power. It may also be used as a shock absorber.

Actuator - A device which converts hydraulic power into mechanical force and motion. (Examples: hydraulic cylinders and motors.)

Back to Top

B

Bleed - The process by which air is removed from a hydraulic system.

Bypass - A secondary passage for fluid flow.

Back to Top

C

Cam Lobe Motor - A hydraulic radial piston motor in which rotational force is created by the outward movement of the pistons against the lobes of a stationary cam.

Cavitation - A phenomenon which occurs when the pressure at a point in a hydraulic system is lowered below the vapor pressure of the oil in the system. This allows bubbles of oil vapor to form in the oil. If this occurs at the pump inlet, the quick pressure rise inside the pump forces these bubbles to collapse violently. This can cause erosion of metal parts, noise and vibration.

Circuit - A series of component parts connected to each other by fluid lines or passages. Usually part of a "system".

Closed Center System - A hydraulic system in which the control valves are closed during neutral, stopping oil flow. Flow in this system is varied, but pressure remains constant.

Controller - A microprocessor that controls electro-hydraulic valve functions.

Cooler (Oil) - A heat exchanger which removes heat from a fluid. (See "Heat Exchanger.")

Coupler - A device to connect two hoses or lines, or to connect hoses to valve receptacles.

Cusion - A device sometimes built into the end of a cylinder which restricts outlet flow and thereby slows down the piston.

Cycle - A single complete operation of a compo- nent which begins and ends in a neutral position.

Cylinder - A device for converting fluid power into linear or circular motion. An "actuator". Basic design types include piston and vane units.

• Double-Acting Cylinder - A cylinder in which fluid force can be applied to the movable element in either direction.
• Piston- Type Cylinders - A cylinder which uses a sliding piston in a housing to produce straight movement.
• Rotary Cylinders - A cylinder in which fluid force is applied to produce circular motion. Single-Acting Cylinder-A cylinder in which fluid force can be applied to the movable element in only one direction.
• Vane-Type Cylinder - A cylinder which uses a turn- ing vane in a circular housing to produce rotary movement.

Back to Top

D

Displacement - The volume of oil displaced by one complete stroke or revolution (of a pump, motor, or cylinder).

Drift - Motion of a cylinder or motor due to internal leakage past components in the hydraulic system.

Back to Top

E

Energy - Three types of energy are available in modern hydraulics (of the normal hydrostatic type):

• Potential Energy - Pressure energy. The static energy of oil which is standing but is pressurized and ready to do work. Example: oil in a loaded accumulator.
• Heat Energy - Friction or resistance to flow. (An energy loss in terms of output.) Example: friction between moving oil and the confines of lines or passages produces heat energy.
• Kinetic Energy - The energy of the moving liq- uid. Varies with the velocity (speed) of the liquid.

Back to Top

F

Filter (OIL )- A device which removes solids from a fluid.

Flow Meter - A testing device which gauges either flow rate, total flow, or both.

Flow Rate - The volume of fluid passing a point in a given time.

Fluid Power - Energy transmitted and controlled through use of a pressurized fluid.

Force - A push or pull acting upon a body. In a hydraulic cylinder, it is the product of the pressure on the fluid, multiplied by the effective area of the cylinder piston. It is measured in pounds or tons.

Friction - The resistance to fluid flow in a hy- draulic system. (An energy loss in terms of power output.)

Back to Top

H

Heat Exchanger - A device which transfers heat through a conducting wall from one fluid to another. (See "Cooler, (Oil)".)

Horsepower - The work produced per unit of time.

Hose - A flexible line.

Hydraulics - The engineering science of liquid pressure and flow.

Hydrodynamics - The engineering science of the energy of liquid pressure and flow.

Hydrostatics - The engineering science of the energy of liquids at rest. (All the systems covered in this manual operate on the hydrostatic principle.)

Back to Top

I

Inert Gas - A non-explosive gas.

Back to Top

L

Line - A tube, pipe, or hose for conducting a fluid.

Back to Top

M

Manifold - A fluid conductor which provides many ports.

Moter (Hydraulic) - A device for converting fluid energy into mechanical force and motion - usually rotary motion. Basic design types include gear, vane, and piston units.

Back to Top

0

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.

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.

Back to Top

P

Packing - Any material or device which seals by compression. Common types are U-packings, V- packings, "Cup" packings, and a-rings.

Pipe - A line whose outside diameter is standardized for threading.

Piston - A cylindrical part which moves or recip- rocates in a cylinder and transmits or receives motion to do work.

Port - The open end of a fluid passage. May be within or at the surface of a component.

Pour Point - The lowest temperature at which a fluid will flow under specific conditions.

Power Beyond - An adapting sleeve which opens a passage from one circuit to another. Often installed in a valve port which is normally plugged.

Pressure - Force of a fluid per unit area, usually expressed in pounds per square inch (psi).

• Back Pressure - The pressure encountered on the return side of a system.
• Breakout Pressure - The minimum pressure which starts moving an actuator.
• Cracking Pressure - The pressure at which a relief valve, etc., begins to open and pass fluid.
• Differential Pressure - The difference in pressure between any two points in a system or a component. (Also called a "pressure drop.")
• Full-Flow Pressure - The pressure at which a valve is wide open and passes its full flow.
• Operating Pressure - The pressure at which a system is normally operated.
• Pilot Pressure - Auxiliary pressure used to actuate or control a component.
• Rated Pressure - The operating pressure which is recommended for a component or a system by the manufacturer.
• Static Pressure - The pressure in a fluid at rest. (A form of "potential energy.")
• Suction Pressure - The absolute pressure of the fluid at the inlet side of the pump.
• Surge Pressure - The pressure changes caused in a circuit from a rapidly accelerated column of oil. The "surge" includes the span of these changes, from high to low.
• System Pressure - The pressure which overcomes the total resistances in a system. It includes all losses as well as useful work.
• Working Pressure - The pressure which overcomes the resistance of the working device.

Pulsation - Repeated small fluctuation of pressure within a circuit.

Pump - A device which converts mechanical force into hydraulic fluid power. Basic design types are gear, vane, and piston units.

• Fixed Displacement Pump - A pump in which the output per cycle cannot be varied.
• Variable Displacement Pump - A pump in which the output per cycle can be varied.

Back to Top

R

Regenerative Circuit - A circuit in which pressure fluid discharged from a component is returned to the system to reduce flow input re- quirements. Often used to speed up the action of a cylinder by directing discharged oil from the rod end to the piston end.

Remote - A hydraulic function such as a cylinder which is separate from its supply source. Usually connected to the source by flexible hoses.

Reservoir - A container for keeping a supply of working fluid in a hydraulic system.

Restriction - A reduced cross-sectional area in a line or passage which normally causes a pres- sure drop. (Examples: pinched lines or clogged passages, or an orifice designed into a system.)

Back to Top

S

Solenoid - An electro-magnetic device which positions a hydraulic valve.

Starvation - A lack of oil in vital areas of a sys- tem. Often caused by plugged filters, etc.

Strainer - A coarse filter.

Stroke - (1) The length of travel of a piston in a cylinder. (2) Sometimes used to denote the changing of the displacement of a variable delivery pump.

Surge - A momentary rise of pressure in a hy- draulic circuit.

Symbols, Schematic - Used as a short-hand representation on drawings to represent hydraulic system components.

System - One or more series of component parts connected to each other. Often made up of two or more "circuits".

Back to Top

T

Terminal Expansion - Expansion of the fluid volume due to heat.

Torque - The turning effort of a hydraulic motor or rotary cylinder. Usually given in inch-pounds (in- Ibs) or foot-pounds (ft-lbs).

Tube - A line whose size is its outside diameter.

Back to Top

V

Valve - A device which controls either 1) pressure of fluid, 2) direction of fluid flow, or 3) rate of flow.

• Bypass Flow Regulator Valve - A valve which regu- lates the flow to a circuit at a constant volume, dumping excess oil.
• Check Valve - A valve which permits flow in only one direction.
• Closed Center Valve - A valve in which inlet and outlet ports are closed in the neutral position, stopping flow from pump.
• Directional Control Valve - A valve which directs oil through selected passages. (Usually a spool or rotary valve design.)
• Electro-Hydraulic Valve - A valve that is opened and closed by a solenoid.
• Flow Control Valve - A valve which controls the rate of flow. (Sometimes called a "volume control valve.")
• Flow Divider Valve - A valve which divides the flow from one source into two or more branches. (In- cludes "priority" and "proportional" types.)
• Needle Valve - A valve with an adjustable tapered point which regulates the rate of flow.
• Open Center Valve - A valve in which the inlet and outlet ports are open in the neutral position, al- lowing a continuous flow of oil from pump.
• Pilot Valve - A valve used to operate another valve or control.
• Pilot Operated Valve - A valve which is actuated by a pilot valve.
• Poppet Valve - A valve design in which the seating element pops open to obtain free flow in one direc- tion and immediately reseats when flow reverses.
• Pressure Control Valve - A valve whose primary function is to control pressure. (Includes relief valves, pressure reducing or sequencing valves, and unloading valves.)
• Pressure Reducing Valve - A pressure control valve which limits outlet pressure.
• Pressure Sequence Valve - A pressure control valve which directs flow in a preset sequence.
• Priority Flow Divider Valve - A valve which directs oil to one circuit at a fixed rate and dumps excess flow into another circuit.
• Proportional Flow Divider Valve - A valve which directs oil to all its circuits at all times.
• Relief Valve - A valve which limits the pressure in a system, usually by releasing excess oil.
• Rotary Directional Valve - A valve designed in a cylindrical shape. When the valve is turned, it opens and closes drilled passages to direct oil.
• Selector Valve - A valve which selects one of two or more circuits in which to direct oil, usually operated manually.
• Shuttle Valve - A connecting valve which selects one of two or more circuits because of flow or pressure changes in these circuits.
• Spool Directional Valve - A valve designed as a spool which slides in a bore, opening and closing passages.
• Thermal Relief Valve - A valve which limits the pressure in a system caused by heat expansion of oil.
• Two-, Three-, Four-, or Six-Way Valve - A valve having 2, 3, 4, or 6 ports for direction of oil flow.
• Unloading Valve - A valve which allows a pump to operate at minimum load by dumping the pump's excess oil at a low pressure.
• Volume Control Valve - A valve which controls the rate of flow. Includes flow control valves, flow di- vider valves, and bypass flow regulators.

Valve Stack - A series of control valves in a stack with common end plates and a common oil inlet and outlet.

Velocity - The distance which a fluid travels per unit time. Usually given as feet per second.

Vent - An air breathing device in a fluid reservoir.

Viscosity - The measure of resistance of a fluid to flow.

Volume - The amount of fluid flow per unit time. Usually given as gallons per minute (gpm).

Back to Top

ABBREVIATIONS

ANSI -American National Standards Institute ASAE-American Society of Agricultural Engineers (sets standards for many hydraulic components for agricultural use)

°F- degrees Fahrenheit (of temperature) ft-ibs-foot-pounds (of torque or turning effort) gpm - gallons per minute (of fluid flow) hp-horsepower

I.D. - inside diameter (as of a hose or tube) ISO-International Standards Organization O.D.-outside diameter (as of a hose or tube) psi - pounds per square inch (of pressure) rpm - revolutions per minute

SAE - Society of Automotive Engineers (sets standards for many hydraulic components)