Basic Hydraulic Systems and Components

Basic Hydraulic Systems and Components
اسم المؤلف
The Army Institute for Professional Development
7 يوليو 2021

Basic Hydraulic Systems and Components
The Army Institute for Professional Development
Army Correspondence Course Program
Table of Contents
Section Page
Subcourse Overview . i
Terminal Learning Objective . ii
Administrative Instructions . iv
Grading and Certification Instructions . iv
Lesson 1: Hydraulic Reservoirs, Filters, Pumps,
Accumulators, and Motors 1
Practice Exercise . 19
Answer Key and Feedback . 22
Lesson 2: Basic Construction and Operation of Hydraulic
Actuating Devices, Flow Control, and Directional
Devices . 25
Practice Exercise . 43
Answer Key and Feedback . 46
Lesson 3: Hydraulic Pressure-Limiting, Controlling, and
Sensing Devices . 49
Practice Exercise . 55
Answer Key and Feedback . 58
Examination 61
Appendix: Glossary . 69
Student Inquiry Sheets
Accumulator–device for storing liquid under pressure, usually consisting of a chamber separated into a
gas compartment and a liquid compartment by a bladder, piston, or diaphragm. An accumulator also
smooths out pressure surges in a hydraulic system.
Actuating cylinder–device that converts fluid power into linear mechanical force and motion.
Actuating cylinder, double-action–actuating cylinder in which both strokes are produced by pressurized
Actuating cylinder, single-action–actuating cylinder in which one stroke is produced by pressurized
fluid and the other stroke is produced by some other force, such as gravity or spring tension.
Angular piston pump–hydraulic pump that has the cylinder block placed at an angle to the drive shaft
plate where the pistons are attached. The angular configuration causes the pistons to stroke as the pump
shaft is turned.
Baffle–metal plate installed in a reservoir to keep the fluid from swirling and surging.
Bladder–synthetic rubber bag inserted in an accumulator to hold the air charge.
Bypass valve–valve used to allow fluid to go around a filtering element if the element becomes clogged.
Cam pump–type of hydraulic pump that utilizes a cam to cause stroking of the pistons.
Check valve–valve that permits fluid flow in one direction, but prevents flow in the reverse direction.
Closed-center valve–type of valve that has its pressure passage blocked to fluid flow when the valve is
in the OFF position.
Cracking pressure–pounds per square inch pressure at which the valving device of a pressure relief
valve clears its seat just enough to permit fluid to seep through.
Diaphragm–synthetic rubber device that divides an accumulator into two separate compartments, one
for air and the other for
Displacement–volume of fluid that can pass through a pump, motor, or cylinder in a single revolution or
Double-action actuating cylinder–See Actuating cylinder, double-action.
66 AL0926Drive coupling–device that transmits torque from a driving unit to a hydraulic pump drive shaft.
Efficiency–ratio of output power to input power, generally expressed as a percentage.
Energy–ability or capacity to do work.
Filter–device used to remove contaminants from hydraulic fluid.
Fixed-displacement pump–pump in which the volume of fluid per cycle cannot be varied.
Fluid–any liquid, gas, or mixture thereof.
Hydraulics–that branch of mechanics or engineering that deals with the action or use of liquids forced
through tubes or lines under pressure to operate various mechanisms.
Irreversible valve–device used in conjunction with a servo to block feedback.
Land–smooth machined surface on the spool of a spool selector valve.
Micron–millionth of a meter, or about 0.00004 inch.
Open-center valve–type of valve that has its pressure passage open to return when the valve is in the
OFF position.
Orifice–device used to restrict the flow of fluid in order to slow the operation of a component.
Pilot valve–valve used to control the operation of another valve, the spool in a selector valve.
Piston–that part of an actuating cylinder, servo, or motor that the hydraulic fluid works against. In a
pump, the pistons work against the fluid.
Poppet–valving device similar to the valves found in an automobile engine.
Port–opening for the intake or exhaust of fluid.
Power–rate of doing work or expending energy.
Pressure–amount of force distributed over each unit area expressed in pounds per square inch (psi).
Pressure reducer–device for lowering the pressure in a hydraulic system to allow a component to
operate at a lower pressure than the rest of the system.
67 AL0926Pressure relief valve–pressure control valve used to keep system pressure from exceeding
predetermined limits.
Pressure switch–electrical switch operated by the increase or decrease of fluid pressure.
Priority valve–valve used to route fluid to those components requiring immediate completion of action
when a reduction in normal system flow and pressure occurs.
Pump–device that converts mechanical energy into fluid energy.
Ratchet valve–valve used with double-action actuator cylinders to aid the cylinder in holding a load in
the position selected by the operator.
Reservoir–container that serves primarily as a supply source of the fluid for a hydraulic system.
Selector valve–valve used to control the direction of movement of an actuating unit.
Servo–device used to convert a small movement into a greater movement or force.
Sloppy link–point of interconnection between control linkage, pilot valve, and servo piston rod in a
Standpipe–pipe located in a reservoir where the main hydraulic system draws its fluid.
Stroke–distance a piston moves in its bore from bottom to top, a single movement of a piston from one
end of its range to the other.
Thermal expansion–increase in volume of a substance due to temperature change.
Variable-delivery pump–type of pump in which the volume of fluid per cycle can be varied
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