The Parts of a Hydraulic Cylinder

by Pauline Gill
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Strateger Graphics

Hydraulic cylinders generate linear force and motion from hydraulic fluid pressure. Most hydraulic cylinders are double acting in that the hydraulic pressure may be applied to either the piston or rod end of the cylinder to generate either extension or retraction force respectively. Hydraulic cylinders are used in many mechanical applications wherever high linear forces are necessary. With a relatively small hydraulic motor, a large cylinder may generate tens of tons of force, due to almost unlimited mechanical gain capability and inherent stability of hydraulic systems.

Hydraulic Cylinder Parts

Hydraulic cylinders consist of a smooth bore round tubular cylinder, a freely moving piston with several polymer seals, a highly polished round piston rod and a rod support bearing along with several tight-fitting seals to seal the sliding rod where it exits the cylinder. The top of the cylinder as well as the end of the piston rod have clevis fittings which allow angular movement of the device the cylinder is attached to. Each end of the cylinder has a threaded or compression fitting opening where the hydraulic pressure tubes are connected from the cylinder control valve.

Cylinder Operation

When the control valve handle is moved toward the extend cylinder position, hydraulic fluid under high pressure--usually 500 lbs. to thousands of lbs. per square inch is allowed to flow from the hydraulic pump to the piston side of the cylinder, while the oil under the piston by the rod side of the cylinder is allowed to flow from the cylinder and back to the reservoir. If the handle is pushed to the retract position, the pressurized oil is sent to the rod side of the cylinder, retracting the cylinder and pushing the oil on top of the piston and back to the reservoir.

System Description

A typical system employing hydraulic cylinders to accomplish work requiring large forces--such as a hydraulic power excavator shovel used to perform construction excavation--consists of the engine-driven hydraulic pump, oil reservoir, cooling and filtering, operator controls, tubing and the hydraulic cylinders.

Hydraulic Gain

This provides a much higher output force from the cylinder than the force applied by the motor, albeit at a proportionately lower speed. In the case of hydraulic jacks, rams and presses, this is exactly the goal of using hydraulic cylinders--tons of force available at slow to moderate speed of inches per minute.

Common Applications

Hydraulic cylinders are used in virtually all types of construction equipment including excavators, bulldozers, back hoes, cranes and graders. In buildings, hydraulic elevators use large compound telescoping cylinders. Large trash dumpsters use hydraulic cylinders on mechanical rams to compact trash. In the automotive and transportation industry, hydraulic brakes for drum and calipers for disc brakes consist of hydraulic cylinders. In industry, forklifts and jacks are widely used.

Domestic Applications

Log splitters use hydraulic cylinder rams to split cords of firewood for winter without you having to sprain your back, swinging a mallet.

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