The hydraulic cylinder generates linear force and motion from flowing pressurised hydraulic fluid. 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. The hydraulic cylinder is 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 the inherent stability of hydraulic systems.
Hydraulic Cylinder Parts
A hydraulic cylinder consists 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 attached to the cylinder. Each end of the cylinder has a threaded or compression fitting opening where the hydraulic pressure tubes are connected from the cylinder control valve.
When the control valve handle is moved towards the extend cylinder position, hydraulic fluid under high pressure--usually 500 to thousands of pounds 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 out of the cylinder and back to the reservoir. If the handle is pushed to the retract position, the pressurised 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.
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.
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.
The hydraulic cylinder is 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.
Bottle jacks lift cars and trucks with ease while log splitters use hydraulic cylinder rams to split cords of firewood for winter without spraining your back swinging a mallet.