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The cylinder is made of a seamless tube or casting that has a true, smooth interior surface. The piston circumference has a mechanical seal, usually in the form of “0-ring” or “T- ring” seals. The piston rod on double acting cylinders is chromed or hard surfaced and polished and passes through an end cap. The piston end cap provides mounting for the cylinder end. Ports into the cylinder are positioned in the end caps. System relief pressure and piston diameter determine the piston capacity within design limits. The piston speed is determined by the volume of the oil flow.

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(Fig. 13) shows a single acting cylinder. Oil directed to the piston will extend the rod to raise the load. Oil contained in the cylinder will hold the load, and oil released back to the reservoir will allow the load to lower by its own weight or by mechanical force such as springs. Double acting cylinders (Fig. 14.) have both
piston surfaces exposed to oil, and can move loads in two directions.

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When valving directs the circuit pressure to the piston end, oil at the rod end is directed to the reservoir as the rod extends. To change direction, the oil under pressure is directed to the rod side of the piston and oil on the piston face is directed back to the reservoir.


Oil pressure and flow volume must be present for cylinder response. Oil pressure is measured in pounds per square inch. Using (Fig. 15) as an example, let us suppose the piston has a surface area of 3 square inches. If circuit pressure (controlled by a relief valve) were 500 P.5.1., the cylinder would move a weight of 1500 pounds (500 X 3 1500). To move the weight, a specific quantity of oil must flow into the cylinder. This quantity is usually measured in gallons per minute. Positive displacement pumps are rated in G.P.M. at a specific rotation speed.

In (Fig. 15), cylinder A with a capacity of ½ gal. would take 6 seconds to stroke fully with a flow rate of 5 G.P.M. Cylinder B with a capacity of 1 gal. would take 12 seconds at 5 G.P.M. Equal pressures at 5 G.P.M. would allow cylinder B with twice the piston area to move double the weight of cylinder A. With these examples, we can see how pressure affects capacity and volume affects speed. Engineers designing hydraulic systems choose components which will perform their jobs according to the needs of the equipment. Pump volumes, cylinder and line capacities, valving, and operating pressures all need to be
considered in their specifications.

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[Source: Simplicity publication, Hydraulic Systems Training Information, #840172, Principles and Operation of Tractor Hydraulic Systems]



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