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HYDRAULIC CYLINDERS
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.
PRESSURE AND
FLOW
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]
