Hydraulic Drive System - Hydraulic Cylinder

Hydraulic Cylinder

Hydraulic cylinders (also called linear hydraulic motors) are mechanical actuators that are used to give a linear force through a linear stroke. Hydraulic cylinders are able to give pushing and pulling forces of millions of metric tons with only a simple hydraulic system. Very simple hydraulic cylinders are used in presses; here, the cylinder consists of a volume in a piece of iron with a plunger pushed in it and sealed with a cover. By pumping hydraulic fluid in the volume, the plunger is pushed out with a force of plunger-area pressure.

More sophisticated cylinders have a body with end cover, a piston rod, and a cylinder head. At one side the bottom is, for instance, connected to a single clevis, whereas at the other side, the piston rod is also foreseen with a single clevis. The cylinder shell normally has hydraulic connections at both sides; that is, a connection at the bottom side and a connection at the cylinder head side. If oil is pushed under the piston, the piston rod is pushed out and oil that was between the piston and the cylinder head is pushed back to the oil tank.

The pushing or pulling force of a hydraulic cylinder is as follows:

• F = Ab * pb - Ah * ph
• F = Pushing Force in N
• Ab = (π/4) * (Bottom-diameter)^2
• Ah = (π/4) * ((Bottom-diameter)^2-(Piston-rod-diameter)^2))
• pb = pressure at bottom side in
• ph = pressure at cylinder head side in

Apart from miniature cylinders, in general, the smallest cylinder diameter is 32 mm and the smallest piston rod diameter is 16 mm.

Simple hydraulic cylinders have a maximum working pressure of about 70 bar. The next step is 140 bar, 210 bar, 320/350 bar and further. In general, the cylinders are custom built. The stroke of a hydraulic cylinder is limited by the manufacturing process. The majority of hydraulic cylinders have a stroke between 0, 3, and 5 meters, whereas 12-15 meter stroke is also possible, but for this length only a limited number of suppliers are on the market.

In case the retracted length of the cylinder is too long for the cylinder to be built in the structure, Telescopic cylinder can be used. One has to realize that for simple pushing applications telescopic cylinders might be easily available; for higher forces and/or double acting cylinders, they must be designed especially and are very expensive. If hydraulic cylinders are only used for pushing and the piston rod is brought in again by other means, one can also use Plunger cylinders. Plunger cylinders have no sealing over the piston, if the cylinder even exists. This means that only one oil connection is necessary. In general the diameter of the plunger is rather large compared with a normal piston cylinder, whereas a hydraulic motor will always leak oil. A hydraulic cylinder does not have a leakage over the piston nor over the cylinder head sealing so that there is no need for a mechanical brake.