There are two types of hydraulic cylinders:
Although they are described briefly here, vane-type hydraulic cylinders are nearly obsolete.
In hydraulics, both single-acting and double-acting cylinders are used. In both types, a piston is moved linearly within the cylinder bore when it is subject to pressurized oil. In application, a dump truck ram would require only a single-acting cylinder, whereas COE cab-lift cylinders would have to be double acting.
Single-Acting Cylinders. In a single-acting cylinder, hydraulic pressure is applied to only one side of the piston. Single-acting cylinders may be either outward- or inward-actuated. When an outwardactuated cylinder has hydraulic pressure applied to it, the piston and rod are forced outward to lift the load. When the oil pressure is relieved, the weight of the load forces the piston and rod back into the cylinder. One side of a single- acting cylinder is dry. The dry side must be vented so that when oil pressure on the pressure side is relieved, air is allowed to enter, preventing a vacuum. In an outward-actuated single cylinder, the “dry side” or vent chamber is on the rod side of the piston.
When an inward-actuated cylinder has hydraulic pressure applied to it, the rod is pulled inward into the cylinder. This means that oil pressure acts on the rod side of the piston and the vent chamber is on the opposite side.
A seal on the piston prevents oil leakage from the wet side to the dry side of the cylinder and a wiper seal is used to clean the rod as it extends and retracts. To prevent dirt from entering the dry side of the cylinder, a porous breather is used in the air vent.
A ram is a single-acting cylinder in which the rod serves as the piston. The rod is just slightly smaller than the cylinder bore. A shoulder on the end of the rod prevents the rod from being expelled from the cylinder. A ram-type cylinder eliminates the need for a dry side because oil fills the entire inner chamber when actuated.
Double-Acting Cylinders. Double-acting cylinders can provide force in both directions, which means that oil pressure can be applied to either side of the piston within the cylinder. When oil pressure is applied to one end of the cylinder to extend it, oil from the opposite side returns to the reservoir. This is reversed when the cylinder is retracted. Good examples of double-acting cylinders would be those required to raise and lower truck cabs on COE applications.
Double-acting cylinders may be balanced or unbalanced. In the balanced double-acting cylinder the piston rod extends through the piston head on both sides, giving an equal surface area on which hydraulic pressure can act. In an unbalanced double-acting cylinder a piston rod is located on one side of the piston. The result is that there is more surface area on which hydraulic pressure can act on the side without the rod. It should be noted that actual balance or unbalance of these cylinders depends on load. If the loads are not equal on either side, the balances will vary.
Vane-type cylinders may be found in some much older hydraulic systems. A vane-type cylinder provides rotary motion. It consists of a cylindrical barrel, within which a shaft and vane rotate when pressurized oil enters. As the shaft vane rotates, it closes off an inlet passage in the top plate, leaving only a small orifice to discharge the oil; this slows the rotating vane as it comes to the end of its stroke. Most vane-type cylinders are double acting. Pressurized oil would be directed through one chamber to swing left, the other to swing right. Double-acting vane-type cylinders can be used in applications such as backhoes because they enable a boom and bucket to be swung rapidly from trench to pile. An alternative to a double-acting vane cylinder would be a pair of cylinders.