Different Types of Hydraulic Cylinders

Hydraulics in its applications is generally concerned with, moving, gripping, lifting, and rotating motions with force. Devices that actually achieve this objective are called actuators.

Depending upon the motion they transmit, the actuators are classified as

Linear actuators convert hydraulic energy into straight-line motion. They include various types of hydraulic cylinders.

Rotary actuators convert the hydraulic energy into rotational motion.

Different types of hydraulic cylinders are classified as

A ram or plunger is a hydraulic cylinder, which is designed to apply force in one direction only. The basic constructional difference between ram and other cylinders is that ram does not have a mostly separate piston and piston rod.
The piston itself is big and has a uniform diameter. This construction is to avoid buckling of piston rod due to bending load.

Plunger Cylinder

A common example of a ram-type cylinder is the jack cylinder. The returning force, in this case, is generally the dead weight of load and ram itself.

A single-acting cylinder with a single-piston rod is shown in the figure below.

Single Acting Spring Return Cylinder

Single Acting Single Piston rod cylinders are used for comparatively lower loads than plunger or ram-type hydraulic cylinders.

This cylinder may be gravity return or spring return. This cylinder finds its applications mostly in production tooling. It is also used in automation plants where severe cylinders perform an individual operation in a predetermined sequence.

A double-acting cylinder with a single-piston rod is shown in the figure below.

Double Acting Single Piston Rod Cylinder

The fluid pressure can be exerted on both sides of the piston whenever required. This is the most common type of design and is available in various sizes.

Hence, there are two different piston areas on both sides of the piston due to the presence of a piston rod on one side.

Due to the differential area, the retraction stroke is faster than the extension stroke; as well, the force obtained on extension stroke is more than that of in retraction stroke.

Certain applications such as the feeding of machine tables in machine tools demand motion to be transmitted on both sides of the cylinder. In such an application, double-acting double piston rod cylinders are used.

They have piston rods extending out from both ends of the cylinder. Since the annular area is the same on both sides, it has forward and reverse stroke of the same speed and same force.

Outlined construction and symbol of such a cylinder is shown in the figure below.

Double Acting Double Piston Rod Cylinder

A tandem cylinder is the combination of two or more cylinders working in tandem i.e. coupled mechanically to each other. Each cylinder has its own inlet and outlet ports, but cannot operate independently of other cylinders.

Tandem Cylinder

The advantage of tandeming the cylinder lies in forces addition, greater forces can be obtained at the same pressure without increasing piston sizes, but increasing the area on which the pressure acts by coupling the two or more cylinders.

This finds application, especially where high force requirement with limited space is required.

Total force obtained at piston rod, F=P x A + P(A-a)

Where,

P = Pressure in the cylinder

A = Cross-sectional area of the piston

A = Cross-sectional area of the piston rod

Tandem cylinders are also used in flow dividers.

The telescopic cylinder, as the name implies works similar to a telescope, which can be extended several times of its un-extended length. As shown in the diagram below a telescopic cylinder has a series of rams nested in telescopic assembly.

Telescoping Cylinder Construction and Symbol

Diameter “A” is relatively large, hence during extension stroke, this cylinder extends first. As the ram with diameter “A” reaches the end of its stroke, ram “B” begins to move providing a smaller force than A. When ram “B” reaches the end of its stroke; ram C then starts moving outwards, until its stroke is completed.

A typical application of a telescopic cylinder is dump trucks, where a larger force is required to move initially the load and as the load moves up, force requirement goes on reducing.

A lot of special designs and features have been made to suit a wide range of applications. Three or more cylinders nested together to achieve different speeds and forces, during the stroke. One of the typical examples of nesting the cylinders for hydraulic presses is shown in the figure below.

Nested Cylinder

In a hydraulic press, a fast approach (with less force) to the workpiece is required the slow pressing (with greater force) of the workpiece, and faster retract is required.

Various cylinder designs discussed so far are non-rotating type designs. In industrial processes, certain applications demand that a hydraulic cylinder should rotate freely in addition to providing linear motion. There are not merely as many applications for rotating cylinders as there are for the non-rotating cylinders.

The design of a non-rotating and rotating cylinder is about the same, except that the rotating cylinder has a distributor. The distributor is a stationary assembly that supplies fluid to both ports of the cylinder.

A simple construction of a distributor is shown in the figure.

Rotating Cylinder Distributor

It has a stationary body containing fluid supply connections and has passages drilled in it for feeding oil to both ends of the cylinder. The cylinder is coupled with the stem.