double-acting cylinders

DOUBLE-ACTING CYLINDERS

1. What are Double-Acting Cylinders?

• In double-acting hydraulic cylinders, liquid pressure can be applied to either side of the piston, thereby providing a hydraulic force in both directions.

• The double-acting cylinders are mostly used in applications where larger stroke lengths are desired.

2. Construction

Fig.5.2(a) shows the construction of a double-acting cylinder and Fig.5.2(b) shows its symbolic representation.

The five basic parts of the double-acting cylinders are :

1. Base cap with port connections,

2. Bearing cap with port connections,

3. Cylinder barrel,

4. Piston, and

5. Piston rod.

The components are designed in such a way that the end caps and pistons are common to all cylinders of the same diameter, and only barrels and piston rods need to be changed to construct different length cylinders. The end caps are welded to the barrel and the piston rods are threaded to the piston.

In order to prevent wear and leakage, the inner surface of the barrels must be made very smooth. Usually a seamless drawn steel tube is used as a barrel. The cast iron or steel piston is used to transmit force to the piston rod. Also the piston acts as a sliding bearing in the barrel and provide a seal between high and low pressure sides. Piston seals are provided in between piston and barrel to avoid any leakage. The piston rods are usually made of heat treated chromium alloy to protect it from corrosion and abrasion.

A wiper or scraper seal is fitted on the outside of the piston rod bearing to prevent grit, water or other matter being drawn into the bearing. An internal sealing rings (also called O rings) are fitted behind the bearing to prevent high pressure fluid leaking out along the rod. The end caps have threaded entries for the ports.

3. Operation

During the extension stroke, the pressurized fluid (usually from the pump) enters through extend port. This fluid moves the piston towards left and the fluid present on the other side is pushed out to the reservoir from the cylinder through retract port.

During the retraction stroke, the fluid is allowed to enter the cylinder through retract port and the piston moves towards right and the fluid present on the other side is pushed out to the reservoir from the cylinder through extend port.

It can be seen from Fig.5.2(a) that the volume on both the sides of the piston are different. Since the piston rod is attached at one side only, the cylinder exert greater force when extending [F = P × Apiston] than when retraction [F = P × (Apiston - Arod)]. This results in different pressure levels on either side of the piston and that's why double-acting cylinders are also called as differential cylinders.

Thus the both extension and retraction strokes of the double-acting cylinder provide a linear output motion which can be used to accomplish some useful work.

4. Analysis of Cylinder Force, Velocity, and Power of a Double-Acting Cylinder 

The force, velocity, and power of a double-acting cylinder can be calculated as below: 

Let D and d = Diameters of the piston and piston rod respectively,

Ap and Ar = Cross-sectional area of the piston and the piston rod respectively,

Fext and Fret = Hydraulic force acting on piston during the extension and retraction strokes respectively,

vext and vret = Velocity of the piston during the extension and retraction strokes respectively,

Qin = Cylinder input volume flow rate in m3/s, and

P = Hydraulic pressure in N/m2.

Note 

When Ap = Ar, the double-actig cylinder is known as non-differential cylinder.