Explain the construction and operation of a double acting cylinder.

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                                                             Schematic representation

In the double acting hydraulic cylinder, 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 application where large stroke lengths are desired.

Construction: There are five basic parts of the double acting cylinders;

  • Base cap with port connections
  • Bearing cap with port connection
  • Cylinder barrel
  • Piston
  • Piston rod
The components are designed in such way that the end caps and pistons are common to all cylinders of the same diameter, and only barrel 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 provides 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 wipe 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 ring (Also called O-ring) is fitted behind the bearing to prevent high pressure fluid leaking out along the rod. The end caps have threaded entries for the ports.

Operation:

During the extension stroke, the pressurized fluid enters through extend port. This fluid moves 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 the diagram that the volume on both sides of the piston are different. Since the piston rod is attached at one side only, the cylinders exert greater force when extending than when retracting. This result in different pressure level on either side of the piston and that is why double acting cylinders are also called 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.


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