Pneumatic and Hydraulic Cylinder Parts
Section 3

Pneumatic and Hydraulic Cylinders Packings and Seals

There are numerous packings used in pneumatic and hydraulic cylinders including rod packings, piston packings, cushion needle packings, cover gaskets, cover seals and rotary joint packings for rotating cylinders. These packings include o-rings, V packings, U packings, u-cups, flange packings, diaphragms, gaskets, cup packings, cords and more. Sometimes wipers are housed in front of the packing so that abrasive materials can be removed from the piston rod before it goes through the packing.

Rotating pneumatic cylinders need to have airshaft packings so that the airshaft stem and the distributor are sealed from each other. If the airshaft is small, sealing is not a problem unless the piston rod will be moving at a high rate of speed. If the airshaft has a large diameter and is in a cylinder with a hollow center, sealing can be a problem, especially if high speeds are to be utilized. A cylinder like this will have a large sealing area and the friction with the packing can create a great deal of heat. This cylinder may require a water jacket for the distributor housing to prevent the packing from overheating.

Pneumatic and Hydraulic Cylinders Cushion Assembly

A cushion assembly for pneumatic and hydraulic cylinders is used to help the piston decelerate near the end of its stroke. This is recommended when the piston travels at a high rate of speed or is pushing a heavy load. Without the cushion assembly, the piston would make a sharp contact with the covers causing a great deal of shock between the piston and cylinder covers.

This is how cushioning in pneumatic or hydraulic cylinders works: When a piston is coming to the end of its stroke, the cushion nose or collar enters a chamber with a small diameter. As the cushion nose or collar enters the chamber, the air is trapped in the cylinder body between the piston and cover and this air is then bled off slowly. This process slows the piston down and thus alleviates the shock at the end of the stroke. When the stroke is to be reversed, the air flows freely to the piston so it can move quickly at the start of the next stroke.

Rotating pneumatic and hydraulic cylinders do not need to have a cushion assembly. The speed of the pistons is usually relatively slow and their porting is small. In most applications, there is not a great deal of shock between the piston and the cylinder covers.

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