Pneumatic and Hydraulic Cylinder Parts
Section 2

Pneumatic and Hydraulic Cylinder Packing Glands

Packing glands in a pneumatic or hydraulic cylinder are used to hold the packing in place. There are many designs for packing glands. Some are designed to be adjustable so the tension may be increased on the packing as the packing wears. Others are designed to just hold the packing in place regardless of wear. Some packing glands are made to fit closely to the cover and act as a rod bearing. Packing glands can be held in position by cover plates, snap rings, threads and screws.

Pneumatic and Hydraulic Cylinder Pistons and Piston Seals

The design of a piston in a hydraulic or pneumatic cylinder will vary depending on whether the cylinder will be used for light, medium or heavy duty. Aluminum, brass, cast iron and steel are the most common materials used for the piston. Wherever one part is going to be sliding over another part, materials of different hardness work best. For example, one could use a cast iron piston with a composite body. Depending on the type of packing to be used, pistons may be designed to be of a one-piece, two-piece or three-piece construction. For example, if we are designing a double-acting piston and we will use a u-ring for a seal, the piston can have a one-piece construction. However, a double-acting piston which uses cup packings will need to have a three-piece construction.

Pistons are packed with many types of packing such as cups, chevrons, o-rings, u-rings, formed synthetic packings and piston rings. (As an aside, back in the olden days pneumatic and hydraulic cylinders were packed with everything from rope to rags. Of course, these materials extruded quickly. Keeping a pneumatic or hydraulic cylinder from wearing out was quite a chore!) In factories where a hydraulic cylinder will be working in extremely high temperatures, such as casting machines, furnaces or rolling-mill equipment, automotive piston rings are generally used. These packings are designed to withstand the extreme temperatures. However, these automotive-types rings have a tendency to leak a lot, especially when they are being broken in, and thus they must be fitted closely to the cylinder body.

Pneumatic and Hydraulic Cylinder Piston Rods

A strong polished steel with the right tensile strength is used for piston rods. The finish on the piston rod is very important to the life of the rod packing. Sometimes it is necessary to use hardened or plated rods. Rods are made with three types of connections on the ends --- male thread, female thread or a special rod head design, such as an eye. If the pneumatic or hydraulic cylinder will have a long stroke piston rod, the rod must have a large enough diameter to avoid bucking under the pressure of the movement of the piston rod.

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