Hydraulic cylinders are designed to operate at high pressures and handle high loads in demanding operating conditions. Therefore, they need to be constructed with materials of high strength, expert workmanship, and advanced features to provide ruggedness, high quality, maintenance-free operation, and long service life. Some of the features of cylinder construction are presented below to update or refresh your memory.

Barrel

The barrel of a hydraulic cylinder is made of a high-strength seamless-drawn tube that is precision-machined to a perfect finish. The internal surface of the barrel must be very smooth so that the wear and leakage can be controlled. The barrel should of quality and manufactured to high precision to meet the rigid standards of straightness, roundness, and surface finish.

Piston

The primary function of a piston in a hydraulic cylinder is to transmit the force to the load attached to its piston-rod. Apart from this, it acts as the bearing in the cylinder barrel. Pistons can be manufactured with wide bearing surfaces to resist side loading. The piston must be a perfect fit inside the cylinder barrel. It must be reasonably cylindrical and finely finished for its smooth output motion.

Piston-rod

A smooth, hard, and corrosion-resistant surface is essential for the outer surface of the piston-rod. Therefore, the piston-rod is, usually, made of induction-hardened steel or stainless steel. It may also be chrome-plated with an ultra-fine surface finish to ensure its resistance to wear and corrosion.

End-caps

They are cast from iron or aluminium or made from high-quality steel. They may be designed with square or round shapes to match the barrel shape. They can be fixed by tie-rods, or threaded or welded to the barrel. They also incorporate threaded entries for ports. The end-of-travel shocks in a cylinder can be absorbed with the cushion valves built into its end-caps.

Cushioning

Cushions with tight tolerances can be provided at the head-end and/or cap-end sides of a cylinder for the progressive deceleration of the piston and piston-rod assembly at its end-of-stroke positions.

Seals

Piston seals may be self-compensating to conform to variations in pressure, mechanical deflection, and wear. They may also incorporate wear bands to provide smooth operation, longer bearing life, and high load-carrying capacity. The extrusion of the piston seals can be prevented by using backup washers. The piston-rod pressure seals must be designed to provide efficient sealing under all operating conditions. The piston-rod double-lip wiper seal on the piston-rod must be designed to prevent the ingress of dirt into the cylinder for extending the life of gland and seals. Pressure energized seals are used for the barrel to make sure that the cylinder body remains leak-tight, even under pressure shock conditions. Many classes of seals are available to accommodate the many types of fluids and the varying temperature ranges.

Air Bleeds

Air bleeds can be recessed into one end or both ends of the cylinder body for venting trapped air in the cylinder. For bleeding air, first, move the piston to the end-of-stroke position, and then slightly open the bleeder screw until bubble-free oil emerges. Then tighten the bleeder screw again.

Gland Drain

A gland drain may be provided on a long-stroke cylinder or a cylinder with a constant back pressure to relieve the accumulated fluid behind the gland wiper seal. A transparent tube can be used between the cylinder port and the associated reservoir to monitor an inaccessible cylinder and get an early indication of the need for gland servicing.

Joji Parambath

Author

References:

1. Industrial Hydraulic Systems and Circuits -Basic Level (In the SI Units) by Joji Parambath
2. Industrial Hydraulics -Basic Level (In the English Units) by Joji Parambath
3. Hydraulic Cylinders (In the SI Units) by Joji Parambath
4. Hydraulic Linear Actuators (In the English Units) by Joji Parambath
5. Catalogue on ‘Metric Hydraulic Cylinders Series HMI’, Parker Cylinder

Further Reading