A hydraulic cylinder consists of the following parts:
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The cylinder barrel is mostly a seamless thick-walled forged pipe that must be machined internally. The cylinder barrel is ground and/or honed internally.
In most hydraulic cylinders, the barrel and the bottom are welded together; this can damage the inside of the barrel if done poorly. Therefore, some hydraulic cylinder designs have a screwed or flanged connection from the cylinder end cap to the barrel. In this type the cylinder barrel can be disassembled and repaired in future.
The cylinder head is sometimes connected to the barrel with a sort of a simple lock (for simple cylinders). In general however, the connection is screwed or flanged. Flange connections are the best, but also the most expensive. A flange has to be welded to the pipe before machining. The advantage is that the connection is bolted and always simple to remove. For larger hydraulic cylinder sizes, the disconnection of a screw with a diameter of 300 to 600 mm is a huge problem as well as the alignment during mounting.
The piston is a short, cylinder-shaped metal component that separates the two sides of the cylinder barrel internally. The piston is usually machined with grooves to fit elastomeric or metal seals. These seals are often O-rings, U-cups or cast iron rings. They prevent the pressurised hydraulic oil from passing by the piston to the chamber on the opposite side. This difference in pressure between the two sides of the piston causes the cylinder to extend and retract. Piston seals vary in design and material according to the pressure and temperature requirements that the hydraulic cylinder will see in service. Generally speaking, elastomeric seals, made from nitrile rubber or other materials, are best in lower temperature environments while seals made of viton are better for higher temperatures. The best seals for high temperature are cast iron piston rings.
The piston rod is typically a hard, chrome-plated piece of cold-rolled steel which attaches to the piston and extends from the cylinder through the rod-end head. In double rod-end hydraulic cylinders, the actuator has a rod extending from both sides of the piston and out both ends of the barrel. The piston rod connects the hydraulic actuator to the machine component doing the work. This connection can be in the form of a machine thread or a mounting attachment such as a rod-clevis or rod-eye. These mounting attachments can be threaded or welded to the piston rod or, sometimes, they are a machined part of the rod-end.
The hydraulic cylinder head is fitted with seals to prevent the pressurised hydraulic oil from leaking past the interface between the rod and the head. This area is called the rod gland. It often has another seal called a rod wiper which prevents contaminants from entering the hydraulic cylinder when the extended rod retracts back into the cylinder. The rod gland also has a rod bearing. This bearing supports the weight of the piston rod and guides it as it passes back and forth through the rod gland. In some cases, especially in small hydraulic cylinders, the rod gland and the rod bearing are made from a single integral machined part.
A hydraulic cylinder should be used for pushing and pulling only. No bending moments or side loads should be transmitted to the piston rod or the cylinder. For this reason, the ideal connection of a hydraulic cylinder is a single clevis with a spherical ball bearing. This allows the hydraulic actuator to move and allow for any misalignment between the actuator and the load it is pushing.
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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.
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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
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References:
Further Reading
Industrial Hydraulic Systems and Circuits -Basic Level (In the SI Units)
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Industrial Hydraulics -Basic Level (In the English Units)
Joji Parambath
Hydraulic Cylinders (In the SI Units)
Joji Parambath
Hydraulic Linear Actuators (In the English Units)
Joji Parambath
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