Construction variations for single- and double-acting cylinders are based primarily on how the two end caps are attached to the barrel. Additional variations include wall thickness of the barrel and end caps, and materials of construction.
Tie-rod cylinders (Fig. 2) have square or rectangular end caps secured to each end of the barrel by rods that pass through holes in the corners of the end caps. Nuts threaded onto the end of each tie rod secure the end caps to the barrel. Static seals in the barrel/end-cap interface prevent leakage. A number of variations to this design exist, including use of more than four tie rods on a cylinder, or long bolts that thread into tapped holes in one of the end caps.
2. Tie-rod cylinders are the most widely used design in all of fluid power: industrial and mobile hydraulics and pneumatics. Industry standards exist for basic design, but many variations often applied to make tie-rod cylinders lighter or more compact, or provide other benefits.
The majority of cylinders for industrial, heavy-duty applications use tie-rod construction and usually conform to National Fluid Power Association (NFPA) standards. These standards establish dimensional uniformity so cylinders from multiple manufacturers can be interchanged. However, care should be taken when interchanging cylinders because even though it conforms to NFPA dimensional standards, a cylinder may have proprietary features from its specific manufacturer that may not be available from a different manufacturer.
Welded cylinders have end flanges welded to the barrel and an end cap attached to each flange. End caps are secured in place by bolts that slip through holes in each end cap and thread into tapped holes in each end flange. This construction is lighter and more compact than the standard tie-rod configuration, which explains why welded cylinders find wide application in mobile equipment.
A variation to this construction has each end cap threaded into the end of the barrel. This construction, however, usually cannot accommodate as high a pressure rating as welded and can be more difficult to disassemble and reassemble.
Mill-duty cylinders (Fig. 3) have flanges welded to the ends of the cylinder barrels with end caps of the same diameter as the flanges. Bolts secure the end caps to the flanges. Their construction is similar to that of welded cylinders, but mill-duty cylinders have thicker barrel walls and heavier construction in general.
3. Mill-duty cylinders have flanges welded to both ends of their barrel with an end cap bolted to each flange.
Large mill-duty cylinders often have a barrel wall thick enough for the end cap bolts to be threaded directly into the barrel wall. As the name implies, these cylinders were originally de-signed for use in steel mills, foundries, and other severe-duty applications.
The most common type cylinder is the single-rod end, in which the rod is nearly as long as the cylinder barrel. The rod protrudes from the rod-end cap to transmit the generated force to the load. A double-rod cylinder (Fig. 4) has a rod attached to both faces of the piston with each rod extending through a rod end cap. Double rod-end cylinders are useful for moving two loads simultaneously, and they also eliminate the differential area between the rod side and blank side of the piston. With equal areas (and cylinder volumes) on both sides of the piston, a given flow produces the same extension and retraction speeds.
4. Double-rod cylinders have a piston rod that extends from both ends of the cylinder. This allows moving a load from either or both ends and also eliminates the issues of differential piston areas inherent to standard single-rod cylinders.
Most telescopic cylinders (Fig. 5) are single-acting, although double-acting versions are available. Telescoping cylinders contain five or more sets of tubing, or stages, that nest inside one another. Each stage is equipped with seals and bearing surfaces to act as both a cylinder barrel and piston rod. Available for extensions exceeding 15 ft, most are used on mobile applications where available mounting space is limited. The collapsed length of a telescoping cylinder can be as little as one-fifth its extended length, but the cost is several times that of a standard cylinder that can produce equivalent force. Models are available in which all stages extend simultaneously or where the largest stage extends first, followed by each successively smaller stage.
5. Telescopic cylinders have two or more stages that, when fully extended, can produce a stroke that exceeds the length of the cylinder when fully retracted.
Ram cylinders are a special type of single-acting cylinder that has a rod diameter the same as the piston. Used mostly for jacking purposes, ram cylinders must be single acting because there is no internal cylinder volume to pressurize for retracting the rod. Ram cylinders sometimes are called plunger cylinders and are most often used for short-stroke applications. Most do not use return springs, but rather, gravity or the load to retract the piston rod.