Cylinders

The linear motion and high force produced by cylinders are big reasons why designers specify hydraulic and pneumatic systems in the first place. One of the most basic of fluid power components, cylinders have evolved into an almost endless array of configurations, sizes, and special designs. This versatility not only makes more-innovative designs possible, but makes many applications a reality that would not be practical or possible without cylinders.

Fig. 1. Cutaway view shows key features of a typical cylinder, in this case, a double-acting model for hydraulic service with standard tie-rod construction.

Fig. 2. Cut-away view shows key features of a single-acting cylinder. This particular model is base-mounted with an extension spring for a tool holding application.

Fig. 3. Welded cylinders combine benefits of heavy-duty construction with compact design, which is the main reason why they are widely used in mobile-equipment applications.

Fig. 4. Mill-duty cylinders have flanges welded to both ends of their barrel with an end cap bolted to each flange.

Fig. 5. Non-repairable cylinders generally are constructed primarily of stainless steel, aluminum alloys, and engineered plastics for pneumatic or low-pressure hydraulic service. Shown here are a single-acting, spring-return model, top, and double-acting with double rod-end, bottom.

Fig. 6. Telescoping cylinders have two or more stages that, when fully extended, can produce a stroke that exceeds the length of the cylinder when fully retracted. Shown here is a cutaway of a six-stage single-acting model.

Fig. 7. Short-stroke cylinders have a piston diameter that exceeds rod length. They are used where axial space is limited and high force must be generated from a relatively low supply pressure.

Fig. 8. Tandem cylinders use multiple pistons connected through a common rod to generate relatively high force from a low supply pressure and small bore.

Fig. 9. Duplex cylinders have multiple pistons that are not connected to a common rod. Actuating individual piston chambers achieves multiple strokes.

Fig. 10. By eliminating the conventional sliding piston seal - and its inherent tendency toward stick-slip operation - diaphragm cylinders produce nearly frictionless motion, except for the miniscule amount of friction at the rod seal. The diaphragm's positive sealing also eliminates the potential for leakage around the piston.

Cylinder basics

The most common cylinder configuration is double acting, Figure 1. Routing pressurized fluid into the rod end of a double-acting cylinder causes the piston rod to retract. Conversely, routing pressurized fluid into the cap end causes the rod to extend. Simultaneously, fluid on the opposite side of the piston flows back into the hydraulic reservoir. (If air is the fluid medium, it usually is vented to the atmosphere.)

Because the area of the rod-end piston face is smaller than the cap-end area, extension force is greater than retraction force (assuming equal fluid pressures). Because total cylinder volume is less with the cylinder fully retracted (because of rod volume) than when the cylinder is fully extended, a cylinder retracts faster than it extends (assuming equal flow rates).

Single-acting cylinders, Figure 2, accept pressurized fluid on only one side of the piston; volume on the other side of the piston is vented to atmosphere or returns to tank. Depending on whether it is routed to the cap end or rod end, the pressurized fluid may extend or retract the cylinder, respectively. In either case, force generated by gravity or a spring returns the piston rod to its original state. A hydraulic jack for vehicles represents a common application of a single-acting, gravity-return cylinder.

Single-acting cylinders can be spring-extend or the more common spring-return type. A spring-extend cylinder is useful for tool-holding fixtures because spring force can hold a workpiece indefinitely. The cylinder then releases the workpiece upon application of hydraulic pressure. Spring-applied/hydraulic-pressure-released (parking) brakes represent another common application of single-acting, spring-extend cylinders.

But the most common type of single-acting cylinder uses a return spring. In this version, pressurized fluid enters the cap end of the cylinder to extend the piston rod. When fluid is allowed to flow out of the cap end, the return spring exerts force on the piston rod to retract it. Factory automation - especially material handling - is a common application using pneumatic spring-return cylinders.

Standard constructions

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, Figure 1, 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.

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, Figure 3, 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, Figure 4, 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.

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 designed for use in steel mills, foundries, and other severe-duty applications.

At the other end of the duty spectrum are non-repairable cylinders, Figure 5. These cylinders are designed for economy and have end caps welded to the barrel to make them throwaway components. They cannot be disassembled for repair or seal replacement. However, this design proves very cost effective when high service life is not required. Most of these cylinders have stainless steel end caps and barrel, but because they are intended primarily for light duty cycles, many make extensive use of aluminum alloys and plastics for light weight and economy.

An alternative method of manufacture rolls the tube into a slot on the end caps to mechanically lock the three pieces together. Another alternative design has the end cap welded to the barrel and a rod-end cap secured via threads or a lock ring. These modifications allow disassembling the cylinder for repair but also raise its initial cost.