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.
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.
Fig. 4. Mill-duty cylinders have flanges welded to both ends of their barrel with an end cap bolted to each flange.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.
Cylinders - and all components for that matter - should be readily accessible to ease installation and subsequent maintenance. If a fitting cannot be checked for tightness without first removing adjacent lines, for example, there is little incentive to bother fixing minor leaks that may occur.
Consider all components and fluid conductors of the system to be elastic: they will flex and change length due to changes in fluid pressure, temperature, and strain. These changes are not minor. A pressure pulse to 6,000 psi will elongate a steel cylinder with a 24-in stroke by 0.024 in. If made of aluminum or cast iron, the cylinder can elongate about 2 to 2.5 times as much. If this elongation has not been accounted for in the design of the machine, the system eventually will leak, even if the latest fitting technology has been used. If previous installations have continually leaked, take this as clear evidence that a new design approach would be beneficial.
An industrial cylinder should have a design factor of about 4:1 based on yield at rated system pressure. Many manufacturers of heavy-duty cylinders for mobile equipment specify a 3:1 design factor. A 15,000-psi stress at rated system pressure, with smooth system operation and no pressure pulses, is considered conservative. System pressure spikes that cause 30,000-psi stress often are not alarming, but at 30,000-psi unit stress, steel's dimensional change is 0.001 in./in. of length. For a 30-in. cylinder, a pressure spike of that intensity causes a length change of almost 1/32 in. Dimensional changes in stressed cylinders, or those subjected to wide temperature changes, may further limit allowable working pressures.
Large dimensional changes can seriously affect performance and life expectancy of nonmetallic cylinder seals. For example, extrusive failures of 80 Shore A durometer, synthetic Nitrile seals can occur when clearance exceeds 0.004 in. at fluid pressures over 3,000 psi, or a 0.001-in. clearance with system pressure of psi. Such pressures can easily be reached in systems using differential cylinders or those with meter-out flow controls.
You must consider system shock pressures. If the hydraulic system contains speed control or energy-absorbing devices, pressure spikes can occur that are two to three times above normal system pressure. Therefore, determine the loading the cylinder will experience and then mount accordingly to maintain port seal integrity.
Trouble-free use of fluid power cylinders and their ability to serve and remain leak-free depends, in large part, on properly mounting the component for the particular application. The designer must determine the loading the cylinder will experience and mount it accordingly.
The National Fluid Power Association, NFPA, has standardized on a number of dimensions for square-headed tie rod cylinders to promote cylinder interchangeability between manufacturers. Part of this standardization program includes cylinder mounting styles, which generally provide:
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Straight line - Cylinders with fixed mounts that absorb the force on the centerline of the cylinder are considered the best for straight line force transfer. Tie rods extended, flange, or centerline lug mounts are symmetrical and allow the thrust or tension forces of the piston rod to be distributed uniformly about the cylinder centerline, Figure 11. Mounting bolts are subjected to simple tension or shear without compound forces; when properly installed, cylinder bearing sideloading is minimized.
Cylinder tie rods are designed to withstand maximum rated internal pressure, and can be extended at either end and used to mount the cylinder. When the tie rods extend at both ends of the cylinder, one end can be used for cylinder mounting and the opposite end can support the cylinder or be attached to the machine members.
Flange mounts also are extremely good for straight line force transfer applications. Three styles available are head rectangular flange, head square flange, and a larger and thicker rectangular head with its own mounting holes; the same three versions are available for the cap. Selection of a flange mount depends partly on whether the major forces applied to the load result in compression or tension on the piston rod.
Fig. 13. Side mounted cylinders include side lug (a), side end angle (b), side and lug (c), and side tapped (not shown). These mounts produce a turning moment as the cylinder applies force to the load.Straight line, force not absorbed - Side mounted cylinders do not absorb force along their centerlines. These mounting styles have lugs on the end closures and one style has side-tapped holes for flush mounting, Figure 13. The plane of their mounting surface is not through the centerline of the cylinder; for this reason, side mounted cylinders produce a turning moment as the cylinder moves the load. This turning moment tends to rotate the cylinder about its mounting bolts. If the cylinder is not well secured to the machine, or the load is not well guided, side loads will be applied to the rod gland and piston bearings.
To avoid this problem, side mounted cylinders should have a stroke length at least as long as the bore size. Shorter stroke, large bore cylinders tend to sway on their mounts with heavy loading especially with side lugs, end lugs, and end angle mounts.
Side mount cylinders depend wholly on the friction of their mounting surfaces in contact with the machine to absorb the forces the cylinder produces. The torque applied to the mounting bolts should equal that of the tie rod torque as recommended by the manufacturer.
For heavy loads or shock conditions, side mounted cylinders should be held in place with a key or pins to prevent shifting. A shear key - consisting of a plate extending from the side of the cylinder - can be supplied with most cylinders. It should be placed at the proper end to absorb the major loading, that is, at the rod end with the load in tension and at the cap end with a thrust load. This method may be used where a keyway can be milled into a machine member. The key takes shear loads and provides accurate alignment of the cylinder.
Side lug mounts are designed to allow dowel pins to pin the cylinder to the machine. When used, pins are installed on both sides of the cylinder but not at both ends.
fig. 14. pivot mounts absorb force along centerline and actuate loads that travel through arc. cap trunnion (a), intermediate fixed trunnion (b) can locate anywhere between head and cap, and head trunnion (c) are versions of this style; only one of these versions is used at one time. the cylinders shown in figures 3 and 6 employ clevis mounting, which is a type of pivot mounting for loads that travel through arc.Pivot force transfer - Cylinders with pivot mounts that absorb force along the centerline should be used when the actuated load travels through an arc. There are two ways to mount a cylinder so it will pivot during the work cycle: clevis or trunnion mounts, Figure 14. Pivot mount cylinders are available with cap fixed clevis; cap detachable clevis; cap spherical bearing; and head, cap, and intermediate fixed trunnion. Special trunnion assemblies that provide gimballing action are available.
Pivot mount cylinders can be used in tension or thrust applications at full rated pressure, except that long stroke cylinders in thrust applications are limited by piston rod column strength. Clevis or single-ear mounts usually are an integral part of the cylinder cap although detachable styles are available and provide a single pivot for mounting the cylinder. A pivot pin of appropriate length and diameter to withstand the maximum shear load at rated cylinder operating pressure is included as part of the clevis mount. The fixed clevis mount is the most popular and is used where the piston rod travels a fixed arc in one plane. It can be used vertically or horizontally.
On long-stroke thrust applications, it may be necessary to use a larger diameter piston rod to prevent buckling or use a stop tube to minimize cylinder side loading in its extended position. Fixed clevis mounted cylinders do not function well if the path of rod travel is in more than one plane. Such an application results in misalignment and causes unnecessary side loading on the bearing and piston. For applications where the piston rod will travel a path not more than 3° either side of the true plane of motion, a cap spherical bearing mount should be used as well as a spherical bearing rod eye. Cap detachable clevis mounts are most often used for air or medium-duty hydraulic service.
Trunnion pivot mounts also are used when the piston rod travels an arc in one plane. Trunnion pins are designed for shear loads only and should not be used with bending stresses. The support bearings should be mounted as close as possible to the trunnion shoulder faces.
Head trunnion mounted cylinders usually can be specified with smaller diameter piston rods than cylinders with the pivot point at the cap or at an intermediate position. On head trunnion mounted long stroke cylinders, the designer should consider the over-hanging weight at the cap end of the cylinder. To keep trunnion bearing loads within limits, stroke lengths should be not more than five times bore size.
An intermediate fixed trunnion mount is the best trunnion mount. It can be located to balance the weight of the cylinder or anywhere between the head and cap to suit the application. Its location must be specified at time of order because its location cannot be easily changed once manufactured.
Proper installation begins with machine layout; here are some rules:
Many fluid power cylinders incorporate cushions to absorb the energy of moving masses at the end of a stroke, including the masses of the piston and rod, the load being moved, and the fluid medium operating the cylinder. When the cushion operates, the additional thrust is imposed on the cylinder assembly and it will change length. What about the fluid conductors?
Consider protecting exposed rods from abrasion and corrosion that could destroy the rod surface and, in turn, the rod seal. In especially dirty environments, protect the rod with a cover such as a rod boot or bellows.
Flexible wall cylinders have evolved from designs that were originally made for vibration and isolation mounting. They consist of metal mounting plates fixed to a reinforced rubber chamber that extends and collapses, respectively, as it is pressurized and vented. They have generous lateral misalignment allowances and can actuate through an arc without a clevis mount.
Some precautions should be exercised when applying these cylinders. First, mechanical stops should be provided to limit the length of extension. Otherwise, an overrunning load could pull an end plate off the cylinder. Mechanical stops should also limit retraction, thereby preventing crushing the elastomeric portion of the cylinder between end plates. Alignment of these cylinders is much less critical than with conventional cylinders. However, relative torsional rotation between the end caps should be prevented to keep from having the elastomeric portion fail due to excessive shear stress.
An alternate design resembles a length of flexible hose sealed at both ends. With no pressure, the hose is flat; pumping air into it expands the hose into a tubular shape. Maximum stroke is approximately the ID of the inflated hose. Using a long length of such hose can generate very high force from a relatively low pressure. However, actuation force decreases with stroke length. This is because as the hose expands, it becomes more round, so a smaller area is in contact with the load to apply the force.
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