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-end cylinder, Figure 5, 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.
Most telescoping cylinders, Figure 6, 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 15 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.
Ram cylinders are a special type of single-acting cylinders that have a rod OD the same diameter 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 srpings, but, rahter, gravity or the load to retract the piston rod.
Short-stroke cylinders, Figure 7, generally have a rod length that is less than the piston diameter. It is used where high force must be generated from a relatively low supply pressure. Short-stroke cylinders also fit into a narrow axial space but require substantial radial width. These cylinders lend themselves to air-operated, automation machinery.
Tandem cylinders, on the other hand, are designed for applications where high force must be generated within a narrow radial space where substantial axial length is available. A tandem cylinder, Figure 8, functions as two single rod-end cylinders connected in line with each piston inter-connected to a common rod as well as a second rod which extends through the rod-end cap. Each piston chamber is double acting to produce much higher forces without an increase in fluid pressure or bore diameter.
Duplex cylinders also have two or more cylinders connected in line, but the pistons of a duplex cylinder, Figure 9, are not physically connected; the rod of one cylinder protrudes into the non-rod end of the second, and so forth. A duplex cylinder may consist of more than two in-line cylinders and the stroke lengths of the individual cylinders may vary. This makes them useful for achieving a number of different fixed stroke lengths, depending on which individual pistons are actuated.
Diaphragm cylinders, Figure 10, are either of the rolling diaphragm or the short-stroke type. Both use elastomeric diaphragms to seal the barrel-piston interface. The short-stroke type uses an elastomer sheet secured between halves of the cylinder body and is commonly used for truck and bus air-brake applications. The rolling diaphragm cylinder has a hat-shaped diaphragm that rolls into the cylinder barrel as the piston advances. Both types require very low breakout forces, have zero leakage, and are single-acting, spring returned.
| Fig. 11. Fixed cylinder mounts that provide straight-line force transfer are: (a) tie rods extended, and extended both ends (a + b); (c) head rectangular flange, (d) head square flange; and (e) rectangular head which provides same service as (c) but uses entire head rather than an added flange. Cap flange mounts are the same as (c) and (d) but bolt to cap (not shown). Rectangular caps also are available. |
| Fig. 12. As with other NFPA standardized mountings, centerline lug mounts provide striaght-line transfer of force. |
| 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. |
| 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. |
General system design
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 6000 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.