The cutaway view and symbol in Figure 15-3 is for a typical industrial-grade tie-rod cylinder. This cylinder includes all the standard features available from most manufacturers. The names of the parts are what most fluid power glossaries propose, while the names in brackets may be in common use.
The cap end and head end seal off the tube ends with tube-end seals. Tie rods hold the assembly together. The tie rods are tightened to a torque that will resist as much as five times the cylinder’s rated pressure. Tie-rod construction gives the package some flexibility or stretch without permanent deflection or damage. The piston provides the area for fluid to work against. The piston seals stop bypass that would waste energy. The piston rod transmits the force on the piston to the outside of the envelope and is attached to the work mechanism. The rod bushing and rod seal keep the rod aligned and stop fluid leaks to atmosphere. Cap end and rod end cushion plungers block high fluid flow near the end of stroke to allow smooth, no-shock stopping. Cushion-adjusting screws make it possible to adjust stopping speed, while cushion-bypass checks let the piston move rapidly as the cushion plungers are leaving their chambers.
The symbol on the left is the detailed symbol for a hydraulic cylinder with adjustable cushions on both ends. This cylinder also could be as shown as: non-cushioned, cushioned rod end only, or cushioned cap-end only. (When the energy triangles at the ports are blackened, the cylinder is pneumatic.)
The simplified symbol shows less detail but represents the same unit. The 2:1 information over a single rod end cylinder indicates that the rod area is half that of the piston. (Cylinders with 2:1 area ratio will be discussed later in this chapter.)
The spring return and extend cylinders in Figure 15-4 illustrate another method of moving cylinder pistons and rods for some applications. The cutaway views show typical construction (using a tie-rod cylinder as the basic unit). Many other designs are available but essentially use similar parts. Notice that the pistons have mechanical stops to keep the spring from compressing enough to bottom out. Breather ports for air operation or connections for tank drains for hydraulic cylinders are commonly found at the spring end. Most manufacturers indicate that the spring is only capable of returning the piston and rod. It may not be capable of returning the external load. Springs can be less than reliable and difficult to monitor -- especially when they are internal. Because there usually is little savings in hookup or operation, use these cylinders with care.
The tandem cylinder in Figure 15-5 can produce almost twice the force from the same diameter, but it is a little over twice the length. The two cylinders can be independently piped or drained to give extra force in one direction only or both directions. The center heads have guide bushings and seals for both sections so a different fluid can also be used in either end. (See Chapter 17 to learn how tandem cylinders allow oil to control speed and air as the power source. Circuits for matched and unmatched tandem cylinders can be found in the author’s upcoming e-book Fluid Power Circuits Explained.)
The tandem cylinder in Figure 15-5 has a common rod for both pistons. The tandem cylinder in Figure 15-6 has two separate pistons and rods and two different stroke lengths. This combination can be used to get three positive stops from an air or hydraulic cylinder with no special valves or controls. The stops are mechanically fixed, so the stop positions are in the same place every time. However, the stop positions only work for one situation. A four- or five-way directional valve at each cylinder plus flow controls are all that is normally required to operate this circuit.
Special consideration must be used in circuit design for the unattached tandem cylinder in Figure 15-6. If the long-stroke cylinder is not restrained while the short-stroke cylinder extends, it can overtravel and miss the exact position. This problem is exaggerated with horizontal or vertical rod-down applications. Meter-out flow controls or counterbalance valves can eliminate the problem, but could increase cycle time in some cases.
The cap-to-cap mounted cylinders in Figure 15-6 depict another way to use pneumatic or hydraulic cylinders to obtain positive positioning without special valves or equipment. Two four- or five-way valves and flow controls usually make this circuit operate smoothly.
Some designers specify double-rod end cylinders such as those shown in Figure 15-7. These cylinders cost about twice as much as single-rod cylinders and the design has a second place for fluid to leak. In most cases the reason for using them can be accomplished by other methods with equal or better results. If you must use a double-rod end cylinder, remember to allow space for the extra rod and the safety hazard it can cause. Also, the rod reduces the area on the working side of the piston, so a larger bore or higher pressure is necessary in many cases.
A double-rod end cylinder might be specified so that the force and speed in both directions is the same when flow and pressure are equal. This may be true, but flow controls and a reducing valve can accomplish the same result at a reduced cost and in less space. Another alternative is a regeneration circuit, used when producing the exact speed and force in both directions is not critical. (Regeneration circuits are covered extensively in the author’s upcoming e-book Fluid Power Circuits Explained.)
It may appear that double-rod end cylinders reduce rod flexure when the cylinder is fully extended. The rods in their bushings and the piston in its bore provide snug bearing points -- but allow some play. As the piston nears the end of stroke, two of the bearing points get closer together, so lateral movement at the extended end of the rod can increase. It is supposed that the opposite rod will reduce lateral movement and hold the attached load closer to a centered position. However, from the cutaway it is obvious the distance between the piston bearing and the opposite rod bushing almost eliminates any centering effect of the piston. A better way to reduce lateral movement of the extended rod is to stop the piston short of full stroke – either by an internal stop tube or externally by machine members. This arrangement requires a longer cylinder but gives the desired results at a lower cost.
A main reason for using double-rod end cylinders is to mount limit switches to show cylinder position. A special bracket opposite the attachment end holds the limit switches and a doughnut-shaped protrusion on the rod contacts them as the piston strokes. For the same price (and consuming a lot less space), most cylinder manufacturers offer limit switches that attach to the head and/or cap and are activated by cushion plungers. Another signal indicator -- especially for pneumatics – is a Hall-effect switch and a magnetic piston to activate it.
All of the above cylinder-position indicators have one potential major flaw. If the part attached to the rod end gets disconnected for any reason, the machine still will cycle when the cylinder moves...even though the disconnected load may be in the way. If at all possible, mount limit switches on the machine member so its position is never misinterpreted.
Non-rotating rod cylinders
The cylinders in Figure 15-8 incorporate some method to keep the piston and rod from rotating as it strokes. A standard cylinder may try to turn as it extends and retracts, causing it to unscrew from its workpiece. In some applications, the cylinder is expected to orient the work piece it is driving and keep it aligned with mating parts. All the designs in Figure 15-8 attempt to accomplish this non-rotating function in different ways. At best they can keep the rod from turning but none can perfectly guide it when outside forces are acting to turn it. This is especially noticeable on long-stroke cylinders. It is always best to guide the workpiece externally and only use the cylinder to cycle it. Note that the oval-piston design offers the ability to mount cylinders side by side with minimum rod center distance between them and still produce ample force.
The cylinders in Figure 15-9 take up less space on long-stroke applications because they only need mounting space slightly longer than their stroke length. Conventional piston-and-rod cylinders require space more than twice their stroke length -- and can be difficult to conveniently place on many machines.
The earliest long-stroke design is the cable cylinder – shown at top left in Figure 15-9. A coated cable fitted with a work piece attachment, wrapped around two pulleys, and attached to a piston in a bore produces reciprocating motion as fluid -- usually air -- enters and exhausts through the ports. These cylinders are usually 4-in. bore or less, and may have strokes up to 30 ft (or more in certain configurations). Cushions may be specified when required. The cable is coated with nylon or Teflon so it can slide through seals with minimal damage to them. However, the coatings are prone to cracking and eventually will cut the seals until they leak. (The symbol for the cable cylinder is adapted from a manufacturer’s catalog because ISO does not show one.)
The rodless cylinder, top right in the figure, was introduced in the late 1970s. It is even more compact than a cable cylinder and avoids the coating wear problem. It consists of a piston in a bore that has a slot open to atmosphere along its whole length. A seal blocks air from escaping through the slot while the piston is not present. A second contamination seal keeps debris from filling the slot. The fluid seal and contamination seal pass through slots in the piston in slots as it reciprocates. The work piece attachment (connected to the piston) reciprocates to move machine members as fluid enters and exhausts the cylinder. Bores up to 2 12 in. and strokes as long as 33 ft are available from several manufacturers. Cushions may be specified when required.
The band cylinder is an alternative to the cable cylinder. Its smooth steel band passes through seals instead of a coated cable. The magnetic-drive cylinder uses magnetic attraction to keep the piston and workpiece attachment connected. It operates at pressures up to 120 psi and will maintain connection up to 180 psi.
One manufacturer has a modified rodless cylinder with a toothed belt and pulley arrangement to drive the workpiece attachment. It offers the option of an external output shaft to which a brake can be fitted to stop and hold position. This output shaft can also drive an encoder to show work piece position or can connect to another unit for synchronization. It also could act as a low power rotary actuator. It is available in 1- or 1 12-in. bores and up to 177-in. stroke.