Sizing pneumatic cylinders
The procedure for sizing air cylinders is very similar to that for sizing hydraulic cylinders. One major difference: most plant air systems operate around 100 to 120 psi with approximately 80 psi readily available at the machine site. This gives little or no leeway for selecting operating pressure.
Also, because a compressor is part of the plant facilities, the number of cubic feet per minute (cfm) of air available for a single air circuit usually is many times that required. It is good practice though, to check for adequate flow capacity at the machine location.
The other items needed to design an air circuit are maximum force required, cylinder stroke, and cycle time. With this information, sizing cylinders, valves, and piping is simple.
To calculate the cylinder bore required, use the formula given at A in Chart 6-2. Notice the 1.25 multiplier on the force line. For an air cylinder to move at a nominal rate, it needs approximately 25% greater thrust than the force required to just offset the load. When the cylinder must move rapidly, provide a force up to twice that required to simply balance the load.
The reason for this added force can be illustrated by the example of filling an empty tank from a tank at 100 psi. When air first starts to transfer, the high pressure difference between the two tanks produces fast flow. As the pressures in the tanks get closer, the rate of transfer slows. The last 5 to 10 psi of transfer takes a long time. As the tank pressures get close to equal, there is less reason for transfer because the pressure difference is so low.
At a system pressure of 80 psi, if an air cylinder needs 78 psi to balance the load, there is only a 2-psi differential to move fluid into the cylinder. If the cylinder moves at all, the motion will be very slow and intermittent. If pressure differential increases – either from higher inlet pressure or lower load – the cylinder starts to move smoothly and steadily. The greater the differential, the faster the cylinder strokes. (Note that once cylinder force is twice the load balance, any increase in speed due to higher pressure is minimal.)
Substituting the 1.25 multiplier in the formula produces a cylinder bore of 1.72 in. minimum. Choose a 2-in. bore cylinder because it is the next standard size greater than 1.72 in.
To size the valve, use the flow coefficient (or Cv) rating formula. (The Cv factor is an expression of how many gallons of water pass through a certain valve . . . from inlet to outlet . . . at a certain pressure differential.) Valve manufacturers use many ways to report Cv and some may be confusing. Always look at the pressure drop allowed when investigating the Cv, to be able to compare different brands intelligently.
The formula indicates that a valve with 1/8-in. ports is big enough to cycle the 2-in. bore cylinder out 14 in. and back in 4 seconds.
There are many charts in data books as well as valve manufacturers’ catalogs that take the drudgery out of sizing valves and pipes. There are several computer programs as well to help in proper sizing of components.
Cylinder circuits with four positive stopping positions
To stop a cylinder stroke accurately at different points in its travel, use a hydraulic servo system. Particularly for constantly changing intermediate stopping positions, a servo system works best. However, with only one constant mid-stroke stopping point, the circuit shown in Figure 6-2 will work well. A pair of cylinders with different strokes is attached at their cap ends. (This arrangement might be as simple as two off-the-shelf cylinders with their cap end flanges bolted together. Many manufacturers furnish this cylinder arrangement as a unit, using long tie rods to make the mechanical connection.) Because the cylinders have different strokes, it is possible to stop the load accurately at four positions. For instance, if cylinder C has a 2-in. stroke and cylinder D has a 4-in. stroke, the positions are home, and two, four, and six inches from home. If both cylinders have the same stroke, the positions are home, half extended, and full extended.)
This positioning arrangement works the same with air or hydraulic circuits, and always requires two valves. Air cylinders might bounce at fast speeds, but would quickly settle at an exact position. Note that the cylinders also move, so use flexible lines and provide some way to guide the cylinders.
Figure 6-2 shows the circuit at rest. The valves could be double-solenoid (as shown), single-solenoid/spring-return, or spring-centered. The cylinders are both fully retracted, in Position #1.
When valve A shifts, as in Figure 6-3, cylinder C strokes to Position #2. This position is always the same because the piston bottoms out against the cylinder’s head end. Adjusting the rod attachment can make slight position variations. Machine wear could make such adjustments necessary.
In Figure 6-4, valve A shifts to retract cylinder C while valve B shifts to extend cylinder D . This accurately places the load in Position #3. Finally, both cylinders extend as shown in Figure 6-5, moving the load to Position #4.
After both cylinders extend fully, they can return to home or either of the mid-stroke positions as required. (The circuit designer might choose air logic or electrical controls, with palm buttons numbered one through four – to allow an operator to pick any cylinder position at any time.)
Using more than two cylinders can provide a greater number of stopping positions, but controlling more positions requires more circuitry. This still may be is less expensive than a servo system. Lower cost and easier maintenance may offset the greater versatility of a servo system in some applications.
Air or hydraulic tandem-cylinder circuits with three positive stopping positions
A tandem cylinder consists of two double-acting cylinders in one envelope. It has four fluid ports, and the piston rods may be attached or unattached, depending on the application. Most unattached-rod tandem cylinders have unequal strokes, while attached rod tandems have equal strokes. Some tandem cylinders have different bores, again depending on the need.
Figure 6-6 shows a rigidly mounted, unattached tandem cylinder in a multi-positioning circuit, with the cylinder and valving at rest. This circuit produces three positive positions. Note that the load must be resistive – or made that way with valving. Cylinder C has a 2-in. stroke and cylinder D has a 6-in. stroke. This combination gives a positive home position, plus two inches, and six inches extended. Valve A could be single-solenoid/spring-return or a double-solenoid detented (as pictured). Valve B must allow cylinder D to float – to avoid reducing the force of the stroke to Position #2.
Shifting valve A, as shown in Figure 6-7, extends cylinder C through its full stroke, moving cylinder D and the load to Position #2. If travel speed is too fast and/or resistance is low, cylinder D may overshoot Position #2. If this occurs, add a flow control for air or a counterbalance valve for hydraulic service to offer resistance while cylinder C is stroking.
To extend the tandem cylinder fully, valve B shifts, as in Figure 6-8, porting fluid to the cap end of cylinder D . Cylinder D then extends fully to Position #3. Positions #2 and #3 are positive. They will be rigid in a hydraulic circuit and typically spongy in an air circuit.
To retract the load, both valves return to home position, Figure 6-9. Cylinder D retracts fully and also pushes cylinder C home. Vent cylinder C’s rod port to atmosphere if it is air operated, or drain the port to tank on a hydraulic cylinder.
Assembling more than two cylinders this way creates more positive stopping positions when needed. Always make the first cylinder the one with the shortest stroke, with each added cylinder’s stroke longer.
Using tandem cylinders to increase force
On occasion, a cylinder already in service is undersized for a new material or product, and there is no room in its location for a larger-diameter cylinder. One way to produce more force is to use a tandem cylinder with the same bore and mounting dimensions as the original cylinder. A tandem cylinder almost doubles the force of the single cylinder. The tandem cylinder mounts exactly as before, with the same rod diameter and thread. The only dimensional difference is that the tandem cylinder is more than twice as long. (Normally an attached tandem cylinder is best for doubling force, although not in all cases.)
Figure 6-10 shows a tandem cylinder circuit that produces additional force on the extension stroke. Normally the retraction stroke needs minimal force, so vent or drain the rod side of the single-rod cylinder. Piped this way, fluid volume only increases on the extension stroke. With a 6-in. cylinder bore and a 2-in. rod diameter, the tandem cylinder’s force is 90% more than the original single cylinder.
The circuit in Figure 6-11 uses an unattached tandem cylinder in a circuit that allows standard force or increased force as needed. For low force only, energize valve A. Oil volume and force are the same as in the original circuit. For almost double force, energize valves A and B.
The dual-force circuit in Figure 6-12 uses almost the same volume of oil as the single cylinder it replaces. Pipe directional valve A (supplied by the pump) to single rod-end cylinder D that is part of an attached tandem cylinder. When directional valve A shifts to extend the cylinder, oil flows to cylinder D. As cylinder D extends, it moves cylinder E. Cylinder E is fitted with a flow line from rod end to cap end through directional valve B. All the oil in cylinder E transfers to the opposite side of the piston, so the cylinder is full for the double-force portion of the stroke. Check valve C holds backpressure in the transfer circuit while cylinder E is moving and allows oil to flow to tank during the extra-force portion of the cycle. Extra force comes in when directional valve B shifts, sending oil to the push side of cylinder E’s piston and allowing the opposite end to flow to tank.
When changing to a tandem cylinder for extra force, always check the rod diameter for column strength. All manufacturers show maximum force capabilities for a given rod diameter. When rod size increases, maximum force decreases due to less area on the double rod end cylinder. When using an oversize rod, purchase it with an undersize thread rod so it attaches directly to the machine member without modification.
Caution: make sure the cylinder mounting can withstand the extra thrust. Most cylinder manufacturers' literature gives maximum force capabilities for a given mounting style. Because certain mounting styles have a lower pressure rating, a tandem cylinder may only accept slightly more than half the rated pressure. Change the mounting style if the reduced pressure generates too little force. Also, realize the extension speed of the double force portion of a tandem cylinder arrangement is approximately half the speed of a single cylinder.
Circuit with unmatched tandem cylinders for high speed and force
Many press applications require long strokes for loading parts with only a small portion of the stroke operating at high tonnage at the end. A 10-in. bore cylinder might be required for tonnage, while a 4-in. bore cylinder could provide all the force necessary to move to and from the work. Conventional circuitry often uses high volume at low pressure and high pressure at low volume for an application of this type. A regeneration circuit (Chapter 17 will cover regeneration circuits) could reduce the high-volume pump flow by half, but fast cycling still requires high flow.
Large cylinders with prefill valves and push back cylinders are one way to overcome the requirement for large fluid volumes. (Chapter 7 will explain decompression and prefill valves.) Due to their high cost, prefill valves normally are found only in circuits with 20-in. or larger bore cylinders.
The circuit in Figure 6-13 illustrates another way to operate at high speed for extension and retraction at low force, with high tonnage available at any point along the extension stroke. The unmatched tandem cylinder has attached piston rods so the small-bore cylinder can retract both the large-bore cylinder and the load. The small-bore cylinder needs only a small volume of fluid to extend and retract at high speed, while both cylinders can produce high tonnage.
Energizing the extend solenoid on valve A in Figure 6-14 causes the small-bore cylinder to extend rapidly, in regeneration. This moves the large-bore cylinder and platen downward. As the platen lowers, oil in the large-bore cylinder transfers through valve B to the large-bore cylinder’s opposite end.
When the load meets resistance or contacts a limit switch, Figure 6-15, valve B’s solenoid also energizes – sending pump flow to both cylinders. During this part of the cycle, speed slows and tonnage increases. (The large-bore cylinder transferred oil during the high-speed portion of the cycle to ready it for the high-force portion of the stroke.) During the high-force portion of the cycle, oil from the mounting end of the large cylinder returns to tank. Because the large bore cylinder exhausts during this part of the cycle, it receives fresh oil for every high-pressure stroke.
To retract the cylinder at high speed, energize the retract solenoid on valve A, Figure 6-16. The pump retracts the small-bore cylinder, which also retracts the large-bore cylinder and platen. While the large-bore cylinder retracts, fluid in it again flows from end to end, so the cylinder stays full. Backpressure check valve C in the tank line keeps oil from draining to tank when it is lower than the cylinder.
Note externally drained pilot-operated check valve E at the rod end of the small-bore cylinder. With a running-away load, some means is needed to hold the cylinder in place while the circuit is at rest. This cylinder might free fall when the directional valve centers without some way to keep it from trying to regenerate. If the load is heavy, use an externally drained counterbalance valve to stop the pilot-operated check valve from chattering.
One potential problem with this arrangement is the length of the tandem cylinder. For long strokes, the more-than-double length of the tandem cylinder could cause height or length interference. Also, the rod size of the large-bore cylinder determines the smallest bore of the small cylinder. For example: if the double rod-end cylinder has a 10-in. bore with a 5-in. rod, then the smallest single-rod cylinder would require a 7-in. bore.
For the arrangement just shown and sized, the force at 3000 psi is approximately 292,000 lb. A pump flow of 30 gpm would result in a cylinder cycle time of about 15 seconds . . . with a 40-in. travel stroke and a 3/4-in. tonnage stroke.
Short closed height with double-length movement using two cylinders
Some machines need long strokes but lack space to mount long-stroke cylinders. Using telescoping cylinders is feasible for some applications, but high cycle rates usually eliminate them from consideration. Also, most telescoping cylinders are single-acting and depend on gravity or other outside forces to return them. Another drawback to telescoping cylinders is that the smallest-diameter ram must able to generate enough force to move the load. This means all other sections must be larger so they will need to be supplied with high flow for high speed.
Figures 6-17 and 6-18 show two air cylinders facing in opposite directions with their bodies attached side by side. This orientation makes the total stroke additive, while the retracted length is that of a single cylinder. (Assuming that both cylinders have a 20-in. stroke, the platen’s starting position in Figure 6-17 is about 20 inches lower with this arrangement than it would be with a single 40-in. stroke cylinder.) Many applications use standard NFPA-design cylinders in such an arrangement. With this circuit there is constant force and speed, compact mounting, and double-acting operation. The only special requirement is to specify valves that give smooth action and control. If the circuit used two directional valves, the platen could have three positive positions (if required). With different stroke lengths, these cylinders could stop the platen positively in four positions.
Using a single valve for extra stroke only requires meter-out flow controls at each cylinder port for near-simultaneous movement. This arrangement works smoothly and eliminates jerking when the cylinders bottom out at different times.
With hydraulic cylinders, use a spool-type flow divider for simultaneous movement and closely synchronized end of stroke stopping. (Chapter 11 will cover flow divider circuits.)
Because both cylinder bodies move, use flexible fluid lines. Also, arrange to guide the platen or machine member to keep excess side-loading off the cylinders.
Side-by-side cylinder mounting does not work as well in high-force applications because the higher side-load forces will wear out bushings and cause premature seal leakage. The side-by-side configuration works best in low-force pneumatic applications.