Comparing Single- and Double-Acting Hydraulic and Pneumatic ...
Comparing Single- and Double-Acting Hydraulic and Pneumatic ...
BOOK 2, CHAPTER 6: Fluid power cylinders
Throughout this manual, many circuits show cylinders in a variety of applications. An explanation accompanies each example – noting the pumps, valves, and peripheral hardware used to do the work. Every design description also attempts to cover the limitations of a particular circuit and show other ways to perform the same task. This section covers several types of cylinder applications that do not fall under a particular heading.
Normally air cylinder circuits are less expensive than hydraulic circuits because there is no need for a power unit. An air compressor usually is part of the plant facility and compressed air is a commodity similar to electrical power. However, the cost of operating an air-powered machine may be four to seven times more than a hydraulically operated one.
Another disadvantage of air is the fluid’s compressibility. Hydraulic circuits are very rigid, while air circuits are quite spongy. This lack of control makes it almost impossible to accurately stop and hold an air cylinder in mid-stroke with standard air valves alone. After an air cylinder stops, it may start creeping or be forced out of position almost immediately.
When it comes to brute force, air cylinders fall far behind hydraulic cylinders because they normally operate only at 80 to 100 psi. Getting high force from low pressure requires large areas . . . with attendant large valves and piping. A general rule might be to look at hydraulics when an operation requires a 5-in. bore or larger air cylinder to develop the required force. However, another factor is how often the cylinder must cycle. Air circuits with very low cycle rates and long holding times could be more economical than hydraulics, but the faster the cycle time, the more it costs to operate an air cylinder. Another consideration is the operating environment. Around food or medicine, potential contamination from hydraulic oil could be a serious problem. Look at each application to see which fluid system best suits it.
Sizing hydraulic cylinders
Chart 6-1 provides an exercise in sizing a simple, single-cylinder hydraulic circuit with straightforward parameters. The example covers the basic requirements for sizing a hydraulic cylinder to power a specific machine.
Of course, in the real world of circuit design, experience, knowing the process, the environment, the skill of the user, how long will the machine be in service, and other items will affect cylinder and power unit choices.
Before designing any cylinder circuit it is necessary to know several things. The first is the required force. Usually, the force to do the work is figured with a formula. In instances where there is no known mathematical way to calculate force, use a mock-up part on a shop press or on a prototype machine to estimate the force requirements. If all else fails, an educated guess may suffice. (The sample problem in the chart requires a force of 50,000 Llb.)
The second requirement is the total cylinder stroke. Stroke length is part of machine function, but it is needed to figure pump size. Use a stroke of 42 in. in this problem.
Third, how much of the stroke requires full tonnage? If only a small portion of the stroke needs full force, a hi-lo pump circuit and/or a regeneration circuit could reduce first cost and operating cost. This cylinder requires full tonnage for the complete 42-in. stroke.
Fourth, what is the total cylinder cycle time? Make sure the time used is only for cycling the cylinder. While load, unload, and dwell are part of the overall cycle time, they should not be included in the cylinder cycle time when figuring pump flow. Use a cylinder cycle time of 10 seconds for this problem.
Finally, choose maximum system pressure. This is often a matter of preference of the circuit designer. Standard hydraulic components operate at psi maximum, so choose a system pressure at or below this pressure. If the company that will operate the machine has operating and maximum pressure specifications, adhere to them. Remember that lower working pressures require larger pumps and valves at high flow to get the desired speed.
In the example in Chart 6-1, the square root of the maximum thrust, divided by the maximum system pressure, divided by 0. gives a minimum cylinder bore of 5.641 in. Obviously, a standard 6-in. bore cylinder should suit this system.
To figure pump capacity, take the cylinder piston area in square inches, times the cylinder stroke in inches, times 60 seconds, divided by the cycle time in seconds, times 231 cubic inches per gallon. This indicates a minimum pump flow of 61.7 gpm. A 65-gpm pump is the closest standard flow available. Never undersize the pump because this formula figures the cylinder is going at maximum speed the whole stroke. In the real world, the cylinder must accelerate and decelerate for smooth operation, so travel speed after acceleration and before deceleration should actually be higher than this formula indicates.
Figure horsepower by multiplying flow in gpm by pressure in psi by a constant of 0.. This comes out to 75.79 hp . . . and is close to a standard 75-hp motor. This should provide sufficient horsepower because the system pressure does not have to go to psi with the cylinder size used.
The tank size should be at least two to three times pump flow. For the example, 3 X 63 equals 195 gallons. A 200-gal tank should be satisfactory. When using single-acting cylinders or unusually large piston rods, size the tank for enough oil to satisfy cylinder volume without starving the pump.
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.
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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.
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.
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.
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 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.
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