Other types of linear actuators
The actuators in Figure 15-10 depict other ways of producing linear force. The rolling diaphragm, diaphragm, and bellows actuators are single acting. The rolling diaphragm is capable of long strokes but not long life at high cycles. The diaphragm is designed only for short-stroke applications only but can have high force due to large areas. All these single-acting devices use some internal or external method to retract them. Using vacuum and weight are two of the methods used for retracting bellows. The double-acting rolling diaphragm operates like a double-acting piston cylinder but is not designed for long life at high cycles.
Sizing linear actuators
Because most cylinders are round, the formula for figuring their area that most remember is: π r2. Another formula for circle area is .7854 D2 which does not require changing to radius before figuring area. There are also data books that have charts for all standard size cylinders. To determine maximum cylinder force, multiply the known area by the operating pressure in psi. The standard formula is written F=PA where F is force, P is pressure and A is area.
For pneumatic cylinders, the usual working pressure is 80 psi even though most plants cycle their air compressor between 115 to 125 psi. The reason for the lower working pressure is because line losses, especially during high flow surges, make it impossible to maintain compressor output pressure over the entire air supply system.
Operating pressure for hydraulic circuits often falls between 1500 and 2500 psi because most components are rated at 3000 psi. Lower pressures require larger actuators at higher flows and over-sizes the entire system. Pressures over 3000 psi often require special components and are usually relegated to very high force applications.
A factor that is often overlooked when sizing cylinders, especially pneumatic ones, is the force figured by the aforementioned formula is not available until after reaching full pressure. A pneumatic circuit does not reach full pressure quickly like a hydraulic circuit does. For example, when tank pressure is high and tire pressure is low it is easy to rapidly fill a flat tire from a portable air tank. However, as tire pressure starts approaching tank pressure, pressure drop decreases and the transfer rate slows to a crawl as rising tire pressure gets closer to lowering tank pressure. Pressure equalization can take several minutes, adding to cycle time when the tire is an actuator. The 3 1/4-in. bore air cylinder in Figure 15-11 will not move the load attached to it because force required and force available do not allow enough pressure drop to move fluid from inlet to the piston. The 4-in. bore gives nominal speed while the 5-in. bore would give fast speed.
To get nominal cycle times, always size air cylinders at least 25% above workforce whether moving a load or applying force. When cycle time must be fast, oversizing up to 100% could be required. Anything above 100% will quickly lessen the effect and is not worth the expense.
Even with hydraulic cylinders, pressure buildup may be fast but is not immediate. Oversizing of 10% is usually enough to achieve reasonably fast cycles in most situations. Other things that slow hydraulic actuator cycle time and pressure buildup include trapped air, slow shifting valves or valves with long spool overlap, and using pressure-compensated pumps without an accumulator.
Typical cylinder seals
The cutaways in Figure 15-12 show the standard ways of sealing pistons and rods from fluid bypass. O-rings and U-cups are resilient and continuous seals that can stop all fluid bypass so a cylinder can be stopped and held almost indefinitely. Cast-iron piston rings do not seal completely but do a good job when system shock or high heat is part of the operation.
O-ring seals are inexpensive but do not have long life in most applications. They must be setup with interference so as they wear they continue sealing. This also adds to breakaway and running friction. They are bi-directional and should not be used in pairs. With two O-rings on a piston, fluid can bypass and get trapped between the seals causing a lot of friction. At pressures above 500 psi, always specify backup rings to keep the O-rings from extruding and being damaged. Always keep O-rings well-lubricated because they can stick and roll in their cavity. When they start rolling they wear rapidly.
U-cups are long-life, low-friction seals because their flexible lips can be heavily preloaded without much increased friction. These seals are pressure energized, which means pressure inside the “U” forces the seal material tightly against the sealing surfaces. U-cups usually use backup rings at pressures above 1500 psi to keep them from extruding.
Material for the resilient seals discussed here is commonly Buna N rubber for pneumatics and many hydraulic cylinders. When synthetic fluid or high heat is encountered, resilient seals often are made of Viton. Buna N is good for temperatures up to 250° F with mineral oil and water glycol. Viton works at temperatures of 400° to 450° F and any fluid used in air or hydraulic circuits. Other seal materials such as leather, Neoprene, and Teflon are available and are standard for some suppliers.
Automotive-type cast-iron piston rings in sets of two or four seal well but not completely. A general rule is that they will leak approximately one cubic inch per minute per inch of bore per one thousand psi. Different end effects have been designed to reduce leakage at the expansion joint. Placing the expansion joint 180° from each other for consecutive rings helps slow leakage.
Cylinders that move at high speed need some sort of deceleration method to keep them from slamming when they reach end of stroke. Some applications with high speed and heavy loads may need valves and limit switches to give enough time to bring the load to a smooth stop. For most cases standard cylinder cushions work well. They are 3/4- to 11/16-in. long and can be in the head or cap end or both. Cushions add cost on most cylinders so should not be specified when unnecessary.
Hydraulic cylinder cushions
The cutaway in Figure 15-13 shows a cylinder with an oversize rod and standard cushions on both ends. The cushion-adjusting screw and cushion bypass check valve can only be installed in the cap end because the cushion plunger on the rod end is so large there is no room for them. The necessity of oversize rods is a problem for most manufacturers. It is possible to taper the cushion and get a smooth stop but it also slows the stroke as the cushion plunger leaves its chamber. The best option to control deceleration of oversize rod cylinders’ extend stroke is with external valves or proportional directional controls.
The other problem with oversize rods and rod-end cushions is the high intensification pressure present when working pressure is high and/or there is a heavy overrunning load. Pressure in the rod end can easily reach two-to-four times rated pressure each time the cylinder extends. High pressure can damage tube end seals and piston seals, and stretch the cylinder tube past its tensile limits.
The top cutaway in Figure 15-13 shows the cylinder piston moving to end of stroke with full flow exiting through the cushion chambers almost unrestricted. This means the cylinder can travel fast until the cushion plungers enter their chambers. The bottom cutaway is what happens after the cushion plungers have entered the cushion chambers. Trapped fluid decelerates the piston quickly to a speed set by the cushion-adjusting screws. Deceleration is sudden on the straight cushion plungers shown here because hydraulic fluid is almost non-compressible. The piston continues to end-of-stroke at a preset controlled speed without damage to itself or the machine.
On cylinders with standard and some oversize rod sizes, the rod-end cushion would function the same as one on the cap end. There is always some pressure intensification on the rod end but this is normally not a problem. When the cylinder starts to extend again, the cushion bypass check valve opens to allow fluid to the full piston area so it can extend quickly at full force. Without the bypass valves, takeoff speed would be as slow as deceleration on retract.
The cutaways in Figure 15-13 show a way of making cushions work. In actual cylinder design, cushions are built in many ways but the general function is the same. One company offers a self compensating non-adjustable cushion option without adjusting screws or bypass checks. Another supplier offers tapered or tapered slots in the plungers that give smooth deceleration instead of the sudden slowdown. Tapered cushions only work for a given pressure, load, and mounting position. They must be figured from information collected prior to building the cylinder.