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Pressure- or fatigue-testing machines often require high pressure for long periods of time. Other circuits might need a small volume of high-pressure fluid for a short period while most of the cycle only needs low pressure. Other machines can use air cylinders to manipulate a part but need very high pressure to perform one operation. Some manufacturers make high-pressure rotary pumps — rated up to approximately 10,000 psi — but these pumps are expensive and may heat the fluid. Another choice for low-volume/high-pressure circuits is an intensifier.
When a circuit calls for a small volume of high-pressure oil or air, consider using an intensifier — sometimes called a booster. Most cylinder manufacturers build air- or hydraulic-powered intensifiers. Or you can use off-the-shelf cylinder parts to assemble your own booster. Also, intensification is a natural function of single-rod cylinders and motor-type flow dividers.
Figure 13-1 pictures the symbol for an air-oil intensifier. While the symbol shows two pistons with different diameters, the actual intensifier consists of a piston pushing a rod. The large-area air piston pushes a small-area hydraulic ram against trapped oil. The difference between the two areas gives high-pressure capability at the small ram. This capability is indicated by the area ratio. If the air piston has a 5-in. diameter and the oil piston has a 1-in. diameter, the area ratio is 25:1. With this area ratio, 80 psi acting on the air piston produces 2000 psi at the hydraulic piston.
Stroke length dictates the maximum volume of high-pressure fluid from an intensifier configured as in Figure 13-1. The booster in Figure 13-2 produces the same pressure but an unlimited volume. A reciprocating intensifier takes fluid from a reservoir and forces it into the circuit. In effect, the reciprocating intensifier is a single-piston pressure-compensated pump. The area ratio and air pressure determine the maximum hydraulic pressure. This pump is close to 100% efficient, so oil heating is not a problem. Intensifiers do not need relief valves because they stall at maximum pressure.
The oversize-rod cylinder shown in Figure 13-3 also is an intensifier. Any single-rod cylinder intensifies pressure with the rod end port blocked. The larger the rod diameter, the greater the intensification. For low intensification — say 1.5 to 2 times system pressure — a single-rod cylinder is inexpensive and readily available.
Figure 13-4 depicts the symbol for a motor-type flow divider used as an intensifier. This type intensifier produces a continuous flow of higher-pressure oil at a reduced flow rate. The reduced flow rate is the same ratio as the pressure increase. (A 2:1 intensifier reduces the flow by 50%.) A motor-type flow divider intensifier is less efficient than a piston-type intensifier and is not recommended for applications with long holding periods.
Figure 13-5 shows the symbol for an air-to-air intensifier. These intensifiers produce small volumes of higher-pressure air from the plant air supply. Ratios up to 4:1 are common. Hydraulically driven designs with higher ratios are available from some manufacturers.
Intensifier circuit using standard cylinders
The schematic diagram in Figures 13-6 through 13-9 suggests how to use standard cylinders as an air-hydraulic intensifier. This is a quick way to get high ratio intensification for a rush job. A 6-in. bore air cylinder driving a 1.5-in. bore hydraulic cylinder gives an intensification ratio of 16:1. With 80-psi input air, hydraulic output pressure is approximately 1300 psi.
Mount the cylinders to a beam or machine member and pipe them as shown in the Figures 13-6. This circuit allows a hydraulic cylinder to operate at low pressure during extension and retraction, with a short high-pressure work stroke to clamp, punch, or do other work. The circuit includes shop-made intensifier A, air-oil tank B, air-pilot-operated hydraulic check valve C, solenoid-operated 5-way air valve D, sequence operated 5-way air valve E, and work cylinder F. With solenoid S1 deenergized, the cylinder and intensifier stay fully retracted, ready for a work stroke.
Energizing solenoid S1 on valve D, as in Figure 13-7, directs air to air-oil tank B and exhausts the rod end of cylinder F. Oil from the air-oil tank free-flows through check valve C to extend the cylinder rapidly. Pressure in the line to the cylinder’s cap end remains low as the cylinder moves toward the work, so sequence valve E stays in its normal position. The cylinder extends until it contacts the work.
After the cylinder contacts the work, pressure in its cap-end port increases. Figure 13-8 shows the circuit condition after this pressure buildup shifts sequence valve E. When sequence valve Eshifts, air goes to the cap end of the 6-in. cylinder on intensifier Aand exhausts from its rod end. Cylinder Aextends to stroke the 1-1/2-in. hydraulic cylinder. This forces high-pressure oil to the cap end of work cylinder F. Check valve C is held closed by its spring to block high-pressure oil from going to air-oil tank B. Pressure in the cap end of cylinder F rises to approximately 1300 psi — and is available to power any high-force operation.
The intensifier’s hydraulic cylinder must provide enough oil to move the work cylinder through its high-pressure stroke. A 3.25-in. bore work cylinder with a high-pressure work stroke of 0.75 in. requires a minimum 6.22 in.3 intensifier volume. Calculate volume by multiplying the area of the working cylinder by the length of the high-pressure work stroke. To figure the minimum intensifier stroke, divide the volume required for the work cylinder by the area of the intensifier. In this example, the minimum intensifier stroke is 3.5 in. To make sure there is always enough high-pressure oil to do the job, add 1.0 to 1.5 in. to the intensifier stroke to allow for oil compressibility, hose stretch, and possible future needs. Choose an intensifier stroke of at least 5 in. for this application.
Deenergizing solenoid S1 on valve D, Figure 13-9, directs air to the rod end of cylinder F and to the pilot port of air-pilot-operated check valve C. Check valve C opens, providing oil from the cap end of cylinder F with a free path to tank. Pilot pressure to sequence valve E drops when valve D shifts. When sequence valve E returns to its normal position, intensifier A retracts and fills the intensifier cylinder with oil for the next cycle.
Notice that as cylinder F retracts, only 80-psi air pressure drives it. There is ample hydraulic pressure to extend the cylinder for the high-force work stroke, but only air pressure to retract it. If a higher retracting force is needed (to disengage tooling or for other reasons), external help or other circuit changes may be necessary.
Adjust hydraulic pressure to the cylinder with a regulator in the air line connected to sequence valve E. With a regulator to adjust the air pressure, changing hydraulic force is simple.
Hydraulic cylinder F should have resilient seals that keep oil from leaking to the air side or air to the oil side. Some circuits use two air-oil tanks on cylinder F to prevent aeration of the oil. (Chapter 3 has information about sizing and hooking up air-oil tanks.)