In some of the foregoing air-oil circuits, the usual 80- to 100-psi pressure may not be adequate for some operations. This does not mean a hydraulic pump and all the items related to it must be used. Several manufacturers make air-oil intensifiers that convert 80- to 100-psi shop air into 500- to 40,000-psi hydraulic pressure -- in small volumes of fluid.

Single-stroke intensifiers

The simplest intensifier is a single rod-end cylinder with a large piston rod. As explained in Chapter 15, a cylinder with a 2:1 area ratio rod can have pressure as high as twice system pressure in the rod end. This type intensifier is only available in ratios up to 2:1 unless special oversize rods are specified.

Another simple intensifier can be made by coupling the rod of a large-bore cylinder to that of a smaller-bore cylinder with the same stroke, Figure 17-5. Supplying the large bore cylinder with pressurized air or hydraulic fluid forces the hydraulic fluid out of the smaller bore. The upper cross-sectional view is typical of two cylinders assembled in the user’s plant from stock air and/or hydraulic cylinders. The lower cross-sectional view is a purchased assembly that takes less space and eliminates possible mounting and alignment problems. The purchased unit is limited to piston ratios that can have the same size rod in both cylinders.

Usually these intensifiers are hydraulic to hydraulic with ratios that are less than 5:1 ratio. Later we’ll see a similar design for air-to-air intensifiers with similar ratios. Never operate these types of intensifiers above the cylinders’ rated pressure. For all intensifier designs, output pressure is directly related to the area ratio between the driving piston and the driven piston (or ram).

The cross-sectional view in Figure 17-6 shows typical construction of two types of 25:1 air-oil intensifiers. They consist of 5-in. bore air cylinders with 1-in. rods displacing oil from high-pressure oil chambers. The upper cross-sectional view is a dual-head intensifier that requires some sort of blocking valve to isolate its inlet oil from its outlet oil. This is usually done with a pilot-operated check valve so flow can return when the actuator reverses.

The lower cross-sectional view is a triple-head intensifier that has an integral high-pressure seal to isolate inlet oil from high-pressure oil after the rod moves approximately 2 in. There is no need for external isolation because oil can flow freely either way anytime the ram is retracted.

A single-stroke intensifier must be sized to supply enough oil to make the working cylinder perform its work before the air piston bottoms out. It is good practice to size the intensifier for 10 to 15% more fluid than required. Avoid long fluid conductors if possible because the oil’s compressibility and stretching hose can use up the small-volume safety output quickly.

The circuit in Figure 17-7 shows a typical high-pressure air-oil circuit using the components described so far. This could be a press operation that requires a 10-in. total stroke. The stroke concludes with a 0.25-in. high-pressure stroke that generates 25 tons of force. Based on a maximum pressure of 2000 psi, a cylinder with a 6-in. bore is needed to produce the required force. The piston area of a 6-in.-bore cylinder is 28.274 in.2, so the 0.25-in. work stroke produces a volume of 7.07 in.3 of high-pressure oil. Using a standard 5-in. intensifier with a 1-in. ram, this requires

(7.07) (110%) / (0.7854) = 9.9-in. stroke plus 2 in. more for passing the high-pressure seal for a total stroke of 12 in. The volume of the 6-in. bore X 10-in. stroke high-pressure hydraulic cylinder is (28.275 in.2) X (10 in.) or 283 in.3, so the air-oil tanks should have 6-in. bores and be 12-in. long.

The cycle starts when the solenoid on the 4-way directional control valve is energized to send air to the left-hand air-oil tank. Simultaneously, the valve exhausts air from the right-hand air-oil tank. Oil at air pressure is pushed through the triple-head intensifier to the high-pressure hydraulic cylinder. The cylinder advances rapidly at low force until it contacts the work.

At work contact, pressure builds in the left-hand air-oil tank and in the pilot line to the 4-way sequence valve. With supply-air pressure at 80 psi and the sequence valve set for 65 to 75 psi, the valve shifts and cycles the intensifier. As the intensifier extends, after it travels approximately 2 in., it passes through the high-pressure seal to block low-pressure oil and force high-pressure oil into the cylinder. Pressure in the work cylinder can now go as high as 2000 psi to produce the required 25 tons of force.

When the solenoid on the 4-way directional control valve de-energizes, air exhausts from the left-hand air-oil tank and from the 4-way sequence-valve pilot. The sequence valve shifts to its original position and the triple-head intensifier retracts. Air also is directed to the right-hand air-oil tank to pressurize it for the retract stroke of the high-pressure hydraulic cylinder. After the intensifier retracts past the high-pressure seal, the work cylinder can retract quickly to end the cycle. Note: only 80 psi acting on the area of the work cylinder develops retraction force. While as much as 25 tons of force was generated during the short extension stroke, only 1869 lb are generated during retraction.

The intensifier could be cycled by other means -- such as a limit switch or a pressure switch and solenoid valve combination. It could even be operated manually.

Also note: any of the above units could be cycled with hydraulic oil as the driving force. Usually such hydraulic-to-hydraulic intensifiers are only between 2:1 and 5:1 because the input pressure can be much higher than typical compressed air.