For higher volumes of intensified fluid, several manufacturers make reciprocating units. The cross-sectional view and circuit in Figure 17-8 show a typical single-ram intensifier that uses compressed air for power and pumps oil in the high-pressure side. These units often are supplied in a ready-to-run condition as pictured. They may cycle as soon as air is supplied or they may require an external signal to start. Most reciprocating units supply less that 3-gpm maximum at low pressure and slow to a stop at maximum pressure.
To produce higher pressures, some units incorporate more than one air cylinder in series to raise the intensification ratio. These units also come with pressure chambers and rams on both ends to provide a greater volume of high-pressure oil.
Some manufacturers build reciprocating hydraulic-to-hydraulic intensifiers with ratios as high as 20:1 to generate pressures up to 12,000 psi. These units supply small volumes of high-pressure oil from low- to high-pressure input fluids.
Special air-oil units
Several companies manufacture special self-contained air/hydraulic cylinders with integral tanks and intensifiers that produce low-pressure advance, high-pressure work, and low-pressure retract strokes. Externally, they appear to be over-length air cylinders, but they can have output forces as high as 150 tons.
When an application requires a small volume of high-pressure air, try an air-to-air intensifier instead of a high-pressure compressor. The cross-sectional view and circuit in Figure 17-9 shows the makeup of a 2:1 intensifier that can almost double output pressure. Inlet air is delivered to the driving cylinder by a double pilot-operated valve and to the intensifying cylinder through check valves. As the two pistons stroke to the right, the full area of the left piston and the annulus area of the right piston are pushing the right piston’s full area at almost double force. Thus, air exiting the right piston is at about twice input pressure. The discharged air flows through a check valve and on to the high-pressure circuit.
When the pistons complete their strokes, the one on the right contacts a small integral limit valve that sends a signal to the double pilot-operated valve and shifts it to reverse the pistons’ strokes. The same areas and forces push in this direction but they work against a smaller intensifying area. The intensifier will continue cycling until pressure at the pressure-air outlet port reaches full pressure. At that point, the pistons stall and hold pressure until the downstream pressure drops.
These intensifiers will stroke considerably more slowly at about 80% of their maximum pressure so it is best if the output air pressure is at least 20% above what is required. A regulator at the working machine can control the actual working pressure so less air is wasted.
Intensification ratios and output volumes are functions of piston ratios, bore sizes, and stroke lengths. Outputs up to 250 psi are standard with most manufacturers. Some offer higher pressures. Very high-pressure units use hydraulic cylinders to drive gas cylinders to reach pressures as high as 45,000 psi.
(For more air-oil and intensifier circuit designs, see the author’s upcoming e-book, "Fluid Power Circuits Explained.")