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Special air-oil intensifier cylinder
Some manufacturers build self-contained, air-driven, high-force hydraulic cylinders. These units look like a very long stroke air cylinder. Typically, they have 2 to 10 in. total strokes with 1.0- to 1.5-in. high-force strokes. They often replace a hydraulic unit on a machine that needs high tonnage for one operation on an otherwise air-powered circuit. Because these special intensifiers are self-contained, they only require an air supply and a signal to start them. They have sealed reservoirs so they operate in any position. They normally have an indicator to monitor oil volume for preventive maintenance. According to bore size and stroke length, cycle rates go as high as 150 per minute. The bigger the bore and longer the stroke, the fewer the cycles per minute.
As with other air-oil devices, return power is only cylinder net rod-end area multiplied by air pressure. The unit may have 50 tons to punch a hole but only 0.5 ton to retract the punch. For high retraction force use springs or urethane strippers, or add short-stroke return cylinders.
Figure 13-26 has a cutaway view of the intensifier cylinder at rest. (This view only shows function, not necessarily an actual assembly.) Air piston and rod C with attached hydraulic ram D move rapidly at low force to the work and return the tooling at the end of the cycle. Ram D is the area that intensified oil pushes on to get the short, high-force work stroke. Spring-loaded, floating piston A forms the top of a variable-volume, sealed oil tank. Spring-return air piston B drives its piston rod into trapped oil to intensify pressure for the work stroke. Directional control valve E cycles the advance and return strokes of cylinder C, and supplies air to pilot sequence valve F, starting the high-pressure work stroke.
When directional valve Eshifts, as in Figure 13-27, air piston-and-rod C extends the tooling to the work rapidly. As the piston-and-rod extend, ram D advances and fills with oil from the variable-volume tank. Vacuum forms in the chamber behind ram D, and the spring behind the floating piston forces oil into the void. Piston-and-rod C continues to advance and oil transfers until the work is met. This low-force advance stroke moves quickly (and uses air flow controls when necessary). Seals on ram D separate oil and air.
Figure 13-28 shows the intensifier after contacting the work. When air piston-and-rodC stop against the work, pressure build-up behind the piston shifts sequence valve F. When sequence valve F shifts, shop air extends spring-return air piston B. The first movement of the spring-loaded air piston advances the rod to the flow port connecting the tank to the chamber behind hydraulic ram D. As the rod enters this flow port, it passes through a resilient seal, stopping flow to tank and sealing the chamber behind ram D. This action automatically isolates the low-pressure chamber — eliminating the need for a pilot-operated check valve.
As the spring-return air piston continues to extend, as in Figure 13-29, the rod displaces oil in the chamber behind hydraulic ram D. In this case, the area of spring-return piston B is 15 times the area of the rod entering the sealed chamber. The air piston and rod continue to displace oil and move hydraulic ram D until the pressure behind the ram becomes 15 times greater than the air pressure on the piston. The stroke of spring-return piston B and the diameter of its rod set the maximum high-pressure work stroke. The higher the intensification ratio and the shorter the stroke, the less the high-pressure stroke capability.
Deenergizing directional valve E allows the spring loaded air piston and the work cylinder to return home. The work cylinder returns slowly while spring return air piston B retracts past the high-pressure seal.