is no provision for retracting the ram by fluidpower, when fluid pressure is released, either theweight of the object or some mechanical means,such as a spring, forces the ram back into thecylinder. This forces the fluid back to thereservoir.The single-acting ram-type actuating cylinderis often used in the hydraulic jack. The elevatorsused to move aircraft to and from the flight deckand hangar deck on aircraft carriers also usecylinders of this type. In these elevators, thecylinders are installed horizontally and operate theelevator through a series of cables and sheaves.Fluid pressure forces the ram outward and liftsthe elevator. When fluid pressure is released fromthe ram, the weight of the elevator forces the ramback into the cylinder. This, in turn, forces thefluid back into the reservoir.Double-Acting RamA double-acting ram-type cylinder is illustratedin figure 10-2. In this cylinder, both strokes ofthe ram are produced by pressurized fluid. Thereare two fluid ports, one at or near each end ofthe cylinder. Fluid under pressure is directed tothe closed end of the cylinder to extend the ramand apply force. To retract the ram and reducethe force, fluid is directed to the opposite end ofthe cylinder.A four-way directional control valve isnormally used to control the double-acting ram.When the valve is positioned to extend the ram,pressurized fluid enters port A (fig. 10-2), acts onFigure 10-2.—Double-acting ram-type actuating cylinder.the bottom surface of the ram, and forces the ramoutward. Fluid above the ram lip is free to flowout of port B, through the control valve, and tothe return line in hydraulic systems or to theatmosphere in pneumatic systems.Normally, the pressure of the fluid is the samefor either stroke of the ram. Recall from chapter2 that force is equal to pressure times area(F= PA). Notice the difference of the areas uponwhich the pressure acts in figure 10-2. Thepressure acts against the large surface area on thebottom of the ram during the extension stroke,during which time the ram applies force. Sincethe ram does not require a large force during theretraction stroke, pressure acting on the small areaon the top surface of the ram lip provides thenecessary force to retract the ram.Telescoping RamsFigure 10-3 shows a telescoping ram-typeactuating cylinder. A series of rams is nested inthe telescoping assembly. With the exception ofthe smallest ram, each ram is hollow and servesas the cylinder housing for the next smaller ram.The ram assembly is contained in the maincylinder assembly, which also provides the fluidports. Although the assembly requires a smallspace with all the rams retracted, the telescopingaction of the assembly provides a relatively longstroke when the rams are extended.An excellent example of the application of thistype of cylinder is in the dump truck. It is usedto lift the forward end of the truck bed and dumpthe load. During the lifting operation, the greatestforce is required for the initial lifting of the load.Figure 10-3.—Telescoping ram-type actuating cylinder.10-2
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business