As the load is lifted and begins to dump, therequired force becomes less and less until the loadis completely dumped. During the raise cycle,pressurized fluid enters the cylinder through portA (fig. 10-3) and acts on the bottom surface ofall three rams. Ram 1 has a larger surface areaand, therefore, provides the greater force for theinitial load, As ram 1 reaches the end of its strokeand the required force is decreased, ram 2 moves,providing the smaller force needed to continueraising the load. When ram 2 completes its stroke,a still smaller force is required. Ram 3 then movesoutward to finish raising and dumping the load.Some telescoping ram-type cylinders are of thesingle-acting type. Like the single-acting ramdiscussed previously, these telescoping ram-typecylinders are retracted by gravity or mechanicalforce. Some hydraulic jacks are equipped withtelescoping rams. Such jacks are used to liftvehicles with low clearances to the required height.Other types of telescoping cylinders, like theone illustrated in figure 10-3, are of the double-acting type. In this type, fluid pressure is used forboth the extension and retraction strokes. A four-way directional control valve is commonly usedto control the operation of the double-acting type.Note the small passages in the walls of rams 1 and2. They provide a path for fluid to flow to andfrom the chambers above the lips of rams 2 and3. During the extension stroke, return fluid flowsthrough these passages and out of the cylinderthrough port B. It then flows through thedirectional control valve to the return line orreservoir.To retract the rams, fluid under pressure isdirected into the cylinder through port B and actsagainst the top surface areas of all three ram lips.This forces the rams to the retracted position. Thedisplaced fluid from the opposite side of the ramsflows out of the cylinder through port A, throughthe directional control valve to the return line orreservoir.Dual RamsA dual ram assembly consists of a single ramwith a cylinder at either end (fig. 10-4). Fluid canbe directed to either cylinder, forcing the ram tomove in the opposite direction. The ram isconnected through mechanical linkage to the unitto be operated. A four-way directional controlvalve is commonly used to operate the dual ram.When the control valve is positioned to direct fluidunder pressure to one of the cylinders (let’s saythe left one), the ram is forced to the right. ThisFigure 10-4.-Dual ram actuating assembly.action displaces the fluid in the opposite cylinder.The displaced fluid flows back through thedirectional control valve to the return line orreservoir in hydraulic systems or to theatmosphere in pneumatic systems.Dual ram actuating assemblies are used insteering systems of most ships. In some systems,one assembly is used to actuate the rudder in eitherdirection; while in other systems, two assembliesare used for the same purpose.PISTON-TYPE CYLINDERSAn actuating cylinder in which the cross-sectional area of the piston is less than one-halfthe cross-sectional area of the movable elementis referred to as a piston-type cylinder. This typeof cylinder is normally used for applications thatrequire both push and pull functions. The piston-type cylinder is the most common type used influid power systems.The essential parts of a piston-type cylinderare a cylindrical barrel, a piston and rod, end caps,and suitable seals. The end caps are attached tothe ends of the barrel. These end caps usuallycontain the fluid ports. The end cap on the rodend contains a hole for the piston rod to passthrough. Suitable seals are used between the holeand the piston rod to keep fluid from leaking outand to keep dirt and other contaminants fromentering the barrel. The opposite end cap of mostcylinders is provided with a fitting for securingthe actuating cylinder to some structure. This endcap is referred to as the anchor end cap.The piston rod may extend through either orboth ends of the cylinder. The extended end ofthe rod is normally threaded so that some typeof mechanical connector, such as an eyebolt ora clevis, and a locknut can be attached. Thisthreaded connection of the rod and mechanicalconnector provides for adjustment between therod and the unit to be actuated. After the correct10-3
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