flow from the directional control valve. Thisactuator (fig. 10-6) is referred to as an unbalancedactuating cylinder because there is a difference inthe effective working areas on the two sides ofthe piston. Therefore, this type of cylinder isnormally installed so that the blank side of thepiston carries the greater load; that is, the cylindercarries the greater load during the piston rodextension stroke.A four-way directional control valve isnormally used to control the operation of this typeof cylinder. The valve can be positioned to directfluid under pressure to either end of the cylinderand allow the displaced fluid to flow from theopposite end of the cylinder through the controlvalve to the return line in hydraulic systems orto the atmosphere in pneumatic systems.There are applications where it is necessary tomove two mechanisms at the same time. In thiscase, double-acting piston-type actuating cylindersof different designs are required. See figures 10-7and 10-8.Figure 10-7 shows a three-port, double-actingpiston-type actuating cylinder. This actuatorcontains two pistons and piston rod assemblies.Fluid is directed through port A by a four-waydirectional control valve and moves the pistonsoutward, thus moving the mechanisms attachedto the pistons’ rods. The fluid on the rod side ofeach piston is forced out of the cylinder throughports B and C, which are connected by a commonline to the directional control valve. The displacedfluid then flows through the control valve to thereturn line or to the atmosphere.When fluid under pressure is directed into thecylinder through ports B and C, the two pistonsmove inward, also moving the mechanismsattached to them. Fluid between the two pistonsis free to flow from the cylinder through port Aand through the control valve to the return lineor to the atmosphere.The actuating cylinder shown in figure 10-8is a double-acting balanced type. The piston rodextends through the piston and out through bothends of the cylinder. One or both ends of theFigure 10-7.—Three-port, double-acting actuating cylinder.Figure 10-8.-Balanced, double-acting piston-type actuatingcylinder.piston rod may be attached to a mechanism tobe operated. In either case, the cylinder providesequal areas on each side of the piston. Therefore,the same amount of fluid and force is used tomove the piston a certain distance in eitherdirection.Tandem CylindersA tandem actuating cylinder consists of twoor more cylinders arranged one behind the otherbut designed as a single unit (fig. 10-9). This typeof actuating cylinder is used in applications thatrequire two or more independent systems; forexample, power-operated flight control systemsin naval aircraft.The flow of fluid to and from the twochambers of the tandem actuating cylinder isprovided from two independent hydraulic systemsand is controlled by two sliding spool directionalcontrol valves. In some applications, the controlvalves and the actuating cylinder are two separateunits. In some units, the pistons (lands) of the twosliding spools are machined on one common shaft.In other applications, the valves and the actuatorare directly connected in one compact unit.Although the two control valves are hydraulicallyindependent, they are interconnected mechanically.In other units, the two sliding spools are connectedthrough mechanical linkages with a synchronizingrod. In either case, the movement of the twosliding spools is synchronized, thus equalizing theFigure 10-9.—Tandem actuating cylinder.10-5
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