The poppets are actuated by cams on acamshaft (fig. 6-30). The camshaft is controlledby the movement of the handle. The valve maybe operated by manually moving the handle, or,in some cases, the handle may be connected bymechanical linkage to a control handle which islocated in a convenient place for the operatorsome distance from the valve.The camshaft may be rotated to any oneof three positions (neutral and two workingpositions). In the neutral position the camshaftlobes are not contacting any of the poppets. Thisassures that the poppet springs will hold all fourpoppets firmly seated. With all poppets seated,there is no fluid flow through the valve. This alsoblocks the two cylinder ports; so when the valveis in neutral, the fluid in the actuating unit istrapped. Relief valves are installed in bothworking lines to prevent overpressurization causedby thermal expansion.NOTE: In some versions of this type of valve,the cam lobes are designed so that the tworeturn/exhaust poppets are open when the valveis in the neutral position. This compensates forthermal expansion, because both working lines areopen to the return/exhaust when the valve is inthe neutral position.The poppets are arranged so that rotation ofthe camshaft will open the proper combinationof poppets to direct the flow of fluid through thedesired working line to an actuating unit. At thesame time, fluid will be directed from theactuating unit through the opposite working line,through the valve, and back to the reservoir(hydraulic) or exhausted to the atmosphere(pneumatic).To stop rotation of the camshaft at an exactposition, a stop pin is secured to the body andextends through a cutout section of the camshaftflange. This stop pin prevents overtravel byensuring that the camshaft stops rotating at thepoint where the cam lobes have moved thepoppets the greatest distance from their seats andwhere any further rotation would allow thepoppets to start returning to their seats.O-rings are spaced at intervals along the lengthof the shaft to prevent external leakage aroundthe ends of the shaft and internal leakage fromone of the valve chambers to another. Thecamshaft has two lobes, or raised portions. Theshape of these lobes is such that when the shaftis placed in the neutral position the lobes will notcontact any of the poppets.When the handle is moved in either directionfrom neutral, the camshaft is rotated. This rotatesFigure 6-30.—Cutaway view of poppet-type, four-way directional control valve.6-21
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
