DECKMASTERTHEORY OF OPERATIONThe HDC uses a unique double spool arrangement thatserves to separate the null deadband from the feedback,giving both safety against null drift and quick dynamicresponse to command changes.The command signal for the MCV106A is usually aremote hydraulic Control handle, as shown in Figure 2,although other pressure sources could be used. Thehandle modulates supply pressure from the pump (oranother source) so as to apply a proportional differentialpressure across the C1 and C2 input ports.The HDC's null adjust is set with the modulating springcompressed to the equivalent of 12, 120 or 217 psi,depending on the output characteristics of the commandsource, representing the differential pressure required tomove the actuator spool one direction or the other. Thisis a factory setting that defines the width of the actuatorspool deadband and cannot be changed .in the field. Bytightening or loosening the null adjust screw, the fixeddeadband is moved toward or away from the "A" controlport in order to null the valve.As differential control pressure (C1-C2) rises beyond thedeadband, the actuator spool moves in one direction orthe other, pivoting the crosslink about its center. Thepivoting crosslink pushes or pulls the porting spool in theopposite direction of the actuator spool. When theporting spool has moved far enough to open A or B tosupply pressure, oil is ported to the pump servocylindersto move the swashplate. As the swashplate moves, thefeedback link follows, pivoting the crosslink about thestationary end of the actuator spool, driving the portingspool back to its neutral position. Because the feedbacksignal is entered into the control loop after the commandhas been input, response time and accuracy areenhanced.INSTALLATIONA highly reliable connection between the swashplate andthe drag link is necessary for safe operation. Anunreliable connection may result in loss of feedback witha resulting loss of control. Series 3X pumps meet thisrequirement, but all Series 2X units not alreadyequipped with an HDC or EDC must be retrofitted withthe appropriate drag link, press fit pin and retaining ring,replacing the slip fit headless pin and E-rings used toattach the original drag link.Series 3X pumps with serial numbers of 82-34-00000 orgreater will accept the HDC without modification of thepump. These units have a clearance notch cast into theswashplate that provides additional room for linkmovement. Series 3X pumps with serial numbers ofless than 82-33-99999 may not be fitted with the HDCwithout modification of the swashplate by Sundstrand.Prior to mounting any control on a pump, ensure thatboth the control and the feedback link are correct for thepump as evidenced by the series number stamped onthe link and the part number labeled on the control body.See Table A, below.TABLE ATable A correlates the pump series number with theseries number stamped on the side of the feedbacklink protruding from the control.SERIESLINK MARKING2020 No Spacer21, 2221, 222323242425252626272733, 34, 3633, 34, 36WARNINGThe control and feedback link must have the proper Identification In order to be installed.Installing control with an improper feedback link can result in a control failure which can causethe pump plate to move to full angle and remain there independent of signal inputDo not attempt to Install an HDC on a pump for which It was not originally designed withoutusing the full link. assembly. Merely changing the drag link It inadequate. See Ordering;Information for Iinkage assembly numbers.Installing a control not designed for use with the Pressure Limiter on 33, 34 and 36 Series pumpswill prevent the Pressure Limiter from operating
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