PLATE (SEM) 5534352 - Caterpillar

(a) Compensating action for increase in load Referring to the compensating control characteristic curve, we shall assume a large increase instead of a small one, in order to illustrate the action in an exaggerated manner. Suppose the pump is operating at the Q1 -Pd1 point on the curve when the thing being pushed or pulled hydraulically offers a greater resistance than before. Consequently, Pd increases to Pd2. Keeping this change in mind, we have to shift our attention to the schematic Diagram A of the compensating controller.In Diagram A, note that discharge pressure Pd is always admitted into small-diameter chamber (Chamber A) while a certain pressure is trapped in large-diameter chamber (Chamber B). Because of the higher pressure (Pd2) applying to Chamber A and to compensating piston (667), this piston pushes spool (651) to the left to open the port (C liter) to bleed the Chamber B pressure to the drain (through the spool's internal oilway) while Pd2 now in Chamber A breaks the balance. Consequently, servo piston (532), with its tilting lever (531), moves rightward to tilt back the swash plate, making the pumping stroke (of the pistons) shorter. At the same time, this rightward movement of tilting lever (531) causes sliding sleeve (653) to move leftward, the direction for closing the port (C liter), through the feedback lever between lever and sleeve.Thus, a point will be soon reached at which the port (C liter) opened by spool (651) becomes separated from the internal drain port by sleeve (653) to shut off Chamber B, resulting in the state represented in Diagram B: servo piston (532) is now at its position with the swash plate having been moved to a new angular position. This state corresponds to the operating point Q2 -Pd2.We may generalize the above sequence of events as follows:"Over a certain range of discharge pressure, a rise in this pressure automatically results in a decrease in discharge flow." The slope of the characteristic curve signifies this relationship. It is important to note here that there is little or no change in power for the transition from Q1 -Pd1 to Q2 -Pd2.(b) Compensating action for decrease in load Now assume that the load being pushed or pulled hydraulically offers a less resistance than before when the pump is operating at the Q2-Pd2 point. This results in a drop in discharge pressure to Pd1.In the compensating controller of the regulator, the pressure in Chamber A decreases and compensating piston (667) moves back. Spool (651) moves similarly because it is urged by springs (655)(656); by so moving rightward, spool (651) opens the port (C liter), communicating Chamber B to the Pd line and thus admitting the pressure into this chamber. The two chambers are now at the same pressure, Pd2, but, because of the difference in effective area, servo piston (532) moves leftward while causing sleeve (653) to move rightward. See Diagram C.A point will be reached, as before, where the port (C liter) becomes closed while