Written by Camilo H Rueda Wednesday, 14 December 2011 20:26
We will focus on the piston type that are the most common of the two.
Variable displacement axial piston pumps are perhaps the most used pumps in industrial and mobile equipment. What makes them so versatile and broad spectrum of use is precisely the way that they handle the change in the pump displacement. Of course many of them have manual control but in now days the automatic ones are much more common and these are what we are going to start talking about now.
In an axial piston pump the displacement changes decreasing the angle of the swash plate where the heads of the pistons slide. The servo piston is responsible for this and the spring opposes the movement from it to keep the maximum flow, implying that if we want to reduce the displacement of the pump we must put pressure into the servo piston to compress the spring and then to decrease the angle.
The Compensator is basically a control pressure valve that opens the pass from the outlet of the pump toward the servo piston. This function is done in the same way as a simple relief valve does; causing the pressure to push the relief valve spool against the spring where the pressure regulation is. It is valid to note that the required pressure in the servo piston to overcome the spring plate is a low value compared with the maximum pump pressure but it is very common to see values of around 200 psi.
If for instance we adjust the 'special' relief valve to open at 2000 psi, what will happen is that when the pump is working and the necessary pressure does not exceed 2000 psi, we will have a maximum output flow as we can see in the graphic above. Once the system pressure reaches 2000 psi, the compensator opens the way for the pressure (2000 psi) to the servo piston pushing it completely until the angle is 0 or whether the plate is perpendicular to the axis of rotation. At that time, the flow rate that the pump delivers goes from maximum to 0 in a split second and is going to keep it at that point as long as the load requires 2000 psi of pressure out of the outlet port as we can see in the graphic below. If the condition of pressure decreases for some reason, for instance the load was reduced, and then the compensator cuts the connection between the servo piston and the outlet pressure so the spring pushes back the plate to its maximum angle of inclination as it can be seen in the last graphic.
In a practical manner, we can say that this simple compensator controls the maximum pressure of a hydraulic system reducing the flow to 0 when it reaches the Compensator setting. Which is equivalent to the role of the relief valve on a hydraulic system that uses a fixed flow pump with the difference that with the Compensator no power is wasted, nor is generated heat to the system, in short this is a very efficient way to control the maximum system pressure.
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