Insane Hydraulics

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Tackling Torque Limiting Pump Controls - Part Three

Below lies the finished version of my two-spring hydraulic pump torque limiter control simulator. The most perfect one, because it reproduces the behavior of the common adjustment system, which consists of two coaxial springs, placed one inside another, and two adjusting pieces - the external and internal one, which allow you to set the halves of the control curve. The peculiarity of this arrangement lies in the fact that when you adjust the external screw - you are shifting both the threshold pressure of the larger spring (the first half of the pressure/flow curve) and the starting point of the small spring (the second half of the curve). It is very interesting to see how changing these adjustments plots out on the pressure/flow chart.

A few words about the numbers and the settings.

This is an ideal (zero leakage and friction) 100 cm³ unit that is rotating at 1000 rpm and is limited to 300 bar. Obviously, the mechanism that is depicted in the diagram is not real - it's designed to demonstrate the two spring feedback principle. The graphs "stop" at 10 l/min.

The numbers depicted on the External (large spring) and Internal (small spring) Adjustment sliders correspond to the travel of the adjusting screw. 100 corresponds exactly to the 100% of travel of the displacement feedback mechanism. The sum of these two settings is limited to 110. That's because if they went further - we would see the springs compress completely. You can try this by ramping the two settings up to the maximum and then seeing what happens to the springs when you change the displacement from 100 to 10.

The numbers representing the spring constants correspond to the pressure (in bar) that they increment per one percent of displacement shift (in the case of 1000 rpm, one could say that they correspond to bar per liter per minute). Once again - this is a purely theoretical system, and I used these values for the sake of simplicity.

The Spring1 Preset represents the initial compression of the large spring. This means that even when the external adjusting screw is out all the way the spring is still compressed.

The three buttons with power values - 9 kW, 15kW, and 21 kW reset the graphic to default values of spring constants and the spring one preset. This is just to show that with given values of the spring constants and the spring1 preset the adjustment range of such a control is somewhat limited. With these values (and the mechanical configuration of the control) you could use this pump with a 9 - 20 kW prime mover, but going outside of this range would be impossible without exchanging the springs.

And finally - the Override Pressure (the lowest slider). It showcases the function often called power shift, power override, or torque summation. Very often such controls are equipped with another spool or piston, that can create an additional force on the primary pressure spool. Depending on its size, it can be piloted with high pressure (and be used for torque summation) or low pressure (power shift). This additional force effectively slides the pressure/flow curve along the pressure axis. But it is interesting to note that the power curve is not only going up and down, but it is also kind of rotating its lower range half. As you can see - the ideal torque summation control or torque setting override is hard to achieve with pure mechanical means.