Today we will be building (virtually) a simple two-spring torque limiting control for a hydraulic pump. It's going to be fun, I promise! Even if you know torque limiting controls in and out - you may learn something new or maybe even surprising.
There's an interactive graphic at the end of this article - it has buttons and sliders, and I will be constantly referring to it in this article. It's pretty intuitive, especially if you know the theory behind the basic torque limiting control systems of hydraulic pumps, but I still would encourage you to go through this post before you "play" with it.
Why do we even need a torque liming control? I am sure that most of you know the answer, but I will say it anyway - a hydraulic pump with a torque limiter is needed when the prime mover is limited in power, and the pump can stall it if the displacement isn't reduced after a certain pressure is reached.
By the way - very often people call these controls "horsepower" controls. Technically speaking this is not "entirely" correct, because these controls don't take into account the prime mover's speed, but still - the name is quite common, and if the prime mover is running at constant rpm, it's actually OK.
Let us start by looking at a concrete example. A 100 cm³ pump running at 1000 rpm. Let us also introduce some limits - let's limit the maximum pressure to 300 bar, the minimum pressure to 20 bar, and the minimum displacement to 10 cm³, for simplicity. Also - let us forget about volumetric and mechanical efficiencies and assume that we are working with ideal friction and leakage-free unit.
Before doing anything else let us have a look at how an ideal control would behave. If you look at the interactive chart at the bottom of this page, you will see an ideal pressure/flow curve of our pump corresponding exactly to 5 kW. The black line represents the flow/pressure curve, and the red line shows the kW. As you can see, the red line is perfectly flat at all the displacements, which means that our ideal torque liming control will keep the torque of our prime mover perfectly stable.
You can use the slider below the graphic to adjust the power, and see how raising the torque demand moves the power curve up and down, and how the power consumption goes down when the pump reaches its maximum pressure. Don't press any other buttons yet - I am getting there!
OK, so now we know what we want our control to do, but how can we achieve something like that?
The first thing to think of would be to use a simple spring as feedback for our pressure sensing system. This sounds like a good idea, doesn't it? We look at our ideal 5 kW pressure/flow chart and see that we need the control to "start limiting the pressure to 30 bar at full displacement, and to 300 bar at 10 cm³, so, once again, in theory, if we connected a spring to our pump's servo-mechanism in a way that it would compress as the displacement decreases, and then used it to limit the pressure by reducing the displacement (like a pressure limiter in the LS line, for example), and then chose the "right" spring, we could "hit" the two points - the "30 bar at 100 cm³" and the "300 bar at 10 cm³", and thus we would be pretty close to the so-coveted constant power curve, would we not?
So, let's do just that then (virtually) - and this is the moment when I ask you to press the "Show" button on the "Speed Feedback Power Curve" header. Make sure that you have the Ideal Power Curve slider back at 5 kW, and that the second button says "Show Double Spring". Don't press this one yet, please, we want to evaluate a single spring feedback system first.
So, what do we see? Since a spring is a linear device, "fitting" it to the two pressure/flow points we mentioned earlier will indeed cause the pump to reduce the displacement as the pressure rises, but the power curve will be all but flat. In fact - it will have a nasty 15 kW "bump" in the middle. If we connected this pump to a 5 kW motor - it would definitely stall!
Of course, you can change the spring, or set it to a different threshold - but whatever you do (and I invite you to "play" with the sliders now to see if you can approximate the two power curves) you won't be able to squeeze out anything "usable". Even if we push the ideal curve up to 15 kW, you will see that the single spring system will be limiting the torque to 15 kW consumption, but then the pump will be too slow at the extremities of the pressure curve. See - visualizing parameters is so cool!
Clearly what we need is to "bend" our straight line- but how can we do that with springs? Springs are linear devices, right?
A simple (and ingenious) solution would be to introduce a second spring in our feedback mechanism and make sure that it is "loose" in the control and only kick in after a certain displacement point. I will be animating these mechanics in the next article, but for now, you can see how such a double spring system works if you press the button that says "Show Double Spring".
You can use the additional sliders to change the stiffness of the second spring and the displacement it "kicks in". And now you can see, that approximating the ideal curve is a lot easier.
There are three buttons that will put the curves to 5, 10, and 15 kW approximations, and you can see how much the power deviates from the ideal straight line, and how much the spring constants need to change for the curves to become acceptable (within 1 kW of the target).
I encourage you to try other power levels and see how close you can get the two-spring curve to the ideal curve. This is a great exercise because it shows you how challenging it is to design a torque limiting control, and how hard it may be to set different power levels without exchanging the feedback springs.
Try adjusting the power setting without touching the spring constants. Does the power curve still look nice? Do you think the motor will stall?
More interactive stuff in part two.