Today I am finishing the pump horsepower control series with two more ways to achieve a perfect (hyperbolic) pressure/flow curve with the help of Mrs. Parker and her famous PV series, which I believe to be one of the most robust open-loop pumps on this planet. Once again - no affiliations here, I like these pumps for the design and engineering only, and as much as I would love to talk about my “colourful dealership experiences”, I will keep my mouth shut.
So, the double-spring feedback torque-limiting systems are OK-ish, the lever-based feedback systems offer perfect pressure/torque curves with infinite adjustment but also introduce complexity and therefore additional points of failure, but Parker engineers came up with another way to obtain the so coveted perfect hyperbolic function of the torque limiting control - the PV series contour sleeve:
Such an elegant solution - a hardened sleeve, carefully machined to a proper profile, is mounted on the servo-cylinder and is progressively compressing the LS relief spring as the displacement changes.
I believe that in a way this is the best solution because you have the ability to "make" the P/Q curve taking into account the mechanical and volumetric inefficiencies of the unit. That is - if you are meticulous enough to perform the countless tests before approving a contour, but I believe that Parker engineers know what they are doing, and so they did just that.
Of course, if you need to upgrade the prime mover, you won't be able to simply adjust the pump control to a higher setting, because even though the relief cartridge does have an adjusting screw, the only thing it does is slide the P/Q curve along the P axis in a very limited range, and we already know how quickly the torque (power) curve becomes "ugly" when you do that.
I also think that the fact that there is a sliding contact point means this system should have noticeable hysteresis, but then again - what doesn't? I like this solution for its simplicity.
But there's yet another way. A more modern one and the most perfect of them all, because it supposedly gives you unlimited control over all of the pump parameters, including the unlimited torque limitation. I am talking, of course, about the digital controls. Like this one:
This is a PV180 with the UDM proportional displacement control with closed-loop pressure control. As you can see - it has it all - a pressure sensor, a swash-plate position sensor, a proportional DCV plus a proportional relief valve! Bulky and expensive? Yes. Needs a costly control module and careful wiring? Yes. But when you (finally) get it to work - it does it great. I've used these modules on several occasions and their function is perfect. So my general feedback is positive, if I close my eyes on "insignificant stuff" - like needing a cable for an older module, and the dealer saying that unfortunately they are not available anymore, and your best bet is acquiring a new (and expensive) module that uses a common USB cable. Or the dealer saying that you must use an expensive sequence valve (conveniently supplied by them) on the outlet of this pump for it to work properly, "forgetting" that there's a second and very cheap option for cases when a pilot pressure source is available... Yeah... I've seen it all. But I digress, I said I'd keep my mouth shut and I will!
Anyhow - my point is - going digital is the most perfect way. At least from the engineering standpoint. I used to fear these solutions in the past, but the few units that I did apply have been "reliable enough" (at least so far) to justify their existence. And, of course, these controls do dish out perfect P/Q curves as well.