With consistent loads and speeds, it’s a fairly straightforward process to select a pump displacement and to know what the required horsepower (HP) will be. You have performance requirements based on how much force or torque needs to be applied and at what speed. If the load and speed do not fluctuate, it is possible to size actuators, pumps, and prime movers (engines or electric motors) to deliver exactly the desired results with very basic components. Automatically variable displacement pumps with fancy control provisions may not be required.
But what if you need to squeeze every ounce (or kilowatt to be accurate) of work possible out of your prime mover? Let’s say you are operating a large hydraulic press, like the ones used in an industrial baler.
In order to generate massive force, you must apply pressure to a large area. The trade-off is that the larger the area that is being acted on, the more flow (volume) it takes to get an actuator to move a given distance. If you want a cylinder to generate massive force and move quickly, you will need a massive prime mover.
What if you are on a limited budget (or just like being efficient)?
Have you ever heard the phrase: “with a lever long enough, you could move the world?" Well, it is the truth. And it depends upon the same principles that could theoretically allow a household jack to move the world. What isn’t mentioned is the speed at which you would be moving the world.
Variable Flow Based On Available Power
The hydraulic ram has to extend and retract a relatively large distance during which relatively little power is required. If the pump is sized to achieve maximum extension speed during this time, then it will undoubtedly bog down the prime mover once it encounters any real resistance. However, if it is sized to tolerate adequate pressure during the resistance phase, then it will be painfully slow while it is getting there.
The solution to this problem is to vary the flow based on the available power, but not just based on pressure as in a pressure compensated or load sense system, but also based on how much power is available from the prime mover.
This can be achieved with the modern pump control type known as horsepower limiting. This is also commonly referred to as torque limiting. It essentially allows us to grossly oversize a pump and then automatically destroke it to precisely the right amount of flow and torque to match the given input horsepower.
During periods of light resistance, the pump will be wide open and provide massive flow. During periods of medium resistance, the pump will provide medium flow. During periods of massive resistance, the pump will provide minimal flow. All of these flow conditions will be subject to a “cap” of what input power is available.
This can be taken a step further with a technology by Kawasaki called “power shift.” Rock Able wrote in detail on this feature, below. It is able to constantly shift the HP target to match what the prime mover is producing in real time. If an engine is at idle and producing 12 HP, then the pump will receive a signal to only demand 12 HP. If the engine is at high RPM and producing 40 HP, then the pump will receive a signal to correct for 40 HP. If properly configured, it will be impossible to bog down your engine.
Don’t make the mistake of overtaxing the prime mover in your next design project. If a variable power source can just barely create enough power at a specific RPM, then you could end up bogging it down when conditions are anything but ideal. Take the proper precautions and contact Cross Company for design support on your next project!