By Josh Cosford, Contributing Editor
Sometimes, the selection or use of hydraulic pumps is not as linear and straightforward as selecting them from a catalog. You may already have an old or spare pump kicking around that was surplus from a long-dead machine, or perhaps you’re in need of something quickly, and your local farm supply store has a limited selection to choose from.
Occasionally, we must make do with what we have or at least attempt it, but you still must be aware of the limitations of the pumps available to you. You may have pulled a pump off an industrial power unit and wish to install it on a gasoline-powered small engine. Or, conversely, will that old tractor pump work on the trash compactor you’re making for the shop?
There is more at play here than just the mechanical capacity to speed up or slow down a pump, which are both limitations to many pump designs. You must also factor in the flow rate change that occurs when you speed up or slow down a pump. The flow rate is a product of the pump displacement (size) and the pump’s rotational speed. For any given displacement, speeding up the pump will increase flow and slowing the pump will decrease flow, which is common sense until you consider that some people mistakenly believe bigger pumps make higher pressure.
Before you use any pump at a speed other than the last previously known, be sure you’ve cross-checked its part number to decipher the pump displacement. For example, if you’re using a 5 in.³ displacement gear pump from an old power unit that had a 1,200 rpm electric motor, know that it will produce three times the flow on your 3,600 rpm gas engine.
Running some quick math for my above example, you will go from 26 to 78 gpm of theoretical flow. That’s a massive jump that needs to be factored into not only the capacity and requirements of your system but also the available power of your gas engine. Assuming 3,000 psi working pressure, you just went from 46 hydraulic horsepower to 138 instead. Even if your 1,200 rpm electric motor was rated for 50 hp, you also need to consider efficiency.
It’s likely a premium-efficiency electric motor making 50 hp could output the required power because of its high efficiency and safety factor. However, gas engine manufacturers either overstate horsepower or use outdated industry standards, leading to vastly underpowered engines that are either overrated or not rated at all. To avoid lawsuits, many engine manufacturers no longer provide power ratings at all. The point here is you’d likely need a “250 horsepower” gas engine to power your pump at 3,600 rpm.
It goes without saying that if your hydraulic pump was transplanted in the opposite direction, you could be vastly disappointed by the flow rate you experience when turned by a slower, 1,200 rpm electric motor. Worse still, there are applications where pumps are run from gearboxes, pullies or PTO drives. You may even experience a circumstance where pumps simply do not operate as desired when run under a minimum speed.
Vane pumps, for example, often have a minimum speed requirement. Their vanes take advantage of the centrifugal force pushing them against the pump casing. Even designs employing springs still require a minimum speed to work effectively. Be sure you’ve factored in how a reduced pump speed will affect the performance of your machine, as you may be vastly disappointed.
Finally, even if you’re keeping your current gas or electric motor and varying its speed, be sure that the pump is capable of the changes. Gas and diesel engines vary their speed easily through throttle input changes, and even electric motors can be used relatively cheaply these days using VFDs and other technology. Large pumps tend to have reduced maximum rpm, but confirm your pump is capable of an increase in speed.
Veteran fluid power professionals will find this article elementary and obvious, but the unfortunate reality in today’s fluid power industry is that untrained or poorly trained technicians and mechanics do their best to make do with what they have. Understanding how changes in pump speed will affect the performance of your hydraulic system is an important first step in hydraulic education.
