By Josh Cosford, Contributing Editor
Hydraulic power units are both the source of hydraulic energy and fluid conditioning. The hydraulic reservoir provides a mounting surface for the hydraulic pump and its prime mover while offering up real estate to mount filters, coolers, valves and accessories. In addition, the reservoir stores the hydraulic fluid, providing a surface to radiate heat while evacuating air.
Hydraulic power units range from those as large as shipping containers to some small as a bread box. The compact hydraulic power unit packs a lot of punch into a small package, including most of the critical components within reason. The motor, pump, valve head, breather cap and valves pack themselves into one convenient and economical package.
Most compact power unit manufacturers offer standard designs for common machineries, such as auto shop hoists, dump trailers, snow plow attachments, bin tippers, forklifts, lift tables, dock levellers, and power tailgates, just to name a few. What you’ll notice in common with these applications is their low-duty cycle applications, where their functions are used infrequently, slowly or both.
It’s common for these units to come with dc motors with 12 or 24 V and 120 or 220 Vac units with no more than a few horsepower. Their modular design allows a designer to select the pump size, electric motor, reservoir type and size, and end head that each suits their application. The end head is a combination component that provides the reservoir, pump and motor mounting point. The end also includes cavities and circuitry to install valves to create simple hydraulic circuits.
Most compact power unit manufacturers offer a D03 valve pad option atop the end. Just as with industrial stack valves, simple hydraulic circuits are easy to create or modify. In addition, modular mounting pads allow installing more valves in series or parallel, and installation is as simple as a few longer tie-rods to hold the valve pads together.
Most units come installed with relief valves, often just a ball and spring design. In addition, a check valve installed after the pump prevents loads from dropping when the motor shuts off. This basic configuration is typical to nearly every compact power unit design and would make for the most economical.
Reservoirs tend to be small, ranging from a quart up to five gallons or so. The size depends less on cooling and contamination settling and more on satisfying differential cylinder volume. You may have to choose between a vertical and horizontal style reservoir, each offering slightly different advantages. The vertical reservoir needs its breather cap to be close to the end head yet with a long suction tube to reach down to the bottom of the reservoir. Meanwhile, the horizontal style makes do with a short suction tube and flexibility with its breather cap location.
The pumps used in compact hydraulic power units are almost exclusively gear pumps, and they are close-coupled directly to the end head and use a small metal splined drive coupler or sometimes a tang shaft. Displacement options range from small to downright tiny. It’s not unheard of to see 0.25cc/rev of displacement or less. Because of limited horsepower on most power units, high-pressure systems must make do with a minimal flow rating.
Compact hydraulic power units may not be your best option for high duty cycle applications. Still, if you usually use traditional hydraulic power units, you’ll be surprised by the economic advantage these units offer. Even if you require a more complicated circuit, you can simply order one of these units with pump, motor and reservoir and instead plumb up your own control and conditioning circuit. For example, the pressure port may lead to your own custom valve manifold and then return through the high-quality inline filter of your choice – your options are endless.
Filed Under: Components Oil Coolers, Engineering Basics, Mobile Hydraulic Tips, Reservoirs