Lumberjacks. Miners. Search and rescue crews. What do they have in common? Well, they’re obviously some of the most hardcore people in America, but other than that, they also all work some with extreme machinery in their daily jobs. But where does a man find enough power to lift a massive tree, move huge volumes of earth, or drag a car out of a canyon? The answer is that the machinery they use are almost universally driven with a hydraulic power unit — a motor that converts the motion of liquid into mechanical force.
A hydraulic motor, at its essence, is fairly simple: a reservoir of hydraulic fluid, a power unit, and a machine that can be moved by any form of rotation. The power unit — usually a small electric device — pressurizes the fluid. Because fluid cannot be compressed, any amount of pressure from the pump causes the fluid to move, usually through a series of valves designed to make sure that the fluid can’t move backwards and harm the pump.
At the far end of the line, the fluid moves into a piston, which extends as it fills up, and voila! — a log is lifted easily off of the earth, swung about, and dropped onto a waiting truck. Or, perhaps, the fluid moves through a propeller, which generates a rotational force that is then fed into a series of gears that slows the rotations-per-minute but adds a huge amount of torque, and the winch on the far side of those gears pulls a truck out of a lake. The number of potential applications is huge. After the fluid has done its job, it returns back to the reservoir along a different line, ready to be used again the next time the pump comes online.
Electric motors can only generate more power by being built bigger — so to get an electric motor to haul a ton of earth directly, you need one that is absolutely massive. But by using a hydraulic power unit — which is, ultimately, an electric motor, just used in conjunction with a hydrostatic system — the same job can be accomplished with a much smaller device.