One of the strengths of hydraulics is in its unsurpassed versatility to create and control motion. You can do many things accurately and with tight control, but the plethora of available components and manufacturers can be dizzying, especially when so many companies make many versions of the same thing. I’d say pumps are the most confusing product for the average fluid power learner to get their heads around, but hydraulic motors are a close second.

I’m going to discuss the latter, today, for no other reason than I feel like it. Motors come in various shapes and sizes, and all most have in common with each other is they have shafts and can be described as “round-ish.” Some hydraulic motors are downright tiny – by hydraulic standards anyway – and can fit inside a coffee cup, while other motors are the size of a rhino and weigh more than 20,000 lb.

A hydraulic motor uses hydraulic pressure pushing against a surface area, rotating either radially or axially about its centre. The force sweeps through radii, creating torque, and both the surface area pushed against and the moment arm relative to the radius create a swept volume, usually expressed in cubic inches- or cubic centimetres per revolution.

In terms containing less jargon, hydraulic pressure pushes on gears, vanes or pistons to create rotational torque around a spinning shaft. The amount of torque the motor can produce is a combination of its size (piston, vane or gear size, and how far out they are from the middle of the spinning shaft) and how much pressure you throw at it.

One of the seven wonders of the mechanized world is the power density of hydraulic actuators. Sure, the control system for a cylinder or motor can fill a three-car garage and produce enough heat to melt the polar ice caps, but hydraulic actuators cannot be touched in their ability to produce a lot of force from a small package. Electric actuators are limited on a quantum level when every molecule is already magnetically polarized and cannot create any more force, and even NASA’s experimental super-conductor motors can achieve only 3.6 hp/lb.

However, hydraulic actuators are subject to a much, much slower progressing version of Moore’s Law (where computer chip power doubles every eighteen months), in that average system pressure increases at a steady rate over the years, and appears to have no factors capping the rise. I’ve spoken in the past about Bosch Rexroth’s A2F5 bent-axis piston motor, which puts out 34 hp in a package weighing just 5.5 lb. And other motors in this line are capable of over 10 hp/lb. Awesome!

There are about six relatively common hydraulic motor configurations making up 90% of the applications in use, although countless versions of each are available. I’ll start with the simple ones, and ramp up the complexity.

The simplest motor design is the gear-style. It is essentially the exact thing as a gear pump, but with a high-pressure shaft seal. Meshed, counter-rotating spur gears (one idler, one driver) turn pressure into movement. Most gear pumps can be converted to gear motors by swapping in that high pressure shaft seal, although they are often unidirectional without some sort of case drain.

The most common motor — especially on mobile equipment — is the low-speed, high-torque motor, or just LSHT motor. These motors come in various levels of refinement, some of which are “disposable” quality, while others are large, relatively efficient units. These motors are inside gear style units with a rotating, off-centre rotor and stator.

Gerotor-style LSHT motors are inefficient little gremlins, especially outside of a limited pressure and flow sweet-spot.  They don’t spin very fast, probably related to high friction and internal leakage. They are quite inexpensive, however, so they are common. Improving on this design is the “geroller” motor with rollers in the stator to improve efficiency, although they still like to operate in a narrow window of efficiency. Upgrading further, is the disc valve motor, which also uses a roller stator. Instead of the spool valve on most LSHT motors, these use a flat disc valve, and can actually be fairly efficient, if properly chosen. LSHT motor manufacturers provide excellent data to help choose a motor which will operate within its efficiency window.

Vane motors are somewhat rare, although I’m not sure why because they are more efficient than the first two I’ve discussed. They do have their limitations, however. A vane motor has a slotted, rotating hub with rectangular vanes inserted around the circumference. As you would suppose, pressure acts upon the vanes, creating torque. On top of being respectably efficient, they’re generally reliable and easy to repair, however, they are often limited to medium pressure and speed applications, depending on the model. They’re not terribly inexpensive, either.

There are a few types of piston motors, but I will discuss only the radial piston motor and bent-axis piston motor. A radial piston motor is best described as a hydraulic version of an old-school airplane engine, with pistons reciprocating in/out from the centre of an offset shaft. Radial piston motors are extremely efficient, some as high as 95% efficient, and are available in a wide array of sizes. Any application requiring epic applications of torque would benefit from a radial piston motor; some can output millions of pound-feet of torque. They operate at high pressure, and some are more efficient as pressure rises, to a point.

The bent-axis piston motor uses an axially arranged rotating piston assembly, although the pistons act on an angle to the output shaft; it looks as though someone took a cylindrical motor and bent it in the middle. This motor configuration allows for high pressure, high speed and unusually high radial shaft loading, even without the use of an overhung load adapter to absorb the forces. They are also available in a wide range of sizes, form 5 cc/rev up to 1000 cc/rev. If you need high torque and high speed, these motors are the best option. The Rexroth A2F5 motor I previously mentioned is capable of 10,000 rpm, and even their 1000 cc version can spin to 1600 rpm. Excuse me while I pick pieces of my brain from the floor, as it is blown! Yes, I'm a nerd; haven't you figured that out yet?

There are enough variables in motor selection to choose any one of these styles for a particular application, even if cost was not an option. I would generally prefer a piston motor in most applications, because of efficiency alone … unless I was paying for it from my own pocket, at which point a low-speed, high-torque motor will do fine!