Hydraulic pumps and rams are a tremendously flexible way of multiplying force: just think of a simple bottle jack, which lets anybody lift a car, or a hand-operated hydraulic press which can apply tens of tonnes of force.
Hydraulic motors can be seen as the rotating counterpart to the hydraulic ram: a way to generate enormous continuous torque without the complication of gearboxes.
For example, Parker’s latest V16-270 bent-axis motor weighs just 93kg, but can produce a maximum continuous power of 555kW.
They can also be designed to handle hostile conditions and shock loads, making them suitable for really punishing applications such as industrial shredders.
When specifying a hydraulic motor, it’s most important to decide what combination of torque and speed characteristics you need, and whether you anticipate shock loads in the system. Conversely, do you need a motor with a particularly smooth power delivery? And does it need extras such as air-operated control valves, or a built-in parking brake?
Hydraulic motors can be specified as explosion-proof for ATEX applications, and many can use biodegradable fluid, or one of the non-flammable fluids on the market.
Most hydraulic motors have some sort of rotating element (or rotor) turning inside a housing, with the pressurised oil passing between them and applying rotational force. The housing may be stationary (that is, a stator) or may itself rotate inside a casing.
Whatever the motor, the key measurement is displacement: how much fluid flow is required per revolution. For a given displacement, the speed of the motor is proportional to the flow rate of the hydraulic supply: double the displacement, and the speed of the motor will halve. The torque of the motor is proportional to displacement and to pressure (or rather, the pressure drop across the motor): double the displacement, and the torque will double; double the pressure drop, and the torque will double. For example, the Hägglunds Atom has a specific torque of 40Nm/bar.
In practice, these relationships are not quite so linear, and there are limits on rotational speed and pressure (the Atom will handle a peak pressure of 420bar).
Vane motors have a cylindrical rotor incorporating sprung radial blades (or vanes) which wipe against the inside of an eccentrically-machined housing. Fluid pressure pushes against the vanes, turning the motor (some fluid pressure is bled off to maintain the vanes’ force against the housing). The motor is extremely compact and its rotation is smooth, but the vanes require expensive machining and are not tolerant of fluid contamination.
A gear motor has two meshing spur gears in a closely-machined figure-of-8 housing; as the fluid passes from one side of the 8 to the other via the outside of the gears, it pushes the teeth around. This is a relatively low-torque, high-speed motor. The mechanism is simple and relatively inexpensive, but there are internal losses from oil bypassing the gears, so it needs substantial oil flow.
Gerotor and Geroler motors, sometimes known as orbital motors, have a ‘generated rotor’ with n lobes (where n is typically 6) rotating eccentrically within a stationary housing with n+1 lobes (that is, 7, or 6+1). The rotor is effectively a gear wheel travelling within a ring gear, and it drives an output shaft at 1/(n+1) of the rotational speed: this gearing effect means that these produce very high torque at relatively low speeds.
The Geroler motor (it’s an Eaton trademark, but other firms make gear-and-roller motors) uses free-spinning rollers to form the inner lobes of the stator, reducing friction and wear significantly. They are particularly recommended for use at lower speeds than Gerotor types.
OTHER MOTOR TYPES
There are different types of piston-operated hydraulic motors, but none uses a crankshaft like an internal combustion engine. The pistons act on a cam, and are actuated via control ports – either disc valves or spool valves – which may change the speed, direction or flow characteristics. One clear advantage of piston motors is variable displacement: by altering the stroke of the piston, or by shutting some pistons off, you can allow the motor to run at different levels of torque and/or speed.
Axial-piston motors come in different types too. First is the swashplate motor, with pistons spaced around (but parallel to) the axis of rotation: these press against a swashplate (the cam) angled at around 20º, pushing it round. Most are fixed-displacement models, but the displacement may be varied by altering the angle of the swashplate. Swashplate motors are compact and can operate at high speed but are potentially noisy.
The second type is the bent-axis motor. This has free-sliding pistons (actuated by pressurised oil fed via control ports) which are mounted at an angle to the drive plate. The plate itself is mounted square on the output shaft. Displacement is varied by altering the angle of the pistons relative to the output shaft. A choice of fixed, two-step variable or continuously-variable displacement is available. Some have a zero displacement mode, for an ‘idle’ or ‘freewheel’ function which reduces energy losses. Like swashplate motors, these work well at high speeds – sometimes more than 10,000rpm: they offer more flexibility in displacement (the piston angle may be as much as 40º), but are costlier. The thrust bearings on the drive shaft are critical components that must withstand considerable forces.
Radial piston motors offer considerable flexibility, as well as mechanical efficiency of up to 98%, and provide high torque at relatively low speeds. They typically have a rotating cylinder block with an even number of pistons, each with a roller on the end. As these are actuated, they press against the inside of a stationary cam ring; this pushes the cylinder block round towards the next lobe of the cam ring. With careful design the torque delivery can be very smooth, and the effective displacement can be reduced by shutting off the feed to a number of pistons: for example, a 12-piston motor could run on 4, 6 or 8 pistons, or freewheel.
The power can be multiplied by simply adding another block of pistons and another cam ring. Some motors use four or more banks of pistons.
BOX: LOOK AFTER YOUR FLUIDS
Whereas a gas or a vapour such as steam is compressible, hydraulic fluid is effectively incompressible. This means that has to go somewhere – This is why most hydraulic motors have three ports: an input, an output and a case drain port to allow for bypass leakage within the motor. Oil from the drain port flows back to the reservoir, and can be a useful diagnostic tool: if too much oil is draining, it suggests wear in the motor.
Hydraulic motors are generally reliable and have predictable maintenance needs. However, manufacturers point out that high casing pressures and high temperatures (for instance from fluid with too high a viscosity) will shorten the motor’s life. The most critical precaution is to ensure that there are no points where air can enter the hydraulic system; entrained gases effectively make the fluid compressible. This reduces efficiency and leads to cavitation, where rapid depressurisation causes sharp ‘explosions’ in the system, resulting in significant surface damage.