4 Types of Hydraulic Motors
The hydraulic motor is a device that converts the pressure energy of a liquid into mechanical energy and is generally used in modern hydraulic machinery using hydraulic fluid as part of a closed hydraulic circuit. According to its structure type, the hydraulic motor can be mainly divided into 4 types - vane motor, hydraulic gear motor, axial plunger motor, and radial piston motor.
Vane motors
As one type of hydraulic motor, the vane motor consists of a housing with an eccentric bore, which runs a rotor with vanes in it that slide in and out. The force differential created by the unbalanced force of the pressurized fluid on the vanes causes the rotor to spin in one direction. A critical element in vane motor design is how the vane tips are machined at the contact point between the vane tip and motor housing. Several types of "lip" designs are used, and the main objective is to provide a tight seal between the inside of the motor housing and the vane, and at the same time to minimize wear and metal-to-metal contact.
Hydraulic gear motors
The hydraulic gear motor (external gear) consists of two gears, the driven gear (attached to the output shaft by way of a key, etc.) and the idler gear. High-pressure oil is ported into one side of the gears, where it flows around the periphery of the hydraulic gears, between the gear tips and the wall housings in which it resides, to the outlet port. The gears then mesh, not allowing the oil from the outlet side to flow back to the inlet side. For lubrication, the gear motor uses a small amount of oil from the pressurized side of the gears, bleeds this through the (typically) hydrodynamic bearings, and vents the same oil either to the low-pressure side of the gears or through a dedicated drain port on the motor housing, which is usually connected to a line that vents the motor's case pressure to the system's reservoir. An especially positive attribute of the gear motor is that catastrophic breakdown is less common than in most other types of hydraulic motors. This is because the gears gradually wear down the housing and/or main bushings, reducing the volumetric efficiency of the motor gradually until it is all but useless. This often happens long before wear causes the unit to seize or break down.
Axial plunger motors
For high-quality rotating drive systems, plunger motors are generally used. Whereas the speed of hydraulic pumps ranges from 1200 to 1800 rpm, the machinery to be driven by the motor often requires a much lower speed. This means that when an axial plunger motor (swept volume maximum 2 litres) is used, a gearbox is usually needed. For a continuously adjustable swept volume, axial piston motors are used.
Like piston-type pumps, the most common design of the piston type of motor is axial. This type of motor is the most commonly used in hydraulic systems. These motors are, like their pump counterparts, available in both variable and fixed displacement designs. Typical usable (within acceptable efficiency) rotational speeds range from below 50 rpm to above 14000 rpm. Efficiencies and minimum/maximum rotational speeds are highly dependent on the design of the rotating group, and many different types are in use.
Radial piston motors
Radial piston motors are available in two basic types: pistons pushing inward, and pistons pushing outward.
Pistons pushing inward
The crankshaft type with a single cam and the pistons pushing inwards is basically an old design but is one that has extremely high starting torque characteristics. They are available in displacements from 40 cc/rev up to about 50 litres/rev but can sometimes be limited in power output. Crankshaft type radial piston motors are capable of running at "creep" speeds and some can run seamlessly up to 1500 rpm (such as ATO 10 hp radial piston hydraulic motor) whilst offering virtually constant output torque characteristics. This makes them still the most versatile design.
The single-cam-type radial piston motor exists in many different designs itself. Usually, the difference lies in the way the fluid is distributed to the different pistons or cylinders, and also the design of the cylinders themselves. Some motors have pistons attached to the cam using rods (much like in an internal combustion engine), while others employ floating "shoes", and even spherical contact telescopic cylinders. Each design has its own set of pros and cons, such as freewheeling ability, high volumetric efficiency, high reliability and so on.
Pistons pushing outward
Multi-lobe cam ring types have a cam ring with multiple lobes and the piston rollers push outward against the cam ring. This produces a very smooth output with high starting torque but they are often limited in the upper-speed range. This type of motor is available in a very wide range from about 1 litre/rev to 250 litres/rev. These motors are particularly good on low-speed applications and can develop very high power.
