Hydraulic motors come in the same variety as pumps. Many are low-speed/high-torque, some are high-speed/low-torque, and a few are low- or high-speed/high-torque. The main difference between pumps and motors is that a motor is usually capable of having either port pressurized.
High-speed motors can reach 3000 rpm continuous to drive fans, lawn mower blades, and grinders. They usually do not have high torque starting capabilities but most applications they are used on do not require this feature. Low-speed/high-torque motors usually have 75% to 90% of their maximum torque to start. They usually operate at 500 rpm or less. Piston motors of the in-line and bent axis design have high low speed torque and can run at 1500 to 2500 rpm without losing efficiency.
Hydraulic gear motors
The gear motor shown in Figure 15-23 is one of the oldest designs and is built for high-speed/low-torque needs. At first it appears fluid entering the lower port pushes against two teeth to start the gears turning. However, a closer examination shows the left gear has fluid pushing on opposing teeth as it comes out of mesh and only the right gear has any twisting action. After one tooth of revolution, the left gear drives while the right gear is balanced and so on as the motor turns.
Hydraulic gerotor motors
The high-speed gerotor motor in Figure 15-24 has similar characteristics to the gear-on-gear motor just mentioned. This is not a popular design but the gerotor concept with the idler gear held stationery shown next is made by many manufacturers and holds more than 50% of the small-to-medium high-torque/low-speed motor market.
The generated rotor hydraulic motor shown in Figure 15-25 is made high-torque/low-speed by holding the idler gear still and allowing the orbiting gerotor to cycle inside of it. This change causes the orbiting gerotor to make as many power strokes as it has teeth for every revolution of the output shaft. The seven-tooth gear shown makes seven power strokes while the output shaft turns once. A splined drive connection follows the orbiting gear and transmits the rotary motion to the output shaft.
Generated rotor motors give at or near full torque from about 25 rpm and normally do not go higher than 250 to 300 rpm. Maximum output torque is directly related to the width of the gerotor element which may be as narrow as 1/4 in. to 2 in. Pressure ratings as high as 4000 psi are common from most manufacturers.
Gerotor motors can have a selector valve that changes the internal rotary valve output to feed only half the chambers, causing the motor to run at twice the speed and half the torque. The gerotor design is machined with too close tolerances but must have some clearance to allow the inner gear to move. This makes it less efficient and as the gears wear, internal leakage increases. The geroler design has rolling seal points and can be setup much closer and has less wear for longer life. Most of the geroler types also use a plate valve which has less leakage and is wear compensating as well.
Hydraulic vane motors
The hydraulic vane motor shown in Figure 15-26 is a very efficient design and works well for applications at 20 to 3000 rpm. Fluid entering one port pushes against two or four vanes as they extend in the cavities of the cam ring. Internal porting directs pressure and return fluid to the working and exhausting vanes. While half the vanes are being pushed by fluid, the other half are discharging spent oil to tank. The amount of torque is in direct relationship to the vane area exposed to pressure fluid and the distance the vanes are from shaft center. Speed is limited to how much displacement and what size ports the motor has.
The high-speed/medium-torque design with an elliptical cam ring gets full torque at approximately 100 rpm and can go as high as 3000 rpm. Because it covers such a broad speed range it is suitable for many applications where other designs fall short on torque or speed. The low-speed/high-torque design is designed for approximately 10 to 400 rpm and usually eliminates any need for gear reduction. Using a direct drive eliminates maintenance problems and makes a smaller package at the work area.