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.
Hydraulic piston motors
The most efficient and versatile hydraulic motors are piston type but they are also the most expensive. Inline or axial and bent axis types operate smoothly from 10 to 2000 rpm with high torque throughout their speed range. Radial piston types go as low as 1 rpm but usually not higher than 400 rpm. Their main use is very high-torque/low-speed applications.
The cutaway in Figure 15-27 shows typical construction of inline fixed- and variable-displacement hydraulic motors. Low-displacement variable motors may be controlled manually while larger motors need pistons to change displacement. An inline hydraulic motor shaft rotates as fluid pushes against a piston, forcing its shoe to slide up the angled swash plate. A small amount of pressure fluid goes through an orifice in the piston and behind the shoe to keep it from rubbing metal-to-metal while it is producing torque.
With the swash plate at a steep angle, torque is high while speed is usually low. A shallow swash plate angle gives high speed but less torque. Most manufacturers recommend a minimum swash plate angle of 15° to 17° for best results. A maximum angle of 40° to 45° gives good torque and long motor life.
The bent-axis piston motor in Figure 15-28 has the same operating characteristics as the inline motor but is more rugged and capable of higher operating pressures. Since there is no sliding action of piston shoes there is less friction and higher torque for a given energy input. The angle of the cylinder block to the input shaft determines torque and speed ranges. The greater the angle, the higher the torque and the lower the speed. Kidney-shaped openings in both inline and bent-axis motors port fluid to and from pistons as they rotate. Internal leakage is sent to tank through the case drain. Variable-displacement bent-axis motors are available but not commonplace due to expense and size.
The radial piston motor shown in Figure 15-29 uses pistons pushing against an eccentric to produce rotary motion. These motors usually have five or seven pistons with rods and shoes, with half of them pushing against the eccentric while the other half return oil to tank. The shoes have high pressure fluid fed to them from the piston through the rod to keep them from rubbing the eccentric during the power stroke. A rotary valve attached to the output shaft feeds and exhausts fluid to and from the pistons as they turn the eccentric.
Some radial piston motors are made with a moveable eccentric that allows different offset amounts. Usually the offset is full or one-half so a motor with this feature can have higher speed at lower torque for fast movement. Eccentric-type radial piston motors are one type of motor that cannot function as a pump without special inlet considerations.
Other radial piston motor designs are similar in action and torque output, but arrange the pistons in different configurations. One design has the pistons facing in and pushing outward against a cam-shaped housing. The shaft is connected to the machine and the housing rotates. It was originally designed as a wheel motor.
When rotary output is one or two turns or less, a hydraulic motor could be used but repeat stopping accuracy could be a problem. There are several designs of rotary actuators that give rotary output for limited numbers of revolutions (usually under one revolution).
Figure 15-30 shows one way to achieve rotary action when the motion is 90° or less. A clevis-mounted cylinder attached to a lever arm that is driving a shaft gives no less than half the cylinder force times the lever arm length. In the example shown, torque on the shaft would be approximately 4000 lb-in. when the cylinder starts and finishes and approximately 6600 lb-in. when the angle between the cylinder and lever arm is 90°. Often maximum torque is only required at the end of stroke so arrange cylinder mounting to give the greatest torque at that point. Remember, retract force is less than extend force which could cause the cylinder to stall when reversed.
The vane-type rotary actuator in Figure 15-31 is a common design for both pneumatic and hydraulic fluids. The vane has seals around the edges where it contacts the housing to control leakage. Fluid entering from the left, as shown, pushes the vane away and forces fluid out the opposite port as it turns the output shaft. A single-vane rotary actuator is usually limited to 270° rotation or less. A double-vane rotary actuator usually only turns 90° or less. Torque is equal to the vane area times input pressure times the radius from the center of the output shaft to halfway across the vane. The symbol shows a semi-circle to indicate rotary action that is not capable of being continuous in either direction.
Another common rotary actuator is the single-cylinder rack-and-pinion rotary actuator shown in Figure 15-32. Several companies make these units in single- and dual-cylinder models with torque outputs up to and above 300,000 lb-ft. They can be pneumatic- or hydraulic-actuated. This rotary actuator design can turn more than one revolution because turns are in direct relation to pinion gear size and rack gear length. It may also be supplied with cushions, and/or stroke limiters for smooth adjustable stops. Fluid entering ports at the cylinder ends forces the piston to move away and drive the rack gear against the pinion gear. The pinion gear continues to rotate until the opposite piston bottoms out. Reversing flow to the pistons reverses output shaft rotation. Torque is equal to piston area times input pressure times the radius of the pinion gear.
The helical gear rotary actuator shown in Figure 15-33 shows another design that can have more than one revolution of the output shaft. The number of rotations is in direct relation to gear teeth angle and the non-rotating piston stroke. Fluid entering and pushing against one side of the non-rotating piston forces it to move and impart the turning action to the output shaft through the helical gear and nut arrangement. This rotary actuator design is able to have deceleration built in and may be purchased as a double-shaft model.
The chain-and-sprocket rotary actuator in Figure 15-34 depicts another way of achieving limited rotary output with more than one turn capability. The number of turns is in direct relation to the sprocket size and the working piston stroke. It can be used with pneumatic or hydraulic fluids. Fluid entering the CCW port pushes against the working piston and the isolating piston at the same pressure. Because the working piston has more area it will move left while the isolating piston moves right. The effective thrust is pressure times the area of the working piston minus pressure times the area of the isolating piston. The result of these calculations times the radius of the sprocket determines torque.
There are some other rotary actuator designs, but in most cases, they are variations of the ones presented here. For circuits using rotary actuators see the authors upcoming e-book Fluid Power Circuits Explained.