Radial-piston motors, Figure 6, have a cylinder barrel attached to a driven shaft; the barrel contains a number of pistons that reciprocate in radial bores. The outer piston ends bear against a thrust ring. Pressure fluid flows through a pintle in the center of the cylinder barrel to drive the pistons outward. The pistons push against the thrust ring and the reaction forces rotate the barrel.
Motor displacement is varied by shifting the slide block laterally to change the piston stroke. When the centerlines of the cylinder barrel and housing coincide, there is no fluid flow and therefore the cylinder barrel stops. Moving the slide past center reverses direction of motor rotation.
Radial piston motors are very efficient. Although the high degree of precision required in the manufacture of radial piston motors raises initial costs, they generally have a long life. They provide high torque at relatively low shaft speeds and excellent low speed operation with high efficiency; they have limited high speed capabilities. Radial piston motors have displacements to 1,000 in.3/rev.
Axial-piston motors also use the reciprocating piston motion principle to rotate the output shaft, but motion is axial, rather than radial. Their efficiency characteristics are similar to those of radial-piston motors. Initially, axial-piston motors cost more than vane or gear motors cost more than vane or gear motors of comparable horsepower, and, like radial piston motors, have a long operating life. Because of this, their higher initial cost may not truly reflect the expected overall costs during the life of a piece of equipment.
In general, axial piston motors have excellent high speed capabilities, but, unlike radial piston motors, they are limited at low operating speeds: the inline type will operate smoothly down to 100 rpm and the bent-axis type will give smooth output down to the 4-rpm range. Axial piston motors are available with displacements from a fraction to 65 in.3/rev.
Inline-piston motors, Figure 7, generate torque through pressure exerted on the ends of pistons which reciprocate in a cylinder block. In the inline design, the motor drive-shaft and cylinder block are centered on the same axis. Pressure at the ends of the pistons causes a reaction against a tilted swashplate and rotates the cylinder block and motor shaft. Torque is proportional to the area of the pistons and is a function of the angle at which the swashplate is positioned.
These motors are built in fixed- and variable-displacement models. The swashplate angle determines motor displacement. In the variable model, the swashplate is mounted in a swinging yoke, and the angle can be changed by various means — ranging from a simple lever or hand-wheel to sophisticated servo controls. Increasing the swashplate angle increases the torque capacity but reduces drive shaft speed. Conversely, reducing the angle reduces the torque capacity but increases drive shaft speeds (unless fluid pressure decreases). Angle stops are included so that torque and speed stay within operating limits.
A compensator varies motor displacement in response to changes in the work load. A spring-loaded piston is connected to the yoke and moves it in response to variations in operating pressure. Any load increase is accompanied by a corresponding pressure increase as a result of the additional torque requirements. The control then automatically adjusts the yoke so that torque increases when the load is light. Ideally, the compensator regulates displacement for maximum performance under all load conditions up to the relief valve setting.
Bent-axis piston motors, Figure 8, develop torque through a reaction to pressure on reciprocating pistons. In this design, the cylinder block and drive shaft are mounted at an angle to each other; the reaction is against the drive-shaft flange.
Speed and torque change with changes in the angle—from a predetermined minimum speed with a maximum displacement and torque at an angle of approximately 30° to a maximum speed with minimum displacement and torque at about 7-1/2°. Both fixed- and variable-displacement models are available.
Rotary abutment motors
Rotary abutment motors, Figure 9, have abutment A, which rotates to pass rotary vane B, while second abutment C, is in alternate sealing engagement with the rotor hub. Torque is transmitted directly from the fluid to the rotor and from the rotor to the shaft. Timing gears between the output shaft and rotary abutments keep the rotor vane and abutments in the proper phase. A roller in a dovetail groove at the tip of the rotor vane provides a positive seal that is essentially frictionless and relatively insensitive to wear. Sealing forces are high and friction losses are low because of rolling contact.
A screw motor essentially is pump with the direction of fluid flow reversed. A screw motor uses three meshing screws — a power rotor and two idler rotors, Figure 10. The idler rotors act as seals that form consecutive isolated helical chambers within a close-fitting rotor housing. Differential pressure acting on the thread areas of the screw set develops motor torque.
The idler rotors float in their bores. The rotary speed of the screw set and fluid viscosity generates a hydrodynamic film that supports the idler rotors, much like a shaft in a journal bearing to permit high-speed operation. The rolling screw set provides quiet, vibration-free operation.
Selecting a hydraulic motor
The application of the hydraulic motor generally dictates the required horsepower and motor speed range, although the actual speed and torque required may sometimes be varied while maintaining the required horsepower. The type of motor selected depends on the required reliability, life, and performance.
Once the type of fluid is determined, the selection of actual size is based on the expected life and the economics of the overall installation on the machine.
A fluid motor operating at less than rated capacity will provide a service life extension more than proportional to the reduction in operation below the rated capacity.
The maximum horsepower produced by a motor is reached when operating at the maximum system pressure and at the maximum shaft speed. If the motor is always to be operated under these conditions, its initial cost will be lowest. However, where output speed must be reduced, the overall cost of the motor with speed reduction must be considered — to optimize the overall drive installation costs.
Sizing hydraulic motors
As an example of how to calculate hydraulic motor size to match an application, consider the following: an application calls for 5 hp at 3,000 rpm, with an available supply pressure of 3,000 psi, and a return line pressure of 100 psi; the pressure differential is 2,900 psi.
The theoretical torque required is calculated from:
T = (63,0252 3 horsepower)/N
T is torque, lb-in., and
N is speed, rpm.
For the condition T = 105 lb-in.
Motor displacement is calculated as:
D = 2π T ÷ ΔPeM
D is displacement, in.3/rev
ΔP is pressure differential, psi, and
eM is mechanical efficiency, %.
If mechanical efficiency is 88%, then D is 0.258 in.3/rev.
Calculating the required flow:
Q = DN/231eV,
where: Q is flow, gpm, and
eV is volumetric efficiency, %.
If volumetric efficiency is 93%, then Q is 3.6 gpm.
Pressure in these equations is the difference between inlet and outlet pressure. Thus, any pressure at the outlet port reduces torque output of a fluid motor.
The efficiency factor for most motors will be fairly constant when operating from half- to full-rated pressure, and over the middle portion of the rated speed range. As speed nears either extreme, efficiency decreases.
Lower operating pressures result in lower overall efficiencies because of fixed internal rotating losses that are characteristic of any fluid motor. Reducing displacement from maximum in variable-displacement motors also reduces the overall efficiency.