Hydraulic Bent Axis Motors

Bent axis motors were developed to improve the operating and stall torque efficiencies of high-speed motors. The design (Fig. 5.15) operates in a manner similar to that of the axial piston design. A series of cylinders are mounted around the center line of the bent axis. The pistons in these cylinders have a spherical end that fits in a plate attached to the output shaft. Springs hold the pistons against the plate. (These springs are not shown in Fig. 5.15.) Fluid enters the motor and flows into the cylinder that is aligned with the inlet port. The piston extends, pressing against the plate, causing it to rotate. This rotation causes the cylinder carrier to rotate, and the next cylinder is aligned with the inlet port. This piston extends and produces the next increment of rotation. Continuous rotation is produced by the rapid sequencing of these events.


The Rexroth Series 6 axial piston, bent axis motor, Size 80, was chosen as an example of the bent axis design. This motor has a 4.91 in3/rev displacement and maximum speed of 4500 rpm (5000 rpm intermittent). Note that this speed is the highest speed rating of any design discussed. Maximum pressure rating is 5800 psi.

The efficiencies for the bent axis motor (Fig. 5.16) are similar to the efficiencies for the axial piston motor. For both designs, the torque efficiency reaches a maximum at about 3000 psi and remains approximately constant at higher pressures. Volumetric efficiency declines linearly with pressure, so overall efficiency decreases after reaching a maximum of 91% between 3000 and 4000 psi.


Bent axis motors are available as both fixed and variable displacement units. A variable displacement design is shown in Fig. 5.17. The servo cylinder moves the cylinder block in an arc to change the angle between the cylinder block center line and the output shaft center line. When this angle is 0°, shaft output speed is 0. For the first increment of angle increase, shaft speed increases as the angle increases. If flow to the motor is held constant, a further increase in angle will increase motor displacement and thus reduce speed. Potential torque output increases, because motor displacement increases.


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