Radial piston motors produce very high torque at low speed. They are used as wheel motors for large equipment. Pistons operate in radial bores in a stationary cylinder block. The surrounding housing rotates (Fig. 5.18). The rotating housing has two identical cam rings. The pistons each have two cam rollers. An extending piston forces the two rollers against the two cam rings causing the housing to rotate.
A distributor valve is installed in the cylinder block. This valve rotates with the housing. It directs high-pressure oil to the pistons during their work stroke and collects the low-pressure oil during their return stroke.
The distributor valve has two concentric rings. When the valve is set to supply high-pressure oil to both rings, half of the eight pistons are supplied with high-pressure oil at any time. This configuration is identified as full displacement in Fig. 5.18. It gives the maximum torque output.
When the distributor valve is set to supply high-pressure oil to only one ring, 25% of the eight pistons are supplied with high-pressure oil. The four “return stroke” pistons feed oil to the remaining two “power” pistons, and the excess oil feeds back to the pump. Since the two idling power pistons get low-pressure oil, they contribute very little to the output torque. This configuration is shown as half displacement in Fig. 5.18. Speed is doubled for a given pump flow, but output torque is halved.
In the recirculation mode shown in Fig. 5.18, charge pressure is delivered to all pistons. The motor can rotate freely up to rated speed as the vehicle is towed. Freewheeling can also be achieved by connecting all motor ports to the drain line. In this case, the pistons are forced to their inner position by the case pressure, and the housing rotates freely.
Radial piston motors can operate at pressures up to 5000 psi. They tend to be robust. Under normal operating conditions, the design life is 15,000+ hours. The manufacturer states that full torque is available at any speed, including startup.
Applications include heavy load carriers with as many as 124 wheel motors. These motors are also used for a wide range of agricultural, forestry, mining, and construction equipment.