The piston shaft (tapered section shown in Fig. 5.15) does not provide the linkage that ensures that the cylinder block rotates with the output shaft. This linkage is provided with another mechanism. Different manufacturers use various mechanisms to develop this synchronizing torque, and this detail is beyond the scope of this text. The key point is that the full output torque of the motor is not transmitted through the synchronizing mechanism, only the torque to keep the cylinder block turning in synch with the output shaft.
Generally speaking, the maximum speed of an axial piston motor (in-line design or bent axis design) is limited by the requirement to maintain an oil film between the piston and wall of the cylinder. As the cylinder block rotates, centrifugal force throws the pistons out against the cylinder wall. A heavier piston produces a higher centrifugal force and thus a higher potential for the oil to be squeezed out of the clearance between the piston and cylinder bore. Loss of this film results in metal-to-metal contact and a “burn” of the piston, or cylinder bore, or both.
Two design features have been incorporated in bent axis motors to reduce the potential for loss of the oil film between piston and cylinder bore.
1. Lighter pistons are used. Also, the end in contact with the cylinder bore is shaped to minimize the contact area between the piston and cylinder bore.
2. A synchronizing mechanism minimizes the side load on the pistons. This mechanism transmits the torque required to keep the cylinder block in synch with the output shaft.
Given the lighter-weight piston, coupled with the small contact area of the spherical end, and the elimination of the requirement for the piston to transmit much (if any) side force, the bent axis design can run at higher speeds for a given displacement than can the in-line design.
The reader can appreciate that the lightweight pistons (with needed tight tolerances) and the synchronizing mechanism are not easy to build. Bent axis designs are expensive. There are certain performance advantages, and, if these advantages are fully utilized, the bent axis design can be very cost competitive.