It sets the stroke length of each piston and therefore the displacement per revolution. A larger angle means more displacement, giving more torque and lower speed for a given flow; a smaller angle gives less torque and more speed. On variable designs the angle changes while running.

A wide speed range from one unit. High displacement gives maximum torque for starting or climbing, and reducing it as the load falls away gives higher speed at the same flow. That removes the gearbox or second motor a fixed-displacement arrangement would need to cover the same range.

Because a closed-circuit transmission depends on high efficiency, high pressure capability and controllable displacement at both ends. Paired with a variable pump, a variable axial piston motor gives continuously variable drive with good energy performance, which is what mobile machinery needs.

Critical, and it is the main determinant of life. The close-fitting pistons, slippers and valve plate have very small clearances that contamination scores and jams. Most premature failures trace to filtration or fluid condition, so meeting the specified cleanliness target matters more than any other maintenance action.

Internal leakage returned to tank through the case. It rises measurably as internal clearances wear, so it is the most useful diagnostic on a piston motor - measurable without dismantling anything, and a rising trend indicates wear long before performance loss becomes obvious to the operator.

When a cheaper type would do the job. It is the most expensive per unit of torque and the least tolerant of dirt, so a low-speed drive that an orbital motor handles, or a rough duty on a contaminated system that a gear motor would survive, should not be given a piston motor by default.

Yes, though the closed-circuit hydrostatic transmission is where the type is most at home. In open circuit the case drain arrangement and return line pressure need attention, since case pressure above the shaft seal rating is a common cause of leakage that gets blamed on the seal rather than on the plumbing.