Hydraulic Motor Face-Off: Bent Axis vs Axial Piston

Bent axis and in-line axial piston motors are both high performance designs featuring high efficiency at high operating pressures. But which is best? To answer this question, a comparison of relative performance advantages is required.

Bent axis designs can be 'bent' to angle of up to about 40°. Whereas the swash plate angle on an in-line design is typically limited to less than 20°. This means the maximum to minimum displacement range or stroke ratio of a variable-displacement bent axis motor is around twice that of an in-line design. This gives a variable bent axis motor a wider operating torque/speed range than an in-line motor of the same displacement (remember, increasing motor displacement increases output torque and reduces output speed; and vice versa).

On the other hand, not all variable-displacement bent axis motors have the capability to go to zero displacement. And this can be an issue if, say for example on the ground drive of a mobile machine, the motor must be shifted to 'neutral', in order to switch from 4 wheel drive to 2 wheel drive. In-line motor designs have no such limitation.

Bent axis motors-especially those with light-weight pistons, also have the wood over in-line designs when it comes to maximum operating (shaft) speed; a bent axis motor can be spun significantly faster than an in-line design of the same displacement. And there's more. The stall or breakout torque of a bent axis motor is around 5% higher than an in-line motor of the same displacement.

With all the apparent performance advantages a bent axis motor has, why would anyone use an in-line motor at all? In a word, cost. Bent axis motors are more expensive, and in most cases, significantly so. Plus, if you've studied Preventing Hydraulic Failures, you'll be aware that the shaft bearings on bent axis designs are heavily loaded and therefore more prone to premature failure than the shaft-bearings on an in-line design. So not only is the initial cost of a bent axis motor higher; its lifetime maintenance cost is likely to be higher as well.

As with every other component in a hydraulic system, selecting the wrong type of motor can be costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get "Six Costly Mistakes Most Hydraulics Users Make... And How You Can Avoid Them!" available for FREE download here.