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August 22, 2017
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Radial Piston versus Axial Piston Hydraulic Motors

You can’t have a final drive motor without a hydraulic motor.  While there are many different types of hydraulic motors, the primary ones used for travel motors and track drives are going to be either radial piston hydraulic motors or axial piston hydraulic motors.  In this Shop Talk Blog post, we are going to discuss the difference between the two, and the types of applications they are well adapted to.

Quick Review on Hydraulic Motors

Hydraulic motors are one of the most important components in a hydrostatic power transmission system as they convert hydraulic energy into useful mechanical energy in the form of rotational motion and, even more importantly, torque.   When well designed, hydraulic motors are powerful, efficient, and compact.   The basic kinds of hydraulic motors are piston, gear, and vane.  Final drives and hydraulic motors used in construction are usually of the piston variety, and, of these, the most common are radial piston motors and axial piston motors.

Axial Piston Hydraulic Motors

bobcat-331-rotator-group-piston-retaining-plate-final-drive-travel-motor

Axial piston motors don’t generate the same level of torque that radial piston motors do, but they are far cheaper and smaller (a 5-hp axial piston motor is about the size of a soda pop can).  In an axial piston motor, the pistons will be found arranged in a circular pattern in some type of housing (often called a cylinder block) that rotates about its axis by a shaft.  That shaft is aligned with the pistons, and the pistons are all parallel to one other.

There are two different axial piston motor designs.  The first is the swashplate design, where the pistons and drive shaft are parallel.  The second is the bent-axis design, where the pistons are arranged at some angle to the main drive shaft.  In this design, the up and down motion of the pistons is converted to rotary motion through a ball joint.

 

radial-piston-hydraulic-motor-final-drive

Radial Piston Hydraulic Motors

Radial piston motors are low-speed high-torque (LSHT) motors and can generate much more torque than axial piston motors and do not require a gearbox to act as a speed reducer/torque multiplier.  In a radial piston motor, you will find the pistons (sometimes called plungers) arranged perpendicular to the axis of the crankshaft in a star-like configuration.  All pistons lie in the same plane, and as the pistons move linearly their motion is converted to rotary motion by an eccentric shaft.

 

 

Comparing Radial Piston Motors to Axial Piston Motors

The speed range of the radial piston motor is very limited compared to the axial piston motor, but it offers significantly more torque.  This explains why you see radial piston motors used on more powerful construction equipment like full-size excavators and backhoes while axial piston motors are commonly seen on skid-steers, mini-excavators, and compact track loaders.  Radial piston motors, however, are far more sensitive to contamination of the hydraulic fluid than axial piston motors are.

Summary

In short, the two main types of hydraulic motors you see used with heavy equipment are radial and axial piston motors.  The radial piston motors offer much more torque but not as much speed as the axial piston motors.  Both, however, are commonly used to provide the torque necessary to turn the tracks or wheels on many types of heavy equipment.

 


 

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Would you like to learn more?

Here are some other Shop Talk Blog posts you may find interesting …

Travel Motors versus Slew Motors: What’s the Difference?

Hydraulic Drive Motor Contamination: What You Need to Know

Rebuilt Final Drive Motor versus Re-Manufactured Final Drive Motor

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