November 16, 2017
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Primary Cause of Radial Piston Motor Failure

Here at Final Drive Parts, we tend to get quite a few radial piston motors (many belonging to CAT CTLs) — and we’ve noticed a definite pattern of failure.  In this week’s Shop Talk Blog post, we are going to discuss the primary mode of failure for radial piston motors and explain how easily this kind of failure can be prevented.

Review: Radial Piston Motors

Radial piston motors are powerful LSHT (low-speed, high-torque) hydraulic motors that are designed such that they do not require a torque-multiplying gear-box.  Their speed is limited, but they generate much more torque than an axial piston motor can.  Radial piston motors are commonly used on track loaders, excavators, and backhoes, and they get their name from the radial arrangement of the pistons, which are laid out in a star-like configuration perpendicular to the driveshaft.

Major Cause of Radial Piston Motor Failure: Contamination

One of the main components is the piston block, which is located in the center section of the drive motor.  The piston block shown above is in very poor condition, and the source of the extensive damage you see is hydraulic contamination.  Hydraulic contamination is the number one enemy of hydraulic systems, including radial piston motors such as the one shown. Between 75% and 90% of all hydraulic system failures can be traced back to contamination.

Form of Contamination

damaged-cam-ring-final-drive-radial-piston-hydraulic-motorThere are two major types of hydraulic contamination:  abrasive (think sand, grit, metal flakes) and non-abrasive (water, air, chemical degradation).  The type of damage we are talking about in this blog post is caused by abrasive contamination.  There are many different sources of such contamination.  Some contamination is literally built-in and can be leftover casting sand, fibers from a rag used to clean components before assembly, pipe sealant, and even weld spatters.  Contamination such as dust, water, and rust can be introduced during routine maintenance. Then there are contaminants that are generated during normal use and as a result of system failures.  This type of contamination includes metal flakes and shavings, rust particles, and pieces of failing seals. Contaminants can also enter the system through pitted shafts, failed seals, or broken air breathers.

Contaminants can have serious effects on systems, and if they are not filtered out properly, they will result in abrasion, erosion, and cavitation. This, in turn, starts a vicious cycle of damage that results either in thousands of dollars in repairs or even total a hydraulic motor.

radial-piston-hydraulic-motor-final-drive-contamination-02Recognizing the Effects of Contamination

So how do you know when contaminants are starting to damage your radial piston hydraulic motor? The symptoms of contamination-related failure are jumping or lunging or loss of power in one side of the tracks.  If you see these symptoms, then it is time to check your hydraulic fluid and filters before any more damage occurs.

Preventing Damage from Contamination

This cycle of damage can be prevented by two simple maintenance tasks: changing the hydraulic fluid per OEM guidelines, and checking and changing the filters. Believe it or not, these two simple actions will drastically reduce the contamination that your hydraulic motor is exposed to, as well as minimize downtime, increase efficiency, provide more consistent performance, reduce component wear, and save you hundreds if not thousands of dollars in repairs and replacements.  In short, keeping your hydraulic fluid clean is the key to a well-functioning, efficient, dependable hydraulic motor.


The number one cause of failure in radial piston motors is contamination.  This includes the ones found on CAT CTLs and the Bobcat S series. By simply changing out your hydraulic fluid and filters on a regular basis, you can minimize the contamination your motor is exposed to and, as a result, prevent this type of failure, avoid the downtime related to it, and escape the extensive repair costs that are often involved.

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