Insane Hydraulics

Site theme image

A Step Not To Skip Before Opening an Orbital Motor

What is the first thing a mechanic should do before servicing an orbital motor (discarding the answers like wash hands, wear protective gear, clean the working place and such...)? I ask this question because the step I am going to discuss in this post is often skipped, and although most of the time this doesn't produce negative consequences, it potentially CAN.

Before disassembling any used orbital motor one should always confirm its timing (the relation of the shaft rotation to the direction of the port oil flow), which is a five-second step and can easily be done by turning the input shaft and noticing the port that creates suction.

The reason you shouldn't skip this step is very simple - most surely your client will want the repaired motor to be timed the way it was before! Practically all orbital motors can be timed in two manners, but not all orbital motors that crash on your shop bench will be timed according to the standard manufacturer timing pattern (to top it up, the timing pattern can differ between brands and motor models). Once you have the motor disassembled, there's no way of telling "which way" it was timed, which means that if you skipped the "timing check" and re-assembled the motor "by the book", there's a small (but real) chance that after delivering it to your client the motor will start turning the other way - something that can create a hazard and will definitely make you look like an amateur.

I am not sure if it is universally accepted, but I've been calling motors that follow standard Danfoss pattern - right motors, and all the other - left motors. The majority of orbital motors on the market are "right" motors, by the way. Sometimes machine manufacturers will opt for purposefully timing an orbital motor "the other way" to allow for a more compact piping layout (when two hydraulic motors drive the same function, one of them being "right" and the other one "left"). Sometimes a motor can have a long "history" of interventions before ending up at the final owner already re-timed. In any case, it is always the safest practice to check the timing before starting the disassembly, especially when you deal with an unknown brand or a motor that has a reference plate missing.

The same principle can be applied to radial piston motors, that also can be "right" and "left". Due to the often large size and the fact that the pistons inside can be retracted it is often easier to check the timing by turning the motor with compressed air, rather than by turning the shaft. In most cases, due to the valve plate design, it will be possible to mark its position during disassembly and thus define timing, but again not always, so checking the timing before disassembly still remains a winner!

It's always better to say "I did" than "I should have done"!