Insane Hydraulics
Bold and audacious blog about fluid power
- It's one tooth to the right, man!
- No, dude! It's one to the left!
- To the right, I tell you!
- I am pretty sure it's to the left!
- I'm sorry but you're wrong, mate. I'm positive it's to the right!
- ME? Wrong? I've been mounting these motors since I was five!
- Aha! And I've been mounting these motors since I was three!
...and the conversation when on, till they went back to the office and fetched a service manual... Fortunately, no shots were fired that day.
That was me and a colleague of mine, standing in front of an open and up-ended Danfoss OMR orbital motor and arguing how to time it right, only one of us was referring to the tooth on the gear wheel and the other to the tooth on the connecting shaft, which meant that both of us were right.
Orbital motor timing is a peculiar topic for me because I often find that it creates confusion or an unnecessary - I am not even sure which word to use here - complexity in a hydraulic shop when it is regarded as something extremely complicated, that mandates the use of a diagram and can't be done in any other way, and if, for some reason, a mechanic assembles such a motor without a service manual the world will end.
This, of course, is not the case. Imagine only how such a mechanic would feel in the field, where there's no manual around (I know that in the modern smartphone era this is almost impossible, but still..): "Oh my God! I can't mount the motor without a manual... What will I do?!!"
So, in this post, I want to clarify why you shouldn't worry too much about having "proper timing instructions" on hand when you assemble orbital motors. Don't get me wrong - I am absolutely OK with using service manuals, however, I am absolutely against spending twenty minutes looking for documentation or waiting for an "experienced advice" when there is a very easy way to "solve the timing riddle"!
The only thing you do need to know when you have to disassemble and then reassemble an orbital motor is whether it is a right or a left motor, meaning - you have to know which way the shaft should turn in relation to the port flow, and that's it! (which is why it is important to confirm which of the versions is in front of you before you start the disassembly).
If you know this - the timing part is easy. It is easy because there are only two ways you can time such a motor, so you already have a 50% chance to get it right on the first attempt - which makes is a 100% chance if you take two.
And with orbital motors that employ a spool valve to take care of the oil distribution (think Danfoss OMM, OMP, or OMR) it's even easier than that - all you need is a shop air blow-gun. What you do is you assemble the motor in a random way, all except the end cap - and then blow air into a work-port. You will immediately see in what part of the rotor the majority of the airflow is escaping, which will immediately tell you which way it will rotate (if you imagine that this "pocket" is "expanding"). Now - if the rotation is correct - you cap it. If it's not - just shift the rotor's position by one tooth and try again - the timing should be correct now.
Of course, with orbital motors that use a disc valve it can be a little harder - but the "no-manual" gist remains - mount it at random -> try it -> if it is correct -> close it -> if it is not correct -> shift the rotor's timing position (i.e. the position of the rotor in relation to the valve element) by one tooth and try again.
Naturally, you must understand that to check if the timing of an orbital motor is correct (if you can't use the air trick described above) - you don't need to waste time tightening the end-cap screws with the right torque - just tighten them tight enough to compress the seals, put the motor on its side, pour some oil in the ports and rotate the shaft with a tool.
This technique also helps to develop a solid understanding of how orbital motors work. With some practice you can, for example, lay a disc valve on top of the rotor, look at the ports, and tell which way the motor will rotate just by looking at it, which is pretty cool!
I remember till today the first time I worked on an orbital motor - about a week into my first hydraulic job. I had this very experienced mechanic with years of practice opening an orbital motor in front of me. And when I asked him how the motor worked and what made it rotate - in all seriousness the guy responded - well, I know how to change the seals, how to replace the parts and how to time it right, but don't ask me how it rotates - this stuff is for engineers - and he pointed his finger to the office windows directly above us...
You don't want to be like that - trust me!