Insane Hydraulics

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Orbital Hydraulic Motor Principle

Today we come to the point in our "orbital steering saga" (do check out Part 1 and Part 2 if you haven't already) where we look at the orbital section in more detail. Very important and very useful, because low-speed high-torque orbital motors are ubiquitous, and knowing the details of how they work makes you a better tech!

Here we have the orbital section:

Orbital section of a steering valve

It sits on top of our steering valve, and the seven holes that feed it run down the body straight into the ring of 12 holes that are evenly spaced on the outer sleeve of the rotary valve. Looking at the slots machined in the internal spool we can tell that turning the steering column right or left connects 6 alternative holes of the 12 to the P port, and the other 6 to the steered work port (see Part 2 for the hydraulic diagram, if you want more detail).

7 holes that feed the orbital section
These 12 6-mm holes on the rotating spool valve take care of the orbital timing

I've always thought that the best way to learn a concept is to recreate it in a drawing or a model - and this is exactly what we'll do. It's very easy. All you need is to take some measurements and then make a more or less accurate drawing in 2D CAD software.

On a side note - knowing the basics of 2D computer-aided drawing is essential for a hydraulic professional. You'll need it to make diagrams, you'll need it to make technical drawings, and as soon as you start using it - you'll need it for a lot more, I guarantee you that! So do invest some time into learning the basics of 2D CAD. Being able to create dimensionally accurate drawings is very, very handy. And the best part is that basic 2D CAD software is free these days! For example the nanoCAD 5. It's basic, but it's a good place to start - the interface is standard and you'll earn it in no time! I used to recommend Drafsight left and right when there was a free version of it. I still think it's one of the best 2D CAD packages out there, so definitely go for it if you can afford it, but you can do all the basic stuff and more with free programs as well. But I digress... Let's get back to our orbits now, shall we?

So, the outer spool measures exactly 42 mm in diameter, and the 12 holes are 6 mm in diameter. It's hard to take an exact measurement of the diameter of the 7 holes in the steering body, but an approximate measurement of 5 mm is good enough. Now all you need to do is draw a 42-mm circle, then use a polar pattern to place 12 6-mm circles around it, and then another 7 5-mm circles, and then use some trimming to cut out the internal and external semi-circles and, maybe, some solid color fills for a better illustration of alternatively connected holes, like so:

Drawing the orbital motor timing valve

I like hands-on, so my next move after making this drawing would be to print it out on a couple of sheets of paper, and then use scissors to cut out the moving part, and then play with it for a while to figure out how this gismo functions:

Using paper cut outs to study the orbital timing arrangement
Using paper cut outs to study the orbital timing arrangement

But simulating the parts using vector drawings and the magic of JavaScript is even cooler! So I came up with an interactive drawing that perfectly illustrates (IMHO) how an orbital motor works. The toggle switch under the drawing turns on a unique perspective of the orbital rotor function. When it is activated, the animation is exactly the same, but the camera view is rotated so that the pressure and return fields stay stationary. Go ahead, take it for spin (and don't forget to try out the toggle switch)!

Rotate View

How about that unique perspective? As you can see, the orbital section is essentially a combination of a 6-tooth and a 7-tooth meshed gears, and the very clever orbital arrangement can also be looked at as an internal gear pump, that makes 7 full turns for every turn of the main shaft. How cool is that?! This, by the way, also means that we can use the orbital gear to calculate the displacement of our gear pump, and then multiply it by 7 to get the displacement of the orbital motor.

Let's test this. This particular unit has the following dimensions of the small gear:

External diameter of 53.3 mm, internal diameter of 38.3 mm, and gear width of 13 mm, if we punch these numbers into the gear pump displacement calculator (more on gear pump displacement calculation here), we get 14 cm³, and 14 times 7 is 98. And this is an OSPC 100, so our assumption is 98% correct!

I think the interactive drawing pretty much sums it up, but still - the 12 rotating holes plus 7 fixed holes is a beautiful mechanical timing arrangement, and whoever invented it is a genius!