Insane Hydraulics

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Back-Engineering for the Sake of Learning Industrial Hydraulics

Our brain is a muscle, you know, use it or lose it and all, and I kind of like using mine, so in this article I will show you an example of how a routine service procedure can become a brain teaser that has the potential to give you a superior understanding of component operation and design, thus turning you into a better hydraulic tech. This method can be applied to any part that ends up on your bench - in my example, it's going to be this White Drive series 500 orbital motor, that came to our shop for a re-seal.

A few words about why I decided to use this particular model. In this part of the world, when you hear the words "orbital hydraulic motor" the first name you think of is Danfoss (well, I still call it Sauer Danfoss, by the way, and now that I think about it - it's a shame really - Sauer is such a cool name!), the second name is Eaton, and then comes everybody else. So, getting a White Drive is a welcome change. Even though it belongs to the Danfoss family since 2016, we all know that it comes from "the other side of the pond", which makes it a rarity in Europe, especially if you need parts fast and cheap. I could get the front bearing from Danfoss for three bucks, the only problem was the three month lead time! All while multiple US sites offer these by the bucket and boast next day delivery! So yes, it may carry a Danfoss name, but it's not from around, which is just great, because the process that I am about to describe makes the most sense when applied to components that are new to you.

Back to fixing our leaking shaft seal now. In general, there are three approaches to re-sealing an orbital motor:

Approach One

You get the seal kit, look at the motor, and think to yourself: "E-e-e-eh... I can do this, I am that good!", then take out the wrenches and start "wrenching". This works, of course, if you know the motor in and out, but if you don't - you'd better have another seal kit and, maybe even a new motor on standby "just in case"...

Approach Two

You get the seal kit, the service manual, study the manual first, and only then reach for the tools. A much saner approach, especially if you are not familiar with the motor or haven't worked on this model for a while. You should be fine, especially if you don't rush it.

Approach Three

The one I'm advocating here. You get a seal kit, the manuals, study the manuals first, and then, as you go along disassembling the motor, you really look into its function by studying its design and asking the right questions. Call it back-engineering for the sake of learning, if you will. It does take more time, sometimes considerably more time (read - valuable production time) - but it is worth it. It's worth every second of it because you are investing in your knowledge, which is the best investment in the world! So, let me hold your hand as I walk you through the steps of my process.

A small note before we dive in - I understand that to those of you who do these motors regularly, the technical details that I am about to show will be no news. In this case, please, concentrate on the process itself. It's still valid, I promise!

Now - let's get started. Behold - it's Majesty White Hydraulics Series 500 200cc Orbital Hydraulic Motor!

Step 1 - Finding Technical Literature

Before doing anything in the shop - I like arming myself with knowledge first - and by knowledge, I mean all the technical information that I can find. All great techs are great Googlers! Internet is, by far, one of the best (if not the best) tools at our disposal.

Luckily - there are a lot of catalogs for this series available online, and I managed to download both the technical and the service manual pretty fast.

Step 2 - Reading the Literature to Get to Know Your Component

I always like looking into the history of the brand I am working on before anything else. A quick search tells me that the company White Hydraulics was founded in 1976 when it patented its first Hydraulic Orbital Gerotor Motor. The name was changed to White Drive Products in 2005. It is strongest in North America and China, and it was acquired by Danfoss Solutions in 2016 - 2017.

Do I need this information or will I ever have a practical use for it? Probably not. It is still good to know that White Hydraulics motors and White Drive motors come from the same place.

Then I look into the technical stuff. Here are some interesting quotes from the manuals, that arise my curiosity:

"...During startup, pressure causes the balance plate to flex toward the rotor, vastly improving volumetric efficiency. As the motor reaches operating pressure, the balance plate relaxes, allowing the rotor to turn freely which translates into higher mechanical efficiencies..." - Hm... Interesting! I am going to have to look into this flexing balance plate thing.

"...Valve-In-Rotor Design provides cost-effective, efficient distribution of oil and reduces overall motor length..." - Valve-In-Rotor? I wonder what it means?

"...For applications requiring the motor to rotate in only one direction, shaft seal life may be prolonged by pressurizing the “A” port of the motor..." - I have an idea, but I still want to confirm exactly why it is so.

The performance tables from the catalog are very well done, very clear to understand. I, personally, prefer such tables to function diagrams, like the ones you find in Danfoss catalogs, but, of course, both ways are perfectly valid.

Data for the 200 cc model: 300 nominal 370 max rpm, 207/241 bar, 542/633 Nm - not bad, Mr. White! Translating this to the "Danfoss language" - it beats the OMR (and even the OMRX) and sits right below the OMS.

Step 3 - The Actual Disassembly and Assembly

From theory to practice! I will start describing some points that I found interesting from the shaft end and then work my way towards the end cap, but during the actual disassembly, you obviously start from the tail.

Let us inspect the shaft and the front housing.

First thing that I notice - the oil path, which is "curious" (from the Danfoss regular's point of view). You can clearly see that the B port oil goes into the hollow shaft and then up to the rotor through the hollow center of the shaft, literally “washing” the connecting shaft and its spline. This means two important and interesting things. First - the shaft is getting all the lubrication in the world, which is a good thing. Should it ever break it won’t be for the lack of lubrication. And second - we can see that this configuration also means that the B port is connected directly to the shaft seal, and since our motor is rated to 3000 psi, this means that the shaft seal must be super tough. And indeed it is, and is composed of a thousand parts! So you can’t replace it with an off-the-shelf BABSL seal - the moor will spit it out as soon as it starts turning. This also perfectly explains why, while being bi-directional, this motor still has a preferred direction of rotation. Pressurizing the port A connects the shaft seal to the return line, thus subjecting it to a lower stress and wear. Here you can see the port A, and this port here is the B port (the oil goes through the hole in the shaft and then to the gerotor center). A 3000-psi-capable shaft seal also explains why there's no drain.

Let us move "one floor up".

On top of the shaft housing, we have what is referred to in the motor manual as the “manifold”. It is composed of several thin plates with profiled holes that were intricately cut, stacked, and welded together to form a single oil directing manifold with multiple openings. Let us discover what connects to what with our best friend - the shop blowgun. Several "blasts" later I already know that the bottom holes (by saying "bottom" I mean the face that is turned towards the shaft housing) connect the port A to the other side of the manifold - to the ring of these seven small triangular holes. And then we can also see that the internal ring of holes is channeled to the seven work chambers of the rotor, one hole per work camber - like so.

As we already know, to make an orbital group turn, one has to find a way to connect the work chambers of the gerotor to pressure and return in an orderly fashion. Spool shaft motors achieve this in the spool valve integrated in the shaft (for example - the Danfoss OMPs or OMRs), disc valve motors do it with a disc valve, mounted in the tail (think Danfoss OMTs or OMSs), but this motor is clearly different. There’s no rotating spool or disc valve here. It does have, however, a groove cut into the star-shaped rotor, and it is this groove that handles all the timely distribution of oil. Interesting! But how? Well - it’s very easy to see, actually, if you take a piece of transparent plastic, draw the shape of the groove on it, and then place it over the manifold and simulate its eccentric movement. I am very proud of this clever contraption, by the way!

After wiggling our "transparent helper" about the manifold for a couple of minutes it becomes apparent that the irregularly shaped groove cut in the rotor star is our port A, and the hole in the middle is, obviously, port B, and the regular flat circular face in between is what separates them from each other and secures timely distribution, when it slides "in an eccentric fashion", connecting the inner circle of holes (which, as we saw before, are channeled to the gerotor work chambers) sequentially to the motor ports.

My first thought when I saw this was - "how the hell did they design it back then when there was no 3d CAD software around? On a sheet of paper, with a ruler and a pencil?!" Now, this is what I call true engineering. We should be grateful every day for the modern design tools at our disposal!

The second thought was - "Wait a minute… When we want to invert an orbital motor (as you know, there are left and right orbital motors, and you'd better know which one you're working on before opening it) - we must change the position of the timing (oil-directing) element (be it a spool or a disc) in relation to the rotor, but in this case - the rotor and the timing element is the same part, so how do we invert ti? The catalog clearly stated that you could order left and right motors?

The answer is easy if you think about it - you can not invert this motor. That is - you can't invert it using the same parts, however - if you change the way the manifold directs oil - then you can invert it. And indeed - if we check out the parts list - we see that the manifold has two different part numbers - for CW and CCW versions.

OK, so now we know what the "valve in rotor" design mentioned in the manual is, why this motor has a preferred direction of rotation, and how to invert it - time to move another "floor up", and have a look at all the "funky business" with the flexing balance plate, that "vastly improves volumetric efficiency during startup".

There are three small balls, placed in the cavities of the balance plate that sits on top of the rotor. They, apparently, make three check valves, and all the three "point the same way". Then there’s another hole in the plate without a ball in it. Unfortunately, I already had the motor assembled and delivered when I realized that I didn't take pictures of the small balls. But you can still see well - the recessed holes are where the balls work. But why three balls? The motor has only two lines, right?

Let us find out where these check valves are connected to.

I raised the balance plate a bit and used a paper clip to see where the holes "project" when the rotor is moving around. The hole that is closer to the center obviously works with line B. And I managed to trace the other two valves to the line A. The rotor has a hole drilled through the timing groove on the shaft side, and as we saw earlier it is connected to line A. The rotor also has two more grooves machined on the balance plate side. The outer groove has several circular cavities machined into it and is connected to line A (through a drilled hole, remember), and the inner groove is not connected to anything. I think I managed to figure out why there is a need for an "extra" check valve. Since the outer part of the rotor has a bigger amplitude of movement - a single hole is not enough, because even with the extra cavities it gets blocked by the rotor in some positions. So they made two holes at an angle to make sure that line A is always connected to at least one of the check valves. That is my theory anyway, that is what the parts tell me... Look at the cycloid curve the A-line hole is doing. (the bigger one to the right, the one to the left is from the check-valve-less hole).

Now - the endplate. It has a flat ring that sits flush with its surface, and then it has two shallow cavities machined around it. The cavities interconnect because there's a smaller circular cavity machined out in the balance plate. The check-less hole, however, sits right under the flush surface of the endplate, which means that it can only let the oil go through when the balance plate is flexed away from it. I think I am entering the realm of thin oil films here. To be honest - I am not entirely sure what to make of it. Maybe when the motor is stalled and the rotor is stationary, the hydrodynamic film between the rotor and the balance plate is absent, and so the oil pressure from a work line pressurizes the endplate cavity and flexes the balance plate towards the rotor, but when it begins to move and gains hydrodynamic oil film - the forces on the balance place equalize and it flexes back straight? I am puzzled. But I still learned something - I do know where the balls connect and I do know that If you lose one you'll get a nasty cross-port leakage.

So - if used Approach One, I would probably manage to mount this motor, if I had received it in one piece and with original parts. In my case, it came in partially disassembled and with parts missing from the attempts to "fix" the leaking shaft seal, so in this particular case the risk of me mounting something "the other way around" would be very real. The only thing I'd learn from this experience would be to stay away from this brand because it's "hard and weird".

If I used Approach Two - I would manage to get the job done and I would learn how to assemble and disassemble such motors. Not more not less.

Now - by choosing Approach Three, not only did I repair the motor, but I also learned:

I assure you - the next time I work on this model - the repair will be much faster.

Unfortunately, I must end this post on a sad note...

In most industrial production environments that I have seen, such practices are regarded by the management as a punishable waste of time. I run a hydraulic warehouse and a shop, so I can totally see why, but I still consider this as a failure of seeing the bigger picture. In my opinion, such practices should be encouraged, and obtained new knowledge shared. The fact that such attempts to learn something on your own are often actively suppressed was actually one of the reasons I started this blog.

A proper hydraulic workshop should have hydraulic professionals working in it, not mindless part-fitters. Better for production, better for safety, better for egos, and in the end - better for the company!