Insane Hydraulics
Bold and audacious blog about fluid power
This post is called "Rings, Pins and Springs" but I seriously considered naming it "My Love-and-Hate Relationship With Hagglunds." The key word here is, as always, "my" - that is to say - everything that I will present in this article should be regarded as a biased opinion of a self-educated grease monkey, who, by the will of fate, has been overhauling identical Hagglunds tandem motors for about a decade with an average rate of a motor per year. Barely enough to have a formed opinion about a single series - namely - the CA, which, in my case, most of the times is represented by a CA-50 piggybacking a CA-210:
We operate near underground mines, hence - lots of ventilation shafts - lots of raise boring rigs - lots of high-torque hydraulic drives. I can see all the reasons why an OEM would choose a Hagglunds radial motor for their rig - Hagglungs happens to be one of the best manufacturers of compact high-torque radial-piston hydraulic motors available with hollow-shaft configuration, which is super convenient for a raise-boring rig because the high torque reduces the gearbox to a single stage while the hollow shaft allows running the water line straight through the motor.
These motors are compact, strong, and reliable. They come from a place that has been making radial hydraulic motors for ages, and they are backed by the mighty Rexroth itself. What else could you possibly want? I myself would totally choose a Hagglunds radial motor for my rig (if I had one)! And I must say that I love everything about these motors - absolutely everything... except for the said rings, pins, and springs.
But before I begin grumbling about the specific ways the Haggluds CAs have been grinding my gears, I want to give you some context. The tandems that we get in our shop are employed in raise boring rigs - which means that they face very special conditions. First of all - they have to run at high rpm during the pilot hole drilling, and low rpm during the reaming stage - which pretty much mandates the use of free-wheeling on the larger motor. Second - they are mounted vertically, with the shaft facing down. And third - they drive a quarter-mile-long pipe string which is, essentially, a torsional spring that can store (and violently release) tremendous amounts of energy. The combination of these three factors makes the life of these motors especially miserable and occasionally pushes them well beyond the factory-recommended conditions.
So, let us start with the rings.
There actually will be two types of rings, and the first ones that I want to show you are the wear rings of the shaft seals (unfortunately, the pictures can't show how deep these wear gooves are):
Every CA motor carries two shaft seals and two removable wear rings (parts nº 12 in this cutaway view):
I love the fact that somebody thought of making the wear rings replaceable - but they made them of rather soft steel! I opened these motors with a mere 500 hours to check on the wear and was greeted with deep grooves left by seals sliding on the sealing surface of the wear rings. I realize that wear rings are supposed to wear out, but I don't think they should be wearing out that fast!
And before you say something about the possibility of the case pressure being too high - let me show you something else. As I said - these tandems need to run pretty fast during the pilot hole stage, and it's not easy to achieve with the classic high-flow route. For example - a CA210-180 + CA50-50 tandem would have a combined displacement of about 14 liters, which means that running it at 140 rpm would require a flow of 2000 l/min (530 gpm)! Luckily, Hagglunds (like all respectable radial-piston motor manufacturers) provides a free-wheeling manifold option for the CAs, which connects both of the motor lines to the tank and allows for retracting the pistons into the shaft by raising the case pressure to 1.5 bar. When the pistons retract, the motor becomes essentially a shaft connecting the piggyback motor to the gearbox, and, in theory, should have minimal wear during this stage (which, unfortunately, is not the case with the CAs - but more on that later).
So - what does this mean for our rings? This means that the wear of the rings in a free-wheeling CA210 is aggravated by the higher case pressure, and also the fact that the front shaft seal of the 210 is a lot bigger than the one in the 50. Now that you know all that - take a look at the wear rings from the CA210 from the tandem that you saw in the first picture (the motor that I opened a couple of weeks ago) - and I will even place them next to the worn-out rings from the CA50. Notice anything strange?
This image shows you the large ring from the CA210, this one shows the wear ring from the CA50 (on the left) next to the tail wear ring of the CA210 (on the right), and this picture shows the four wear rings together - (once again, the two on the left are from the CA50). Hold on a second... Where's the promised aggravated wear? Indeed - there's virtually no wear on the 210 rings, despite the increased case pressure and the higher linear speed of the larger seal, and here's why - the last time I serviced this tandem (some four years ago, I reckon) the CA50 got the new original wear rings, while the 210 got the old rings repaired with HVOF coating. Ever since we discovered the excessive wear and the fact that a simple coating makes the rings virtually indestructible - we stopped getting the new rings! So yes - the original wear rings don't cut it!
And another thing (and this is the mechanic in me talking) - the rings are replaceable all right, but you need to dismantle the whole freaking motor to replace the front shaft seal! If you decide to build a hydraulic motor with replaceable wear rings for the shaft seals and a tendency to chew through them "with a will" - please design it in a way that allows a mechanic to replace both the seals and the rings without dismantling the unit!
Now - let me show you the second ring type that grinds my gears - the intermediate rings that are placed between the cam rings (obviously, this only applies to the CA210, which is three-cam-rings-high, and therefore carries two intermediate rings, one of which doubles as the mounting interface):
As I said before - when a radial-piston motor is free-wheeling, it is supposed to have zero wear, because when the pistons are retracted the motor turns into a bearing-supported shaft. Unfortunately this is not at all the case with the CA210 when it is mounted vertically. And the culprit is its multi-cam-ring construction. This is a piston assmebly from Hagglunds CA motor:
The rollers are loose (i.e. have no lateral support) and so when the pistons are retracted during freewheeling and the motor is mounted vertically, the weight of the rollers pushes them down. However, while the rollers of the bottom cam ring are sliding over a wide surface of the front flange, which gives them enough support, the rollers of the middle cam ring are supported (and guided) only by the very narrow annuli of the intermediate rings - about a millimeter wide or so. The weight of 16 rollers applied over a narrow edge in a cyclic manner is already bad, but the rotation at higher rpm makes it worse because the rollers tend to tilt and wear not only the bottom intermediate ring but also the top one as well. At least that's what I've been seeing - the worst wear is (quite surprisingly) always on the bottom side (the side that's facing down) of the top-most intermediate ring. The piston rollers literally roll nasty grooves into the edges of the rings, and the displaced steel is then torn into hard fragments during normal operation - and these particles leave pretty deep scratches on the sliding surfaces:
Current Hagglunds manual clearly states that "...Freewheeling in vertical position > 70 rpm may increase the risk of wear in multi cam ring motors (CA 100/140/210). For support regarding increased robustness in vertical freewheeling, please contact your Bosch Rexroth representative..."
I would rephrase this to "Freewheeling in vertical position greatly increases the wear in multi cam ring motors (CA 100/140/210)." This is a fact.
As for the "increased robustness" option - it does exist and it does mitigate the problem to some extent, and it consists of two additional rings, made of very hard plastic, that are inserted inside of the intermediate rings and essentially transfer the weight of upper rollers onto the lower ones and provide wider guiding surfaces (when the pistons are retracted during free-wheeling):
In my experience - the plastic rings help, but the intermediate rings still develop the rolled-in grooves (and the scratches) over time, which means these parts must be regularly replaced (at an eye-watering price, might I add). Seems a shame to have a part that must be replaced every 8000~ish hours in a motor that otherwise would be able to run some 40000 hours problem-free.
Now, let us move on to the pins.
Multi-cam CAs use normal 16x40 slotted spring pins to align the cams and intermediate rings - and I don't like it at all! First of all, driving the pins through the parts always leaves marks that you need to flatten out on each repair, and second - every time I open a CA210, I find most of the pins in shatters:
It seems the cam rings expand and contract, and this cycling load eventually cracks and shatters the hard pins. Solid pins would do so much better here.
And finally - for the springs (be prepared to get blown away because I saved the best for last).
I am talking about the bias springs that live inside the narrow oil channels under the valve plate (one for each of the 20 ports):
Each helical spring is placed under a cap, that provides the hydrostatic interface necessary for balancing the valve plate's compression against the cylinder block - and you may ask "What's wrong with that?" My reply is - balancing with caps is fine, but placing a relatively weak helical spring inside an oil channel is only OK up to a certain flow rate. A high stream of oil flowing through the spring center can cause it to chatter, deform, and eventually, it can even yank it from its place! The HPU, of course, can not create enough flow to even begin disturbing such a spring, but a violent backlash can! It's like pointing a pressure washer jet at a spring if you will. I would not have believed that something like that was possible had I not seen it with my very own eyes! Look closely at these images (old pics from a different motor):
Some of the springs are deformed, and some of the springs are missing altogether. Yes - you read it right - freaking missing! I pulled one of the springs (in an incredibly deformed state) from one of the logic elements of the anti-spike manifold. And after I've done that - I declare that placing helical springs inside an active oil channel is a big no-no!
So, there you have it - my love-and-hate relationship with Hagglungs, which could have been pure love had it not been for the rings, pins, and springs.
These motors are tanks and the "design flaws" that I just described do not apply to normal applications when these motors quietly turn horizontal shafts at constant speeds for decades. And yet, I can't stop wondering how much "tankier" would they become, if some intelligent engineer found a workaround for these... "frailties".