"Wear rings" on a hydraulic piston
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    Default "Wear rings" on a hydraulic piston

    This photo shows the piston that drives the long axis of my J&S 540 surface grinder. I pulled it out because it felt sticky.



    If you look carefully at the photo, you can see why the piston felt "draggy". The piston is ground and hardened with two grooves. The grooves carry split rings, also hardened and ground. The rings are meant to "float": rotate and slide freely in the grooves. The left-hand ring is OK. The problem is that the right-hand ring is stuck in its groove. Easily fixed with some judicious stoning and fitting.

    A few minutes with Google shows that these rings are called "wear rings" and are often made from PTFE or other slippery plastic.

    What's the function of these rings? Does it make sense to keep the existing ones (after they are both free to float)? Or can/should I replace them with modern plastic ones?

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    6 of one, 1/2 dozen of the other. How does the inside of the cylinder look? I hope you let the piston shaft where it attaches to the table loose so it floats in that hole. To correct any misalignment after wear and your scraping. That's why they have double nuts on each side or a nylon lock ring nut so there is .010" or so slop. If the inside of the cylinder is worn or scored, take it to a local hydraulic cylinder re-builder, have it honed and ask them what they recommend.

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    Hi Richard,

    Thanks for the quick answer.

    Quote Originally Posted by Richard King View Post
    6 of one, 1/2 dozen of the other. How does the inside of the cylinder look?
    I don't know yet. I'll pull it off, clean the inside, and have a good look. The machine has had very little use so I am hoping that in spite of the stuck wear ring, the cylinder is still in good shape.

    I hope you let the piston shaft where it attaches to the table loose so it floats in that hole. To correct any misalignment after wear and your scraping. That's why they have double nuts on each side or a nylon lock ring nut so there is .010" or so slop.
    I have not done any scraping to this machine. It was pristine when I got it and has stayed that way. But I did have a problem soon after I got it, when I realized that it was "lifting" slightly at one end, because the piston rod was not aligned properly. It had one normal nut on each side, and was captured tight with no float. I fixed that at the time (probably based on your advice, in fact.)

    If the inside of the cylinder is worn or scored, take it to a local hydraulic cylinder re-builder, have it honed and ask them what they recommend.
    I'll have a good look. Fortunately, if something is needed, there's a company a mile away from me which does exactly this kind of work, and the people there are friendly, helpful, and competent.

    Cheers,
    Bruce

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    OK, a few facts. The cylinder bore is 1.125" ID, and the piston OD (without wear rings) is 1.120". The wear ring groove is 0.99" OD, and the wear rings themselves have a thickness of 0.048". So there is quite a bit of "wiggle room" there.

    Cylinder bore looks quite clean, particularly the center 2/3. The last 1/6 on each end has a less "polished" appearance, probably because has not been used as much.

    I stoned off the burr and sharp edges from the wear rings and the grooves. One of the wear rings was a nice fit in its groove, but the problematic one was a bit too wide and had a tendency to jam. Finally I chucked the whole rod and piston assembly (with some copper around it to protect the finish) into my lathe, spun it slowly, and used some "very fine" green timesaver lapping compound to lap the ring into a nice fit in the piston.

    Cleaned up, reassembled, and then tested it without the table. Definitely smoother than before.

    But I noticed one odd thing. If I run the piston to the end of travel, all seems ok. It lands at the end with no drama. But when I then reverse the direction lever, the piston "hangs" for a moment, then "pops free". That is something I have observed before, when the table is driven to the end, it's not so smooth coming back.

    Maybe when the piston is reaching the end of travel in the cylinder, it's coming up against a stop which is canting or tilting it in the bore. That might also explain how the wear ring got jammed into its groove.

    Has anyone seen this sort of behavior before? My Studer grinder does not do that. I can run the cylinder to the end of travel, and when I reverse it, it glides smoothly out from start to finish.

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    Nope, mine doesn't do that. Could the bore be bigger in that area where it reverses? Maybe it's "slipping" a little there? How does it feel if you drive it to the end under power then reverse with the handwheel?

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    Its a bit more complex than you suspect ....the rings float so as to not transmit any displacing movement to the table.....if you replace the steel rings with plastic,even the hard micarta type ,Im sure the ring will get trapped between the cylinder and piston.A hydraulic shop will not understand unless they deal with machine tool hydraulics,and go bull at a gate with a tractor standard repair .......I think you will find J&S did know what they were doing.......The smaller cylindrical extensions on the piston may be hydraulic shock absorbers ,so the metal parts dont clash ,and the hesitation may be suction in one of the ends .....possibly a ball valve is stuck .

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    Usually there is a dwell at the end of the travel before it reverses. Ballen is in Germany and there are many shops who repair machine tool cylinders. Call them and ask around. Call a machine rebuilder and ask them who they use. Have a Hydraulic print? Have an operation sequence chapter inside your Maintenance manual?

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    Richard, John, thanks for your comments.

    Richard -- here is a hydraulic print from the manual - I hope the PM software does not shrink this into unreadability.



    Parts 14/15/16 are missing from my machine, which does not have a hydraulically-tensioned cross screw. Anyway, it's irrelevant for the table cylinder. Yes, there is a sort of "dwell" at reverse, controlled by the needle valve shown at the bottom of the table reverse valve 3.

    Unfortunately I would have a hard time locating someone locally who knows these machines well. Since there are so many J&S 540s out there, I think it's more likely that one of the forum members whose has been inside theirs can help.

    As a very first step, I want to confirm that the behaviour I am describing is not "normal".

    John -- thank you for your comments. What you write makes sense, and is exactly why I am asking for advice. There's a people in this group with a lot of specialised knowledge, and I am trying to learn from it.

    I don't see any one-way valves in the system apart from one at the pump. Also, when the piston is at the end of travel and reverses, I can feel the "pop" at the end of the cylinder more than in the hydraulic lines. Pretty sure it's mechanical interference inside.

    This weekend I will pull the end cap off the cylinder and check some basic things. Like, is the bore which holds the rod seal concentric with the cylinder? Is the piston rod concentric with the piston body? I have seen enough "monday morning specials" on this machine that I can't exclude these things without checking.

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    This has been bugging me so I spent twenty minutes and pulled it apart again to have a closer look. I think the problem is simple. The piston is a fairly close fit in the end cap:





    There's at most 0.001" or 0.002" of clearance there (0.03mm-0.05mm). That's already an issue, because the OD of the piston body is 0.005" smaller than the inside of the cylinder. So it can "float" to a position that would interfere with the end cap. On top of that, the cylinder has an ID of 1.125", but the end cap is also about 0.006" smaller. So there's even more room for misalignment.



    With that tolerance, it's only the O-ring holding the end cap concentric with the cylinder ID. In addition, the end caps are not located with fiducial dowel pins. They are held down with normal clearance cap screws. Which means that the end block can be cocked with respect to the cylinder.

    I think what's happening is that during the last half-inch of travel, as the piston body enters the end cap, it's rubbing against the sides of that cap. Simple fix would be to increase the clearance inside the end cap by another 0.010", but I don't like that very much, because it might lead to the piston rod being off center and wearing the rod seal. I'd rather try and get the end cap properly centered and axially aligned, and then increase the clearance slightly. So I need to figure out a good way to build up the OD of the end cap by 0.005".

    The design is not what I was expecting. I had thought that as the piston enters the end-cap, it should progressively block off the exit for hydraulic oil, providing a sort of automatic braking or damping. But that's not the case. The oil hole is beyond the reach of the piston, even when the piston is fully in the end-cap. So this design allows the entire table and magnet (weight is probably 80kg = 180 lbs) to crash into the end block at full speed without any kind of damping. Doesn't seem very prudent.

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    What about just adding a small 10° or 15° lead-in bevel on the ends of the pilot diameters? Can't tell from the pictures but is there already a bevel there? It sure doesn't look like they were crashing into each other... I would be surprised if J&S didn't have that figured out long ago - unless there's wear that's causing further misalignment. It's a shame that most manufacturers never put dimensions in the parts manuals for wear parts.

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    Are you sure the piston entering the end cap isn't providing the cushioning you were expecting to see? If the oil is leaving the barrel through the end cap it will have to flow through the restriction created by the small clearance between piston and end cap? The clearance you mention is in the ballpark for the kinds of clearances we use when cushioning. Cushioning like this can also cause the slow extension you mention as oil flow is restricted while the piston is in the end cap. If we want to avoid this in a regular cylinder a bypass mechanism is required.

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    I was thinking the same. I wonder that the small clearance there wouldn't cause the piston to self-center in the bore too. I would think the flow of oil would tend to pull the piston to center.

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    The close fit of the piston end and end cap is there for a reason. It's to act as a shock absorber if the operator tries to run the table into the end-stops. Without it, there would be loud banging sounds and the risk of damage/work shifting on the chuck...


    One shouldn't be running the table as far as the end stops in normal use. How is it behaving with a smaller didtance of travel?

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    Quote Originally Posted by eKretz View Post
    What about just adding a small 10° or 15° lead-in bevel on the ends of the pilot diameters?
    There is a lead-in bevel for both parts. And since they are cushioned in oil they do slide in. But there's a bit of stick-slip friction, and it gets cocked in there, hence the draggy feel and the popping-out behaviour.

    It's not that this is non-functional, and the effect is just some minor misalignment at the end of the travel. But this is a grinder -- if I can see it or feel it, it will reduce accuracy.

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    Quote Originally Posted by tonymor View Post
    Are you sure the piston entering the end cap isn't providing the cushioning you were expecting to see? If the oil is leaving the barrel through the end cap it will have to flow through the restriction created by the small clearance between piston and end cap?
    That's how I was expecting it would work. But that's not how it works. The piston can only enter the end cap about 0.5". Then it is blocked by a shoulder. Beyond that shoulder is another 0.75" of buffer volume with a clear passage to the exit port.

    In other words you can run the piston all the way to the end, and there is still fluid beyond it with an unobstructed path to the hydraulic piping. The piston entering the end cap does not provide any additional cushioning or resistance to flow.

    In fact, once the piston enters the end cap, the effective cross sectional area is reduced. This means that it has the opposite effect of braking. For a given amount of resistance to fluid flow set by the valve, it gets EASIER for the table to move a certain distance (since the fluid displaced per unit of table motion has decreased).

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    Hi Mark,

    Quote Originally Posted by Mark Rand View Post
    The close fit of the piston end and end cap is there for a reason. It's to act as a shock absorber if the operator tries to run the table into the end-stops.
    Nope. The close fit of the piston and end cap is no different than the close fit of the piston in the cylinder. It has NO shock absorbing effect at all. When the piston has fully entered the end cap, there is fluid beyond it with an unobstructed path to the exit.

    So (unless there is a part missing from beyond the piston on my machine, which blocks off the hydraulic port) there is NOTHING that prevents the operator from running the table into the endstop at full speed.

    Does your 540 have something which prevents that??

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    I know I could be reading this wrong as all i can see are the pictures.
    There is still a volume of fluid between the piston seals and end cap as the shoulder of the piston enters the end cap. This volume of oil must escape between the piston shoulder and end cap. I am assuming the oil exits through the end cap?
    The alternative would be if the oil exits the tube and not the end cap, the volume of oil inside the end cap would have to escape past the restriction?

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    Quote Originally Posted by tonymor View Post
    I know I could be reading this wrong as all i can see are the pictures.
    Quote Originally Posted by Mark Rand View Post
    The close fit of the piston end and end cap is there for a reason. It's to act as a shock absorber if the operator tries to run the table into the end-stops.
    Perhaps this photo will clarify things. This is the end cap, looking from the end where the piston enters.



    After it enters the end cap fully, the piston stops on the outer shoulder of this part. The piston does not extend past the inner shoulder visible as a thin ring in the ID. The exit hole for hydraulic fluid is BEYOND that inner shoulder. So the piston NEVER blocks (even partially) the exit hole for the hydraulic fluid. There is no damping, no braking, no buffering.

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    From what i can see, the oil in this cavity here still has to escape once the shoulder enters the end cap. The piston does not have to block the actual port hole, it just has to restrict enough oil flow to cause a pressure increase in some volume of oil that will oppose motion. On the out stroke, oil will have to fill this cavity as it extends out. That oil needs to flow out from the end cap, between the piston shoulder and end cap and then into the cavity.

    tempsnip.jpg

    Floating cushioning design is quite common in our industry. Typically, one of the materials will be harder than the other to prevent damage. The small lead in angle will allow the piston head to self align as it enters.

    Once again I know i may have this completely wrong so i'll leave it here. I won't keep telling you you have cushioning. It just looks like it from here

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    Quote Originally Posted by tonymor View Post
    From what i can see, the oil in this cavity here still has to escape once the shoulder enters the end cap. The piston does not have to block the actual port hole, it just has to restrict enough oil flow to cause a pressure increase in some volume of oil that will oppose motion.
    Please see the previous post, just before yours. The oil in that cavity escapes through the oil port at the end. The piston does not block or restrict that port in any way, even when it is fully inside the endcap!


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