"Wear rings" on a hydraulic piston - Page 2
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  1. #21
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    Yeah i guess i must be missing something. I just cant see how, once the shoulder begins entering the end cap, the oil from the barrel can flow between the shoulder and end cap( with "at most 0.001" or 0.002" of clearance there") without restriction.

  2. #22
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    This is where my head is at. Might help clarify where I'm going wrong.
    capture.jpg

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    Very sorry! All of you are right -- I had forgotten about the "red pool" of oil in the previous drawing. Was only thinking about the "green stuff".

    I'll focus my efforts on making the end cap and the cylinder concentric, so that the piston can enter the end cap without any forces other than those from the oil in the red pool.

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    I see the cushioning plain as day. what I can not wrap my head around is how it sticks and pops free when reversed. I dont think the cushion should be working in reverse since the oil in the end cap can push on the end of the cushion piece and not have to "reverse cushion" by flowing into the tiny clearance created by the cushion.

    Bruce, the cushion is the oil flow being slowed as the piston ring body that extends past the rings goes into the end cap, this will allow only a tiny bit of oil to flow from the main cyl tube into the body of the end cap where the port is. This set up is supposed to cushion on the way in but not out, since the port is never blocked off it has full flow to push in reverse. If anything I would think it would move faster until the cushion plug is out of the end cap, not stick....

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    Rob, when the pressure is reversed, isn't there a tendency to create a "vacuum" in the red area of post #22? After all, that volume is increasing, but to fill it, oil has to move past the 0.001" or 0.002" gap. The cavitation requires 1 atmosphere of pressure to create a vacuum there, and the oil pump here is providing 7 bar.

    I'm guessing that the problem is only that the end cap and the cylinder are shifted from concentric, and that once they are concentric, everything will work as it should.

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    The volume of oil that will cause cushioning on the way in will also need to be replaced on the out-stroke. This oil must flow past the restriction. If the piston moved out without the cavity being filled with oil you would be creating a void(or something stranger, maybe a black hole..)

    My money is on the cylinder performing exactly as it is designed. That slow lift off you see is typical for that kind of cushioning.

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    Ballen, I think what you're surmising might happen if there was no oil pressure pushing on the side of the piston that was inside the end block, but in this case, once the table reverses, the hydraulic system is providing over 100 psi or whatever of oil pressure to that side of the piston... It won't create any suction on the way out.

    Is there any evidence of rubbing on those pilot diameters? What about the inside diameter of the snouts on the end blocks? If that is what is causing your sticky feeling I would think there would be very clear evidence that there's rubbing going on there.

    The B&S uses the same setup for end travel cushioning. If you like I'll measure mine and see how they compare.

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    Quote Originally Posted by ballen View Post
    Rob, when the pressure is reversed, isn't there a tendency to create a "vacuum" in the red area of post #22? After all, that volume is increasing, but to fill it, oil has to move past the 0.001" or 0.002" gap.
    Not sure how I missed that post 22 earlier?

    Yes, it would create a vacuum like you are thinking and I was thinking that must be where the initial stick is from. If you are not experiencing any cushioning when it gets to the end of the stroke maybe the clearance is to tight to allow proper flow for a good cushion. If it is to tight a clearance it would just slam, and then not want to reverse. It seems you should see what the clearance should be, and what oil should be used. I would think that to thick an oil might give the same symptoms. Can you test it with thinner oil?

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    Correct oil is Vacuoline 1405/Vactra 1/ISO 32.

    The vacuum feature is only relevant if you are cranking the table by hand. If using the hydraulic feed, then the "red volume" gets filled in the same way that the 'green volume' does, it's juts that the transfer of oul fromred to green is what controls the speed of the movement.

    As I said before, the behaviour at the very end of the stroke is there for protection. Is the behaviour correct with a smaller travel? That's what is important when grinding.

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    I'm very grateful for this discussion, because I am gradually understanding how it works and what's going on. Thank you all for your patience with me!

    To do more testing, I did something very crude: I cut two shims out of 0.003" (0.075mm) shim stock and made one wrap around each of the end blocks before inserting them into the cylinder. This centers them correctly on the cylinder. That, together with freeing the stuck wear ring has now eliminated the "stickiness" in the travel.

    I can also confirm that the behaviour at the ends of the travel is exactly what everyone here predicted. If I open the speed valve to max, and run the piston right into the end cap, the last 0.4" (10mm) of travel is damped by the piston going into the end cap, just as was explained to me over and over again until I finally understood.

    Last point: when the piston is at the end of travel (cushion used up) and I flip the direction with the speed control at max, there is a moment of hesitation where nothing moves. Then there is a big "pop" and the piston leaps 0.4" towards the center of the cylinder. After that it moves steadily.

    What is happening on the reversal is the following. The pressure is building on the "fill" side of the piston because there is not time for the hydraulic fluid to squeeze through the constriction around the piston. The pressure increases until it exceeds 1 atmosphere. At that moment the red space in post #22 cavitates (becomes a vacuum) and the piston jumps 0.4" to just past the constriction. The vacuum fills with fluid at the speed of sound and controlled motion begins.

    The "jerk" that I see and the "pop" that I hear is the cavitation taking place (jerk) and the cavitation volume filling (pop). If the table is traversing relatively slowly, this doesn't happen, because there is no cavitation, the volume is always filled with fluid.

    If anyone is interested in seeing this, let me know and I will post a few seconds of video.

    The clearances in the "buffer" region are tight enough that if the end caps are not properly centered on the cylinder, then the piston is "unbalanced" in that region and cocks slightly in the bore. I suspect that this is what made one of the wear rings burr the piston and stick in its slot. Or perhaps it was just a lack of fettling at the factory.

    A design which provided damping at the end stroke but eliminated cavitation on the reverse stroke would be nice.

    Since someone asked, the oil is Shell Tonna S3 M32, which is the correct type and viscosity for this machine. Also, I took out my micrometer and bore gauge and checked: the end cap is 0.002" larger in diameter than the mating part of the piston.

    eKretz: there is clear evidence of rubbing on the left side. If you look at the pool of oil around the left end of the piston, you will see that it is black. That's because the rod was exerting uneven pressure on the rod seal because of the mis-centering. The wear from the rod seal has produced black dust which is discoloring the oil gathering under the table.

    Tomorrow I'm going to set up my (self-built) Kingway alignment tool and make sure that the cylinder is parallel to the long travel in both the horizontal and vertical planes. Easy enough to adjust if it's not correct.

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    Just measured the end part of the piston on my machine, it's .994" and the part of the end block where it enters is 1.0005" so about .0065" clearance - the main fit has about identical clearance, 1.371" piston O.D. and 1.377" bore I.D.

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    Quote Originally Posted by eKretz View Post
    Just measured the end part of the piston on my machine, it's .994" and the part of the end block where it enters is 1.0005" so about .0065" clearance - the main fit has about identical clearance, 1.371" piston O.D. and 1.377" bore I.D.
    Does your grinder use 32-viscosity oil or heavier? Is it 6" x 18" travels?

    I'm wondering if the clearances in mine are a bit too small. The piston is ground and I suspect that is correct on mine (and has a main fit which is 0.005" under the bore size, similar to yours). But the end cap is 0.006" small on the OD. It makes me wonder if the lathe operator had the settings off, and made the ID too small by the same amount. That would explain a lot!

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    Quote Originally Posted by ballen View Post
    Does your grinder use 32-viscosity oil or heavier? Is it 6" x 18" travels?

    I'm wondering if the clearances in mine are a bit too small. The piston is ground and I suspect that is correct on mine (and has a main fit which is 0.005" under the bore size, similar to yours). But the end cap is 0.006" small on the OD. It makes me wonder if the lathe operator had the settings off, and made the ID too small by the same amount. That would explain a lot!
    Yes, 32 viscosity. Mine is using Vacuoline 1405. It's 8" × 18" travel.

    I'm not sure about the chances that the clearances could be off, the larger manufacturers usually had pretty good Q.C. - but I suppose it's possible. That is definitely tighter than mine, and mine shows no signs of wear. I should think that the o-ring would be enough to center the tubing/pipe that acts as the cylinder pretty well. And anyway, the rod going through the end block should keep the piston centered pretty well as it approaches the block. If the fit on your machine is that tight in the cushion area, and it's possible to cock the end block due to slack in the bolt holes, maybe it's hanging up and getting tight as it goes into the bore crooked like you were thinking. Mine has dowel pins for alignment on one of the end blocks, but the other floats. However, there's barely any slop in the bolt holes.

    I sent off an email to a B&S field service tech I've been talking to regarding clearances and fits on the pistons and cylinders, if he responds I'll let you know what he says and also ask him about the cushion fits.

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    Quote Originally Posted by eKretz View Post
    I sent off an email to a B&S field service tech I've been talking to regarding clearances and fits on the pistons and cylinders, if he responds I'll let you know what he says and also ask him about the cushion fits.
    Thank you! The machine sounds similar in size and so the amount of cushioning needed to avoid a bad crash should be around the same. How long is the part of the piston that fits into the damping region? What's the maximum table speed? On my J&S540, it's 18m/minute or 300mm/sec = 12"/second = 60 ft/minute.

    Regarding QC, I've found a number of issues on this machine which should have been apparent during at least three or four stages during manufacture, but which were ignored. So I think J&S in the mid 1980s did not have a good quality culture in their factory. They just wanted to get the machines out the door.

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    That part of the piston is about ¾" long. The maximum table speed is spec'd as 100Ft./minute, or ~30M/minute.

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    For my machine that part of the piston is shorter: 3/8" and the speed is slower.

    I've put everything back together and the feel/sound are much better than before. I think the stuck wear ring and the off-center cylinder were to blame. Piston clearances seem OK.

    Everything now feels free, except the last 3/8" of travel where the piston runs into the cushion area. I can run the table at max speed into the end of travel and this provides a safe landing. The part that bothers me is the cavitation pop/jerk when it reverses. I wouldn't be surprised if a later version of this machine incorporates a relief valve for the return stroke.

    Now that I've been running the machine at max travel speed, I've noticed something else odd: when under max load, something in the hydraulic system (pump, relief valve) produces a pretty strong vibration at a few hundred Hz. I'm going to start another thread about it.

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    Is the sticking on one end or both ends of the travel? Did you measure the end cap at the other end of travel and if so are both ends the same?

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    Ballen, If you're interested I could talk you through a fairly simplistic bypass mechanism. Only requires drilling a few holes, a roll pin and a steel ball.

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    Hi Rob,

    Quote Originally Posted by Rob F. View Post
    Is the sticking on one end or both ends of the travel? Did you measure the end cap at the other end of travel and if so are both ends the same?
    I only measured one end (left second end block in place) but the feel/behaviour is identical at both ends.

    Let me describe how it feels when I drive the table by hand to either end and then back. The last half-inch before reaching the end of travel is very stiff. That's OK: it's the cushion which took me so long to understand. The problem (at least in my mind) is after I have reached the very end of travel and reverse. If I proceed very slowly, all is OK. There is a half-inch of stiff travel (cushion in reverse) then all is OK. But if I proceed at normal table-traverse speeds, there is a jerk/pop in that half-inch. That's the red region on post #22 cavitating then filling with hydraulic fluid. I can not be sure that this is how it is supposed to work, but can't see the design doing otherwise.

    Perhaps the intent of the J&S designers was that this last half-inch of travel should never be entered/encountered except in cases of operator error. But it makes it very fussy to set the stops if you want to use the full capacity of the machine with a reasonably rapid traverse.

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    Quote Originally Posted by tonymor View Post
    Ballen, If you're interested I could talk you through a fairly simplistic bypass mechanism. Only requires drilling a few holes, a roll pin and a steel ball.
    Thanks, I'll send you a PM now.


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