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Thread: Troubleshooting hydraulics

  1. #1
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    Troubleshooting hydraulics

    Gleaned this tip from "The hydraulic Trouble Shooting Handbook" written & published by Brendan Casey (www.hydraulicsupermarket.com) in Perth W.A.

    If the piston seals in a hydraulic cylinder are leaking, the cylinder will only extend - it will never contract. Reason being that there is sufficient room in the cap end for the volume of oil in the rod end to move past the seals, but insufficient room in the rod end for the oil in the cap end.

    On first reading, this appears to be counter-intuitive; if there is a fixed volume of oil in a stationary cylinder, it would seem reasonable that oil could seep past a seal in either direction - but even my brain has now seen the logic of the statement

    JV

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    Re: Troubleshooting hydraulics

    Yes that is correct and not that difficult to understand if you draw a small diagram and think about it. I' ve lost count of the times I've heard armchair experts on this and other forums tell someone the reason their machine is dropping is because the piston seals have gone when it's usually a load of cobblers and can't happen unless there is a secondary escape route for the oil too (usually past the spool valve rod). The thing you really need to be aware of with leaking piston seals is the way the pressure intensifies even in a static system when the cylinder converts to a single acting one with a much smaller piston. Once you appreciate the consequences of a piston seal with even a small leak it has implications for system design and explains some things you may have thought were silly construction in the past. Sorry about the typo in the diagram untrained should be untrained. Once you think about the scenario described you soon realise why check valves are typically fitted to the cylinder end and coupled with steel pipes capable of taking many times the pressure of a flexible hose!


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    Re: Troubleshooting hydraulics

    To help explain the dramatic pressure increase from the same applied load, you need to get you head around that the "leak" in the red cylinder is allowing pressure/load to be applied to both sides of the piston and one side will cancel out the pressure/load on its opposite side, but, because they are different surface areas you subtract one from the other (oneside has the rod screwed/welded to it) so then all the imposed weight is acting on the smaller area equal to the diameter of the rod


    there, clear as mud!



    ff

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    Re: Troubleshooting hydraulics

    "there, clear as mud! ", says ff.

    No, can't agree. You've put it well. All that needs to be understood is that lbs of load & psi are different measures (or "metrics", I thnk the venacular is). The smaller a given area to which a standard load is applied, and the higher the concentration of that load, the bigger the figure - in this case the psi is bigger.

    JV

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    Re: Troubleshooting hydraulics

    Quote Originally Posted by john maddock View Post
    "there, clear as mud! ", says ff.

    No, can't agree. You've put it well. All that needs to be understood is that lbs of load & psi are different measures (or "metrics", I thnk the venacular is). The smaller a given area to which a standard load is applied, and the higher the concentration of that load, the bigger the figure - in this case the psi is bigger.

    JV

    Some good info here guys and helps explain things to someone with the physics brain of a dried walnut.

    So this information above informs us that in the event of rod contracting ram creep , and in the absence of an obvious internal leak, the fault is likely to be in the spool.

    I have a tool which demonstrated that theory. Hydraulic folding harrows - I think I inadvertently bent the ram by not using the float position on the spool so when the ram was pressurised by the outer wing forcing it closed over uneven ground, the oil on the piston side could go nowhere. I straightened out the ram rather crudely so it runs free but I assume the piston seals are damaged. It will now lift the harrow to fold but as soon as the spool pressure is off the weight of the harrows extends the ram.
    On the old telehandler, without any crank or linkage on the crowd ram and no obvious external leak, the bucket will creep to tip as the ram extends. This could be simply piston seals?

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    Re: Troubleshooting hydraulics

    On the subject of hydraulic problems (not mine BTW)


    I own a 1983 JCB3CX, when you slew in the left direction the machine operates correctly, when you slew right the arm moves extreemly slow and will not move with any load. If you extend or retracted the boom or bucket to a point where it is bypassing at the valve (and hold it bypassing) then the slew will function correctly. I have swapped the bypass spools ( i think they call them ARVs?) between the left and right direction, but nothing has changed. i have increased the distance that the control lever pushes into the valve block, no difference? has anyone ever had this problem or can anyone point me in the right direction or at the correct post? Any help would be much appreciated. Apologies if you read this post on another forum. Been looking for answers without a great deal of success.
    Thank you

    Someones cry for help on another forum.

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    Senior Member T P's Avatar
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    Re: Troubleshooting hydraulics

    Quote Originally Posted by essexpete View Post
    On the subject of hydraulic problems (not mine BTW)


    I own a 1983 JCB3CX, when you slew in the left direction the machine operates correctly, when you slew right the arm moves extreemly slow and will not move with any load. If you extend or retracted the boom or bucket to a point where it is bypassing at the valve (and hold it bypassing) then the slew will function correctly. I have swapped the bypass spools ( i think they call them ARVs?) between the left and right direction, but nothing has changed. i have increased the distance that the control lever pushes into the valve block, no difference? has anyone ever had this problem or can anyone point me in the right direction or at the correct post? Any help would be much appreciated. Apologies if you read this post on another forum. Been looking for answers without a great deal of success.
    Thank you

    Someones cry for help on another forum.
    As a short term diagnostic test swop the pipes for the slew with another service and use the lever for that to test the slew, if the fault remains in the slew then it's a ram related fault or as I suspect it will go away your fault is in the spool block. The slew function has a special anti-cavitation feature built into the valves or the arv's so it's important not to use the other lever long term as you could drive along the road with the dipper flopping left to right and wipe someone out. How this works is there is some backpressure deliberately held on the return pipe from the spools and a check valve allows a cavitating cylinder to suck oil from it so it is always full of oil both sides of the piston and never able to flop around especially dangerous travelling. This is to cater for a scenario where there is insufficient oil from the pump to fill the backside of a piston as fast as gravity or other force pushes it out of the other side. It's a sort of re-generation. You might well have a broken poppet or spring on the ant-cavitation side of things. Unfortunately there are many different spool blocks/arv's etc used so it's hard to be more precise on the location of the bits and bobs. Later machines tend to have anti-cavitation fitted to all the cylinders whereas in the early days it was only on the critical functions like slew. Sometimes the bits are inside a hollow spool rod itself. If you are really lucky it might just be a chunk of seal that has wedged a poppet open, the reason it works when you activate another valve is probably related to creating a dead end beyond the spool with the fault when you pull the second lever so the escaping oil from the first function cannot return to tank and has to extend it's rod......certainly some ideas there for you to think about/try.

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    Re: Troubleshooting hydraulics

    Quote Originally Posted by Footsfitter View Post
    To help explain the dramatic pressure increase from the same applied load, you need to get you head around that the "leak" in the red cylinder is allowing pressure/load to be applied to both sides of the piston and one side will cancel out the pressure/load on its opposite side, but, because they are different surface areas you subtract one from the other (oneside has the rod screwed/welded to it) so then all the imposed weight is acting on the smaller area equal to the diameter of the rod


    there, clear as mud!



    ff

    And the reason the cylinder cannot drop is that there is nowhere for the oil displaced by the rod to go unless the higher pressure bursts something. In practice if the seal goes on most machines there is such a loss of power that it won't lift the load off the ground, however a catastrophic seal failure with a raised load is a different ball game and why it's bad /dangerous practice to use flexible hoses between a cylinder and a counterbalance valve.

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    Re: Troubleshooting hydraulics

    Thanks TP, I will copy and paste your answer for the guy.

    The cavitation on our old depart hymac dipper ram could be a bit unnerving in certain circumstances.

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    Re: Troubleshooting hydraulics

    I hv only one question , for cylinder which its piston does not have a bypass , if the cylinder is loaded and it is fully extracted , then the oil flow is stoped? .
    I jv a hysradulic ram , it is single acting , when its drain is closed and supply is open , its piston is moving and get full extracted. after this point , it keeps open . the oil flow through it is stoped?



    [QUIOTE=T P;235099]Yes that is correct and not that difficult to understand if you draw a small diagram and think about it. I' ve lost count of the times I've heard armchair experts on this and other forums tell someone the reason their machine is dropping is because the piston seals have gone when it's usually a load of cobblers and can't happen unless there is a secondary escape route for the oil too (usually past the spool valve rod). The thing you really need to be aware of with leaking piston seals is the way the pressure intensifies even in a static system when the cylinder converts to a single acting one with a much smaller piston. Once you appreciate the consequences of a piston seal with even a small leak it has implications for system design and explains some things you may have thought were silly construction in the past. Sorry about the typo in the diagram untrained should be untrained. Once you think about the scenario described you soon realise why check valves are typically fitted to the cylinder end and coupled with steel pipes capable of taking many times the pressure of a flexible hose!

    [/QUOTE]

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