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Old 09-10-2019, 07:32 PM   #1 (permalink)
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Checking for hydraulic cylinder bypassing internally?

Not sure if this is the right forum but i figured shop and tools was the closest.

I have an older Komatsu PC-50UU1 mini excavator.

I might be imagining shit but i think the curl function has gotten significantly weaker. Fluid is at the correct level and isn't losing any so i think the pistion is bypassing internally.

I have no idea how to check this but i was thinking if i could unscrew whatever line is the retract function (bucket un-curl) and then curl the bucket like i normally would against a log and my thumb to pinch it in place i could see if the cylinder was leaking internally by seeing if any fluid comes out of the hole in the piston where i disconnected the line because that would be it bypassed internally.

Would that work or no?
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Old 09-11-2019, 02:09 AM   #2 (permalink)
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If it was bypassing internally then it must be creeping?

You could also install a ball valve on each hose, put the bucket in some position, close the valves and see if the bucket moves over a period of time. You could also use a chain block or ratchet strap to add tension to speed up the creeping.

I've got both bucket and boom creep on my skid steer and I've been planning to try the ball valve trick to see if it's internal to the cylinders or in the valve bank.
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Old 09-11-2019, 08:37 AM   #3 (permalink)
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pressure setting might need adjusted. see that with thumbs a bit
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Old 09-11-2019, 11:12 AM   #4 (permalink)
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Quote:
Originally Posted by Muckin_Slusher View Post
If it was bypassing internally then it must be creeping?

You could also install a ball valve on each hose, put the bucket in some position, close the valves and see if the bucket moves over a period of time. You could also use a chain block or ratchet strap to add tension to speed up the creeping.

I've got both bucket and boom creep on my skid steer and I've been planning to try the ball valve trick to see if it's internal to the cylinders or in the valve bank.
That won't work with a single-ended cylinder, the volume/displacement doesn't stay the same even if it creeps. The open side takes more volume to fill than the shaft side so as it extends it'd have to pull more fluid from somewhere...you'd effectively be preventing your creep via vacuum.
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Old 09-11-2019, 02:58 PM   #5 (permalink)
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Quote:
Originally Posted by vetteboy79 View Post
That won't work with a single-ended cylinder, the volume/displacement doesn't stay the same even if it creeps. The open side takes more volume to fill than the shaft side so as it extends it'd have to pull more fluid from somewhere...you'd effectively be preventing your creep via vacuum.
Good point. Thanks.
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Old 09-11-2019, 03:12 PM   #6 (permalink)
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if the system spends a lot of time operating at relief pressure, the heat generated at the relief can weaken the spring over time and make the relief setpoint lower. Try replacing the relief first.
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Old 09-11-2019, 03:56 PM   #7 (permalink)
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The couple I've had (lift cylinder on a loader specifically) would seem to operate normally but then suddenly drop about 1" when you released the valve. Having enough bypass to make it significantly slow in movement would require the packing to be pretty much not there at all, and even then it should be load dependent. I'd suspect a valve issue before internal bypass.

If you put a gauge on the lines you should be able to see pressure essentially instantly equalize when the valve is released if it's truly bypassing.
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Old 09-11-2019, 04:13 PM   #8 (permalink)
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sounds like piston o-ring needs changed
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Old 09-11-2019, 06:01 PM   #9 (permalink)
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Actuators that I mess with typically have a case drain line back to tank, it is low pressure, the amount of fluid coming out of that port is a direct indication of the seal condition.

You say the the Curl function is weak, If it can't make load that also indicates actuator needs rebuilt.

You are gonna make a big fucking mess, done yet
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Old 09-11-2019, 09:21 PM   #10 (permalink)
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pressure setting might need adjusted. see that with thumbs a bit
It's a manual thumb. I've been wrong before but i don't think this thumb is causing me any issues.
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Old 09-12-2019, 08:26 AM   #11 (permalink)
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if the system spends a lot of time operating at relief pressure, the heat generated at the relief can weaken the spring over time and make the relief setpoint lower. Try replacing the relief first.
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It's a manual thumb. I've been wrong before but i don't think this thumb is causing me any issues.
what isbj60 said. the relief on thumbs are set so the bucket curl can over power them. so that relief is messed with more. but it could be the same thing for the other cylinder.

if the machine as a whole is weaker, its likely either a relief setting or a worn hydraulic pump. if the pump is getting older put thicker fluid in it. i think my log splitter is running straight bar oil anymore, but it will split anything.
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Old 09-12-2019, 10:49 AM   #12 (permalink)
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before i pulled the cylinder to check/replace the o-ring, i'd pull the control valve to replace those o-rings and check the relief valve
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