Hydraulic cylinder air bleed issue.
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    Default Hydraulic cylinder air bleed issue.

    Some 20 years ago I made a hydraulic press for a customer incorporating two upright 7" bore shop made rams with around 12" usable stroke.....They are all loose pieces held together by tie rods.....It suffers from becoming air bound under the pistons through time.......Piston sealing is by a single Caterpillar "O" ring ,and there is also a wear strip of brown fibre ,also Caterpillar parts....Can I maybe relieve the O ring groove so the ring will bleed air on the upstroke ,but still seal pressure on the downstroke?.....and save a complete rework of the piston?

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    I am surprised that under pressure the air doesn't mix with the oil and get returned to tank where the air will separate and vent out, have you tried to pressurize the return stroke a few times? ...Phil

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    Its currently bled by tipping the press over on its side ,and then on a different axis to allow all the air to come out.A couple of tons ,and all the stuff on the bed has to be taken off .....Flow rate is nowhere enough to aerate the fluid ......beats me how the air gets into it in the first place.....suspect run with the tank empty ,as its blown many hoses...Its supposed to max at 3000psi ,but a simple tweak of the relief fixes that.

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    If I am correct, you have a press with single acting rams (hydraulic pressure on one side only) with some means to retract the ram the opposite direction. On single action rams the opposite side of the pressure side should have a return line to Tank (reservoir)
    This return line returns any oil that may bypass the piston as well allows as provides a way to vent air that may find it's way into the piston through the rod end seal. Look at any fork lift the single acting lift cylinder will always have a return line at the top of the piston (non pressure side of ram)that returns to the tank. I have seen venting check valves added to the non pressure side of single acting rams, however these will blow out any oil that leaks past the piston into the non pressure side.

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    Can you just drill the high point for an air bleeder? Buy them or just make one with a cone point set screw.

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    Because of the shape of the pistons ,I d have to use some sort of valve in the piston ....the rams are semi double acting....pressure return ,no springs.....The pistons are /were cast nodular transformer wheels ....clever wheeze ,I thought ,to save the cost of flame cut 7+"dia. disc x3" thick from plate ....plenty strong to take 50 tons ....but shaped to trap a donut of air under the piston.

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    What if you imposed a backpressure on the blind side of the arrangement and stroked it several times? The backpressure needs to be high enough to force the air into solution with the oil so it will get carried to the tank

    Side note- Ive always wondered how they bled large complex hydraulic systems like that of an excavator or the like.

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    My Daewoo forklift has bleeder valves at top of ram, maybe figure a way to incorporate some.

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    if the piston itself is the problem, removing, making new (from burnouts) and re-installing
    should be much less work, than running a rodeo with that press every so often.

    Can you put a slight resistance in the circuit to keep air from going in on that side ?

    Sounds like the weight pulls the ram down a bit, and it is sucking air from the retract side of the cylinder.

    Maybe one of those "load holding cross check valves" so you have to shove the ram down, when pressing.

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    So the air is being trapped under the piston? Is there room for you to drill a small hole through the piston and put a check ball in there? 1/16" or smaller hole, or at least an orifice in a bigger, easier to drill hole. The check valve would open when the ram is being pushed up, letting a tiny amount of oil pass to the upper side of the piston, ideally taking any air with it. The check valve will close on down pressure. The pump will have way more flow than the tiny orifice bleeding oil/air to the top side of the cylinder, and if it looses a tiny amount of capacity on the return stroke, will it matter?

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    New flat ended pistons would be a cure .....but ,I made these presses 20 years ago,and any help is limited to zero cost to me options.....which a modified groove would be...I dont want to come into any modifications at all....Although ,I seem to remember that leather pistons in hydraulic rams will pass air and water ,but seal oil.All the old hydraulic presses had leather buckets and took 8- 10,000psi.................Rob F ,I've already considered some kind of check valve in the pistons ,might be the only cure ,but a O ring that passes air in one direction would be a lot easier .

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    You might be chasing the wrong problem, its the air that should not be there, maybe a loose oil pick up pipe in the tank pulling in air , then it is getting caught in the lower part of the cly...Phil

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    I dont know where the air comes from ...its only a small amount ,but builds up over time ....no issues with the tank ,which is sealed and pressurised.,specifically to stop air issues and cavitation at the pump.Pump is compressed air type,but a quality unit ,not Chinese rubbish,controll valve is a tight spool unit ,zero internal or external leakage .....Maybe tipping the press over is the easiest option .


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