Bleedin’ hydraulics!

[Kiwi BlokeParticipant @kiwibloke62605]

Having done some work on the hydraulics of my tractor and log lifter/splitter, I’m puzzled by the complete lack of any provision for bleeding air out of the systems. I’ve mentioned this to a couple of tractor mechanics, but get blank looks: either it’s never occurred to them, or the answer’s so obvious that I must surely be an idiot. The nearest I got to an answer is that air, but not fluid, will pass the pressure seals, so the system bleeds itself. I find that hard to believe. Even if it is the answer, one would need to have a cylinder orientated with its pressure seal high, so the air was against the seal, but of course that may not be feasible.

Whatever the answer, I was pleased to find that the front-end loader worked fine, with no evidence of ‘bounce’, even after having had to reinstall the cylinders nearly full of air. However, one of the the log lifter’s pressure relief valves squeals a bit. Might this be because the cylinders and pipes are now full of air/oil ‘cream’? (Distant memories of Citroen warning about never pressurising the oleo-pneumatic system if there were any air in it. I believe it took ages for the air and oil to separate).

 

[renardiere7Participant @renardiere7]

Most tractor and excavator hydraulic systems I’ve worked on self bleed. After having the system open the rams are cycled in and out slowly building to full out and full in. I’ve always assumed this just slowly pushes any trapped air into the tank to accumulate there. In fact on my current excavator there is a procedure to follow to pressurise the hydraulic tank with air following oil top ups etc.

[PeteParticipant @pete41194]

I’ve operated hydraulic equipment over most of my life, and none of it needed more than a few cycles on the rams to push the air back to the hydraulic tank. So those systems would self bleed any air out. However that’s for large to very large industrial machines. If its a small and light weight almost consumer grade of tractor. I can’t say all or any brands of those are designed to and would do the same. Logically they should, but I have no experience with agricultural equipment.

[noel shelleyParticipant @noelshelley55608]

Having worked on any hydraulic system make sure that the reservoir is full. Then cycle the system several times fully to expel the air. Now check the reservoir again and top up if need be. If the system has over centre valves in it, it may take many operations over days to get all the air out.

I ve never heard the one about air bleeding past the seals. Depending on the tractor and system, open centre and closed centre are very different systems and are not interchangeable. When bleeding if you have hose failure valves you may have to bleed at hose fittings to stop the valve locking.  Good luck.  Noel.

[Mark RandParticipant @markrand96270]

The difference between an hydraulic system and car brake systems is that the fluid goes from the tank, to the pump to the cylinders and back to the tank. So it’s traveling all the way through the system and will carry air out. In a car, the fluid is effectively static. Once the system is filled, the fluid only moves back and forth and the air stays in the system.

 

Sorry, mansplaining here. 🙂

 

[Kiwi BlokeParticipant @kiwibloke62605]

Thanks everyone, you’re confirming my experience that in practice, it’s not usually a problem. However, without provision for formal bleeding, the air must be got rid of by successive dilution, rather than by purging. My (belated) understanding is that, rather than a big bubble of air being trapped in the cylinder or shunting to and fro along the pipes, the air gets so compressed that it becomes micro-bubbles in the oil, which can readily mix and flow with the oil, and the back and forth flow of the oil successively ‘dilutes’ the contamination of the oil at the far end. So it is bleeding, Jim, but not…

 

[PeteParticipant @pete41194]

That’s the very best and simple explanation of the difference between having to bleed a hydraulic brake system and why it isn’t normally required on most hydraulic powered equipment I’ve read anywhere Mark. 👍

I’ve put equipment back into operation after a cylinder was replaced with a new or rebuilt one. The mechanics either cycled the system while topping off the hydraulic tank, or if they were busy I did it. The large amount of air that also gets compressed I’d definitely describe as a bubble and does get purged back to the tank after only a few cycles and then vented out of the tank. With two people and if there’s a clear sight glass for oil level on the tank, you can see that air entering the tank. I’d never say it couldn’t happen, but I honestly don’t recall any instance where air in the oil was ever an issue on a properly operating and maintained hydraulic control circuit.

[HOWARDTParticipant @howardt]

Worked with many hydraulic systems over the years.  Systems may need to be bled where actuators don’t fully stroke which many don’t and where the pump is low down with elevated pipe work.  The usual way is to just crack open the adjacent fitting allowing aerated oil to escape then retighten with the system operating when the oil flows clear.

[noel shelleyParticipant @noelshelley55608]

An interesting one ! A piece of mobile hydraulic gear, would intermittently land up with air in the cylinder for no obvious reason. it was open centre. Transpired that the return pipe into the tank was too short inside the tank and the oil could IF the machine at a slight angle allow the oil to pass through air before entering the oil, entraining the air that would accumulate in the cylinder. The dip tube inside the tank was lengthened and all was well again.  Noel.

[Dave HalfordParticipant @davehalford22513]

There is no bleed facility on the 10 or 4 ton body work hydraulics systems either, just plug and play

[NealebParticipant @nealeb]

The closest I have ever been to hydraulic systems apart from vehicle brakes is the time I used to drive the tractor and boat lifter at a local boat club. I always wondered about where the inevitable air went when the hydraulics were plugged/unplugged from the tractor.

My thoughts from what I have read so far is that the self-bleeding systems differ from vehicle brakes in that the pressure comes from a separate pump associated with some kind of tank holding oil. The system is operated by valves directing oil into/out of the ram or whatever, and there is considerable flow. This gives air a chance to disperse in the oil and eventually get back to the reservoir where it can be released. In addition, given that there is a pump driving the oil, even if there is air in the system it will be compressed to whatever the system pressure is and presumably allow the ram to operate. Valve operated to release pressure (=oil back to reservoir?) and some air will come out with it. Fairly significant flow of oil needed (volume based on ram diameter/stroke). Vehicle brakes derive all pressure from the limited piston movement in the master cylinder and if some of that movement is needed to compress even a small amount of air then you can run out of movement very easily. Added to that is that the actual movement of the piston in the wheel cylinder is tiny so flow in the system insignificant – hydraulic fluid almost anywhere in the system will never get back to the master cylinder so even if air does tend to “dissolve” (form tiny bubbles?) in the fluid it is still never getting to somewhere where it could be released.

So, big hydraulic systems are self-bleeding because of high fluid flow and separate pressure pump; vehicle brakes are not because of very small fluid flow and limited master cylinder piston movement. Reasonable conclusion?

[Adrian R2Participant @adrianr2]

Tractor hydraulics operate around 2000psi, which means any air in there gets compressed to almost nothing so even if not completely flushed out will not make much odds. The high pressure also means that an external bleed point would be a very undesirable thing.

[noel shelleyParticipant @noelshelley55608]

Hydraulic jacks and presses of the type we may use operate at a WP of up to 10,000 PSI.

Modern tractors and big diggers Etc may well operate at much higher pressures than 2000psi, even up to 5000 psi.

Air in a cylinder can be very dangerous because as the load comes off the piston may suddenly move with great speed and considerable force. On some applications the protector sleeve is not only there to save the hose but also to contain the high pressure oil that may be released in the event of a burst.

The air filter on the reservoir tank is there to prevent dusty air being drawn into the tank, mix with the oil and contaminate the system. The volume of the front of the cylinder and the rear are different by the size of the piston rod. So as the cylinder extends or retracts air is drawn into tank to account for the change.   Noel.

[Adrian R2Participant @adrianr2]

Surely any force from expanding air can’t be greater than force originally applied by hydraulics, and if you had enough in their to cause movement on release it most likely wouldn’t have worked to start with?

I may be wrong or out of date. My experience is from operating rickety kit as a farm lad in my youth, air in hydraulics for me just means something that should move doesn’t until you cycle it a bit. Never had to bleed anything, just top up tank periodically to account for losses due to weeping implement connectors and cylinder seals.

 

[noel shelleyParticipant @noelshelley55608]

When using hydraulics one is normally talking in tonnes of force and often 10 or 20 tonnes. If there is trapped air and the load suddenly comes off the cylinder it will continue to move, initially with what ever load was on it as the air expands. A cylinder with air in it will not respond as it should. The sound the system makes is often a clue that there is air in the works.

Kiwis example of a log splitter is a case where the load will come of rapidly and if there is air present then the ram shoot forward uncontrollably as the air expands, a highly dangerous condition.

[Adrian R2Participant @adrianr2]

I can see what you mean in theory, especially for a single acting ram but I suspect that in practice a cylinder with sufficient air in it to cause a rebound risk would not actually split any logs. Anyway, just an opinion as I don’t have one to inspect or test. A manual splitter probably will have a bleed point or at least a required filling method as cycling by hand is tedious.

I would further say in case anyone reading this ever is tempted to try it that I would not wish to manually bleed any tractor/excavator/plant hydraulic system or be anywhere near oil escaping at system pressure. Shut off engine, release loads and depressurise before doing anything at all.

 

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