How to bleed hydraulics on front end loader?

[Bentrim]

Let me see if I can throw a few monkey wrenches in the mix.
Air can only enter a system on the suction side. As mentioned many times before, check the supply side if you suspect air in the system.
Air in the system is indicated by a bouncy loader. In other words the frame of the loader will bounce up and down when the tractor is driven over a rough place.

An open center system does not need bled, only cycled a time or two.

Control valves can have issues but usually cause few problems, One problem that can happen is the centering spring retaining screw comes loose and does not center the valve spool. This can cause unusual, unexpected movement. Are the ends of both spools equal? Does the spool/lever seem to have too much free travel? If you move valve spool by hand slightly does control improve? Load checks are valves that hold the loader until system pressure exceeds the load. without them when you raise the loader it would drop slightly before starting to raise. There are no O rings that seal inside a valve. The O rings only seal the oil in the valve and keep it from leaking on your shoe.

Pumps only draw oil (and air if available) from the reservoir and move it to discharge port, thru a hose to the valve, from the valve it returns to the reservoir, or to a work port if activated.

Valves can usually be repaired if centering springs break, the centering spring bolt comes loose, or they leak. Valves housings and spools are usually matched or fitted and spools cannot be interchanged or replaced. The only way a valve needs replaced is if it is cracked or damaged.

Double acting cylinders CAN a do leak past the piston. There will be no sign of a leak as oil is only changing sides within the cylinder. How to find out. Listen to the cylinder with a stethoscope or a pipe if leaking you will hear the oil bypassing the piston

From your complaint I would check for a suction leak, check for a loose centering spring and or the cylinder packing.

Good luck I hope it is an easy cheap repair

 

[npalen]

Forget looking for air in the system. Hydraulic oil travels so fast through the components in the circuit that air has got no chance to settle anywhere. I suggest that the FEL falling 3 to 4 inches indicates the problem. Depending upon your circuit design the spool valve, the pilot operated check valve and the cylinder seals should hold the FEL in position almost indefinitely. I don't recall you mentioning the age of your tractor.

So what happens when, with the loader raised high, you let it drop and the pump can't move oil into the cylinder to keep it full? Same when dropping the loader with the engine off.

 

[Bentrim]

So what happens when, with the loader raised high, you let it drop and the pump can't move oil into the cylinder to keep it full? Same when dropping the loader with the engine off.

Some loaders have restrictors that will allow the loader to drop only so fast. That said If the loader drops faster than the pump can replace the oil, the cylinder will fill with available oil the cavitate until the pump catches up.
As when the loader is dropped with the engine off, being the system is sealed oil returning from the cylinder might find its way thru the valve to the other end of the cylinder if not and no other oil replaced the oil, the cylinder will cavitate and be refilled when the tractor is started and loader raised.

 

[npalen]

Some loaders have restrictors that will allow the loader to drop only so fast. That said If the loader drops faster than the pump can replace the oil, the cylinder will fill with available oil the cavitate until the pump catches up.
As when the loader is dropped with the engine off, being the system is sealed oil returning from the cylinder might find its way thru the valve to the other end of the cylinder if not and no other oil replaced the oil, the cylinder will cavitate and be refilled when the tractor is started and loader raised.

Another example would be a 3PH top link cylinder. Letting it drop too fast seems to induce air that has to be worked out by cycling the cylinder slow a couple times.

 

[orangetree]

MadJack,

The usual route for hydraulic fluid, under high pressure, out to the cylinder and then back to hydraulic reservoir via a low ( no pressure) pressure line where entrained air or vapor ( air bubbles or vaporized water if excessively contaminated fluid is in use and fluid is very hot) is separated out. The reservoir is vented to the outside by one way valve.

This cannot account for purging all the air - the unloaded side of the cylinder doesn't pull a vacuum - the volume of air that remains in the hose, valves, (IE doesn't "make it back" to the reservoir or a check valve) is not going to go anywhere [see below...]

Air can get pushed past the seals whereas fluid cannot. Do you think dealers or the factory bleeds loaders when they install them?

^ UNLESS there is another path out. I cannot say definitively that @kennyd 's proposal is INcorrect - but it seem suspect to me.

from a seal manufacturer's site "It is also important that displacement hydraulic cylinders are bled before installation"

additionally, a seal might behave differently per direction: the cylinder seal is very very good at keeping high-pressure fluid IN the cylinder - this says nothing about it's ability to keep air out such as when a vacuum is pulled when a cylinder is forced one way faster than the pump can deliver, as @npalen pointed out. The seal design has some similarities to a one-way valve in this regard.

All that said, as a practical matter Kennyd could be correct, the (relatively cheap quality and/or well-used/worn) cylinders used on most of our tractors could in fact let high-pressure air out. Things flow differently depending on properties & state (think hydrogen at different temperatures, a-la recent SLS rocket launch scrubs).

Now, disclaimer to the above: I design things for a living, but not these things, so the above is me trying to parse easily available information - possibly incorrectly . YMMV.

 

[orangetree]

Another example would be a 3PH top link cylinder. Letting it drop too fast seems to induce air that has to be worked out by cycling the cylinder slow a couple times.

Ignoring the seal issue (do they vent? not vent?) it's entirely possible that this could simply work it's way out "on it's own".

If the air-entrained side of the cylinder is "up" - then any free air would get forced completely out next time the cylinder moves, as the [volume of cylinder] >> [residual volume of dead-headed cylinder + hose + ...].

And even if it's "down" (so "free" air would theoretically be trapped "up" in the cylinder) - filling the cylinder is going to be pretty turbulent flow, I would guess any air in that cylinder is going to be at least momentarily thoroughly mixed - a cycle or two of this would see the vast majority of that fluid flushed, and the air bubbles work themselves out in the tank & filter. Lots of semi-educated guesses here, though, for sure.

and as @Bentrim pointed out, if air can leak past the piston - then it could also find it's way to the "up" side of the cylinder as well.

seems like several paths that don't require air moving the wrong way past hydraulic seals ... if it does move past the seals, seems like this info would be easier to find than "how to bleed a hydraulic cylinder" - which dominates search results?

 

[rScotty]

Opinions are for more for fun, if we really wanted answers I guess we could cook up some experiments.

 

[MadJack]

I think I'll just start by replacing the spool valve thingy.
But thanks to all for getting the old gray matter stimulated!

 

[M59]

Orangetree,

I believe you misunderstood my earlier comment. I did not say the cylinder pulls a vacuum as it is powered ( under pressure ) in both directions ( open and close ). I said if he has air in the system then it is leaking past a seal. That seal could be anywhere in the hydraulic circuit and moreover leaking air past a seal is probably not the issue since a faulty hydraulic control is more likely to be the culprit.

"It would be highly unusual, unless you had a constant inflow of air past a seal, for air to be constantly in the hydraulic system.

Since the Ford 2110 FEL cyclinders are double acting ( you power up and power down ), I believe your issue is more hydraulic control related ( control valve, shuttle/spool valve, unknown restriction, damaged piston, ?? ) related and not air in the system."

 

[rScotty]

I think I'll just start by replacing the spool valve thingy.
But thanks to all for getting the old gray matter stimulated!

Probably what I'd do, too.
A good place to start with research on valves is to go to: Surpluscenter.com and then to hydraulics and then to directional control valves.

You want to match the thread on your hoses (or get adapters), and match as close as you can to the hydraulic gpm flow of your tractor. Buy a valve that has PB (power beyond) along with the associated PB sleeeve fitting, and might as well specify float on one of the spools.

It's your choice as to whether to go for old fashioned two lever control or pay more for the single lever control.
I prefer the two lever myself. It seems simpler & more positive to me. Plius the single lever is just a regular two lever type plus some monkey motion parts anyway.

If you like smooth operation more than low price, the more expensive control valves are said to be better at making very small accurate movements - but I don't have enough experience to confirm that. Maybe someone does. I do tend to buy USA... but anymore that is partly wishful thinking....

rScotty