To Bleed or Not To Bleed ???

[crash325]

I am about to replace a 9 foot hydrolic hose that goes to the back hoe.

Should I just let the air in the line work its self out???
Or should I bleed it??? My thinking leave the fitting a little loose, but that will be pretty messy. Not easy to catch the fluid where it is located.

What you think????

 

[kennyd]

There is no need to bleed anything, the system will purge the air when you cycle the controls.

 

[crash325]

There is no need to bleed anything, the system will purge the air when you cycle the controls.

Thanks - Appreciated
Just worried about air going through the pump.

 

[crash325]

Got line on, not happy with how I routed it, so will change that tomorrow.

After a careful look the back hoe is a closed system, no place for the air to go, so I bled it anyway. A little mess, but did not think air going round & round would be good.

 

[DT86]

Save yourself the mess next time. There really is no need to bleed the line.

 

[crash325]

Save yourself the mess next time. There really is no need to bleed the line.

You and kenny probably both right. Me just a paranoid old man. Seemed to me sort of like air in a brake system.

 

[JD 4520]

You and kenny probably both right. Me just a paranoid old man. Seemed to me sort of like air in a brake system.

They are correct, it's not like a brake system in that regard. Interesting that you had a problem which suggest something isn't quite right but air isn't the problem.

 

[crash325]

They are correct, it's not like a brake system in that regard. Interesting that you had a problem which suggest something isn't quite right but air isn't the problem.

On a closed system how would the air escape???

Its working fine. I just don't like how I routed the hose. Didn't like how was originally routed. Its extra long so the back hoe can be used to remove & install it. Fairly quick & easy with 2 people. Sort of a pain when alone. have to look, move it a little, look again, & so on. Once lined up back the tractor onto it, a little more jerking around to get the pins in.

 

[DT86]

On a closed system how would the air escape???

To quote the movie Joe Dirt..
"Hows a rainbow made? Whys the sky blue? How does a posi track rear end on a Plymouth work? It just does."

 

[mjncad]

All hydraulic systems are closed systems; otherwise they would get contaminated with dirt. Are you saying your tractor uses a closed-center system versus an open-center one? If I'm correct, and I know KennyD knows the answer for certain; but an open-center system uses a constant displacement hydraulic pump, while a closed-center uses a variable displacement pump.

As for where the air goes; it goes back to the sump and is vented to atmosphere via whatever sump venting system is used. Yes, I know this sounds contradictory about having a vent to atmosphere in a closed system; but the sump vent is designed to let air pass while keeping dirt out.

 

[crash325]

All hydraulic systems are closed systems; otherwise they would get contaminated with dirt. Are you saying your tractor uses a closed-center system versus an open-center one? If I'm correct, and I know KennyD knows the answer for certain; but an open-center system uses a constant displacement hydraulic pump, while a closed-center uses a variable displacement pump.

As for where the air goes; it goes back to the sump and is vented to atmosphere via whatever sump venting system is used. Yes, I know this sounds contradictory about having a vent to atmosphere in a closed system; but the sump vent is designed to let air pass while keeping dirt out.

By closed system I meant, the oil in the back hoe does not circulate back to the pump or sump. What is in there stays there. 2 Pressure lines & no return line to pump. There is no place for air to escape.

The 2 lines drive separate functions of the hoe.

 

[john_bud]

On a closed system how would the air escape???

Its working fine. I just don't like how I routed the hose. Didn't like how was originally routed. Its extra long so the back hoe can be used to remove & install it. Fairly quick & easy with 2 people. Sort of a pain when alone. have to look, move it a little, look again, & so on. Once lined up back the tractor onto it, a little more jerking around to get the pins in.

Any air in the line is pumped to the cylinder and while the seals hold the fluid, they allow air to pass into the return line. The return line goes to the reservoir - not the pump. The reservoir is vented so the air will get out - thus the system is not totally closed. Every time you extend a cylinder the reservoir breaths in that volume of air and every time you retract it exhales that amount. The vented filter cap is "ok" but it does let in some dirt and water from the air. That's why you need to change the fluid every so many hours to get that contamination out.


After installing new line or cylinder, etc fully extend then retract the cylinder. Stop the machine, wait a few minutes for the bubbles to all go away then check the fluid level. Simple as pie!

 

[crash325]

Any air in the line is pumped to the cylinder and while the seals hold the fluid, they allow air to pass into the return line. The return line goes to the reservoir - not the pump. The reservoir is vented so the air will get out - thus the system is not totally closed. Every time you extend a cylinder the reservoir breaths in that volume of air and every time you retract it exhales that amount. The vented filter cap is "ok" but it does let in some dirt and water from the air. That's why you need to change the fluid every so many hours to get that contamination out.


After installing new line or cylinder, etc fully extend then retract the cylinder. Stop the machine, wait a few minutes for the bubbles to all go away then check the fluid level. Simple as pie!

There is no return line back to the pump or reservoir.

 

[radioman]

Are you saying you have a single acting cylinder that you are working on? Most tractors or implements have double acting cylinders so there is a "return" line. Hydraulics on a tractor is really a open system. In your case - getting fluid through the hose first and then hook up to the SA with it retracted should take care of most of your paranoia. Any air left will work itself out.
I can see where you might be confused. In a vehicle there is a bleeder because fluid is supposed to go one way in the system. On a tractor, it can go back and forth to the reservoir. When fluid can go back and forth- it can push that bubble of air back to the tank where it can go free. on a car - it has no place to go but through the bleeder screw.

 

[crash325]

Are you saying you have a single acting cylinder that you are working on? Most tractors or implements have double acting cylinders so there is a "return" line. Hydraulics on a tractor is really a open system. In your case - getting fluid through the hose first and then hook up to the SA with it retracted should take care of most of your paranoia. Any air left will work itself out.
I can see where you might be confused. In a vehicle there is a bleeder because fluid is supposed to go one way in the system. On a tractor, it can go back and forth to the reservoir. When fluid can go back and forth- it can push that bubble of air back to the tank where it can go free. on a car - it has no place to go but through the bleeder screw.

Fluid goes from 1 end of the cylinder to the other, back & forth. It never returns to reservoir. It does not circulate through the whole system. 2 Pressure lines, no return line.

 

[JD 4520]

Fluid goes from 1 end of the cylinder to the other, back & forth. It never returns to reservoir. It does not circulate through the whole system. 2 Pressure lines, no return line.

Are you having certain problems with your hydraulics? It sounds like you are trying to solve something and hung up on how the circulation works. Unless you have a very unique / like no one has seen before system, the fluid does flow or should flow how it has been described. Tell us what problem you're experiencing so we can help solve that.

 

[crash325]

Are you having certain problems with your hydraulics? It sounds like you are trying to solve something and hung up on how the circulation works. Unless you have a very unique / like no one has seen before system, the fluid does flow or should flow how it has been described. Tell us what problem you're experiencing so we can help solve that.

No current problems.
There are only 2 lines to the back hoe.
With 1 line disconnected part of the back hoe works.
Reverse & the other part works.

 

[john_bud]

There is no return line back to the pump or reservoir.

No disrespect, but you may have a distorted understanding of what you are looking at.

The "cylinder fluid return" is part of the control valve. When you upset the valve (move it to cause the cylinder to move) pressurized fluid goes down one hose to the cylinder causing it to extend or retract, and the other hose FROM the cylinder has it's fluid directed by the valve out to the tank return. There are variations around that theme, but in general all hydraulic systems work that way.

 

[mjncad]

Assuming your hoe is powered via a Power-Beyond set of connectors on the tractor, one PB line is pressure out to the implement's control valve that directs the oil to various cylinders and back again depending on the control lever(s) position, and the other PB line is return to sump from the implement's control valve. What you are describing would deadhead the pump since the oil is not in constant circulation and the pump would eventually get unhappy from deadheading the fluid.

If for some reason your hoe has an independent hydraulic system powered by the tractor's PTO or a gas engine, you would still have a sump on the hoe for the oil to return to. Whatever air that is in the system goes back to the sump to vent to atmosphere.

 

[crashz]

Crash,

(Nice name BTW) The two lines on the cylinder act as both pressure and return, depending on the action requested by the operator. So while extending, the bottom port receives pressure from the valve body, the top port then returns fluid to the valve body, which then returns it to the vented sump. Reverse fluid flow to retract piston. The valve body receives pressure and has a non- pressure return to sump at all times.

So there are no dedicated low pressure return lines anywhere on a backhoe other than the one from the valve body to the sump. All pressure lines have a secondary function as returns.