purging air from hydraulic lines

[nicknewhall]

I have finished all new hoses and double-acting cylinder on a FEL snow plow. I will be using my 3rd scv, instead of the third function on my loader. this is on a TN75D with the 810TL loader. My question is, after everything is hooked up, how is the air bled from the lines and the cylinder?

 

[Bedlam]

I would just open the valves slowly with low rps and the air will work its way out of the system.
Hopefully someone else will weigh in here, but I think they will have the same directions too.

 

[ToadHill]

I've always just worked the valve slowly un til it operates in a normal manner.

 

[nicknewhall]

Thank you. i thought that might be the case, but I always like a second opinion.

 

[KJM]

Just run the rams to full extension in both directions. As you extend, the other side has no space left and then when you extend that, you get 100% oil. Two cycles and it's done!

PITA with a backhoe main boom cylinder though - you need to have it parked over a darn big hole so you can extend the arm down. Of course, every workshop has a convenient 6 foot hole in the floor to do this

/Kevin

 

[JustGary]

I'm no expert, but my theory on purging/bleeding is that you don't have to run the cylinder all of the way in or out, you only need to make it stop moving so the pressure will build up. So, as long as a backhoe arm won't lift the tractor, you should be OK as long as it will attain full pressure by stalling.

As the cylinder pressurizes, the air gets pushed smaller and smaller into very tiny bubbles. After a few cycles, the bubbles all get displaced by oil and make their way back into the tank where they are free to effervesce out of the oil at normal atmospheric pressure.

Whether that theory is correct or not, the other posters are correct. Just run the cylinder a few times, and it magically starts working!

- Just Gary

 

[nicknewhall]

Thank you one and all. I should have time to try it Monday. I will let you know how it turns out.

 

[john_bud]

Just use it, no need to be slow Expect some noise in the hydraulic tank as the bubbles hit. You will also need to keep it full. I don't know about the less than full operation, but I suspect that is correct. I know I didn't have full motion with the BH boom and it is air free. By the way, It would need to be a much deeper hole than 6 ft.

 

[Soundguy]

Ditto what the others have said.

on instalation of large SA cyls wher ei'm using new hose i will usually leave a fitting loose and slowly pressurize the system witht he valve.. then tighten up and just run her all the way up.. then go for it.

soundguy

 

[J_J]

I may be wrong on this but, shouldn't the cylinder you are trying to purge be horizontal or level so all the air can be purged. It seems that if you purge with the cylinder in an upright position, you will always have some air trapped, maybe on both ends.

 

[john_bud]

I may be wrong on this but, shouldn't the cylinder you are trying to purge be horizontal or level so all the air can be purged. It seems that if you purge with the cylinder in an upright position, you will always have some air trapped, maybe on both ends.

No, that's not correct. It may seem logical, but if you think about the pressure from gravity - it's only a couple psi. But the pressure from the pump is 2000-2500 psi. It way over powers any thing from the cylinder orientation.

jb

 

[J_J]

No, that's not correct. It may seem logical, but if you think about the pressure from gravity - it's only a couple psi. But the pressure from the pump is 2000-2500 psi. It way over powers any thing from the cylinder orientation.

jb

After thinking on this for a while, air will go to the highest point, and if the outlet or inlet for the cylinder is lower, then you will have trapped air, and if pressurized, you will have maybe 2000 lbs of compressed air, and the cylinder will feel spongy. An analogy would be the the brake system on automobiles. Although the brake system is a one way line, the air will not back up through the system, and you have to let the air out. The release valve is at the top of the brake cylinder.

 

[Soundguy]

I wonder how much of that air makes it past the packing...

soundguy

 

[Wayne County Hose]

When you send fluid through a hose, it is travelling very fast. As soon as it enters a cylinder full of air, it shoots across it until it hits the other side and instantly aerates. Now you have a cylinder full of aerated fluid. Eventually, through working the cylinder, the fluid finds it's way back to the tank where it can get rid of the air. The longer the hoses, the longer it will take. Some air will make it past the seal as hydraulic seals, particularly the U or V shaped polyurethane styles, don't seal air very well. However, the aerated fluid won't go through the seal. The best way to get rid of air in the cylinder is just run the machine.

 

[john_bud]

After thinking on this for a while, air will go to the highest point, and if the outlet or inlet for the cylinder is lower, then you will have trapped air, and if pressurized, you will have maybe 2000 lbs of compressed air, and the cylinder will feel spongy. An analogy would be the the brake system on automobiles. Although the brake system is a one way line, the air will not back up through the system, and you have to let the air out. The release valve is at the top of the brake cylinder.

You know it's like I said, it ain't logical but it will fully purge. Just think about a loader lift cylinder. They are mounted pretty near vertical and the fill is on the down side so there is always the potential for a trapped air bubble, yet they get to be air free pretty quick, usually 3 or so full cycles up to down and there is no more air making it back to the tank and after topping off, the tank does not drop it's level. So either all cylinders in that orientation have trapped air or it gets out even if it logically shouldn't.

Some things I just accept...

jb