How do tractor hydraulics self bleed?

[Chris Wilson]

A real newbie question, I'm afraid, sorry, but I can't work out how things bleed air.

Example:

I am putting a secondhand loader on my little MF 35. I am renewing all the hoses and have had the bucket ram rebuilt with a new piston rod. The arm rams are single action. So how does the system bleed air from the lines and the rams themselves when you start a system that is empty of fluid. As far as I can see the fluid will just compress the air in the lines and the single acting rams as the fluid has no (obvious) return path. Where am I being thick? I see this as similar to a brake caliper on a car. You need to bleed the air from the system to get it full of fluid and working solidly. Thanks.

 

[RonMar]

Well on many loaders, the lift cylinders are nearly horizontal when the loader is at it's lowest. IF the line feeding the cylinder is oriented to be on top of the cylinder, as the cylinder fully compresses, most all the air should be forced back to the valve with repeated cycling. It might help if the tractor is pointed down an incline.

It is also a lot harder for the seals to stop air than to stop the fluid for which they are designed, so if you lift the loader to full height and hold the control in the lift position(safety will be relieving the excess above it's set pressure), this will apply full system pressure to a cylinder at it's end stop. Any air should be at the top near the piston seal and may be forced past the seals. It is always a good idea to connect a pressure gauge and verify the safety relief pressure on a new or unknown piece of equipment. The safeties are usually adjustable, and if it can be adjusted, it can be adjusted wrong

Good luck with your "new" loader

 

[Reg]

Wot he said (RonMar).

Plus, pretty much by design air will be returned before fluid is returned.
Some will get entrained, but escape in the tank - eventually.

 

[AndyinIowa]

When you do your first startup after something major (hydraulically speaking) its good to follow a simple procedure:

1. Crank machine a few times without letting it start completely - Does several things. If something was assembled wrong, you can listen for nasty sounds, etc... It also lets the hydraulic charge pump prime and you can look for major leaks (forgetting to install hoses, etc...)

A lot of small OEM's (manufacturers) cold crank for up to 30 seconds.

2. Start vehicle and leave at low idle. Do another leak check all around machine. DON'T use your hand (that gets into a whole other issue)

3. After about 2-3 minutes, if there aren't leaks or nasty noises, start exercising hydraulic functions slowly end to end. This will help get air out of the cylinders, hoses, etc...

4. Don't operate at full throttle for at least 10 minutes. I prefer to run at reduced levels for 30 minutes to get a full thermal cycle complete.


Just a procedure I use when servicing hydraulic systems. Everyone has their own, and authorized service centers (hopefully) have similar procedures.

Andy

 

[Chris Wilson]

Thanks for the replies. I have finished the pipework, and hope to test it tomorrow. Fingers crossed, I'll follow this advice, for sure.

 

[randot]

When you do your first startup after something major (hydraulically speaking) its good to follow a simple procedure:

1. Crank machine a few times without letting it start completely - Does several things. If something was assembled wrong, you can listen for nasty sounds, etc... It also lets the hydraulic charge pump prime and you can look for major leaks (forgetting to install hoses, etc...)

A lot of small OEM's (manufacturers) cold crank for up to 30 seconds.

2. Start vehicle and leave at low idle. Do another leak check all around machine. DON'T use your hand (that gets into a whole other issue)

3. After about 2-3 minutes, if there aren't leaks or nasty noises, start exercising hydraulic functions slowly end to end. This will help get air out of the cylinders, hoses, etc...

4. Don't operate at full throttle for at least 10 minutes. I prefer to run at reduced levels for 30 minutes to get a full thermal cycle complete.


Just a procedure I use when servicing hydraulic systems. Everyone has their own, and authorized service centers (hopefully) have similar procedures.

Andy

That is pretty much what I did when replacing the cylinder on my bucket control on a 30 HP Kioti. No more visible leaks, but now the bucket leaks off after about 30 minutes. Could that be from air still trapped in the lines?

Thanks