How to find a suction leak

[BeezFun]

Is it possible to pressurize a hydraulic system to some low value to help locate a leak in the suction lines? I was thinking of fabricating a cap for the oil reservoir that will have a shrader valve in it so I can hook a compressor to it and put some low pressure (5-10psi?) in the system to find a leak. The reason I want to do this is in the next paragraph.

I've got an aerial lift, hydraulic circuit attached. The problem symptom is that the cage constantly goes out of level, and also moves out of level on it's own even when the system isn't running. In the circuit diagram the cage leveling cylinders are in the lower center area of the drawing, there is a cage cylinder and a knuckle cylinder. The bleed procedure given in manual is to cycle the cage cylinder fully in both directions, holding it at each extreme position for 20 seconds before releasing pressure. That works, the cage leveling operates properly for some time, generally an hour or two. But then the problem recurs. So I assume it's sucking air in the suction lines. I've tightened the fittings, the hoses are relatively new. I'm trying to figure out how to diagnose the problem before I just start replacing hoses and fittings with no idea what the problem is.

 

[JimRB]

When was the last time the cylinders were rebuilt?

 

[arkydog]

if i goes down with it not running then it is an internal leak, not vacuum leak. hydraulics are a pressurized system . suction side is a relative term. its still has pressure in my opinion. cap off the the cylinders and the one capped and still leaks down is the bad one. is usually a cylinder internal on the side going down. i have always rebuilt them in pairs but only me. most people don't

 

[ROUSTABOUT]

if i goes down with it not running then it is an internal leak, not vacuum leak. hydraulics are a pressurized system . suction side is a relative term. its still has pressure in my opinion. cap off the the cylinders and the one capped and still leaks down is the bad one. is usually a cylinder internal on the side going down. i have always rebuilt them in pairs but only me. most people don't

Pairs is the way to go.

 

[BeezFun]

if i goes down with it not running then it is an internal leak, not vacuum leak. hydraulics are a pressurized system . suction side is a relative term. its still has pressure in my opinion. cap off the the cylinders and the one capped and still leaks down is the bad one. is usually a cylinder internal on the side going down. i have always rebuilt them in pairs but only me. most people don't

These aren't lift cylinders, there is no up or down and there's almost no loading. If you read this article , it does a nice job explaining how leveling cylinders work. Also a leaking seal is not consistent with the symptoms I described. Bleeding the air out of the cylinder would not correct the problem if the problem was a leaking seal. I think the reason it moves when the system is not running is because the air is either expanding due to temperature change, or the air is migrating to the highest point in the system, which happens to be the cage leveling cylinder, so the air gets compressed and the cage moves.

Also, as an aside, leaking seals can't cause cylinders to drift in the direction of retraction. You could remove the seals and the cylinder still wouldn't drift retract because of the difference in volumes of fluid on the two sides of the piston. If it drifts in that direction it's either a hose/fitting or the valve leaking.

 

[oldnslo]

Beezfun,
I presume the knuckle cylinder is attached directly to the mast linkage and is mechanically pulled or pushed to drive the cage cylinder as a master slave system. Is this a correct assumption?

Which cylinder is moving in which direction?

I also item 19 a check and item an over center or counterbalance valve in the system. Depending on how these cylinders are attached to frame you could possibly have an internal circuit leak that would allow these two cylinders to move in unison causing the basket to tip or the directional control valve connected to the knuckle cylinder could also allow that cylinder to drift again depending on how the cylinder is connected. The check and over center valve should prevent the cage cylinder from moving if they are working properly.

I would be very surprised if a pump suction leak would only effect this one function and none of the others.

 

[BeezFun]

Beezfun,
I presume the knuckle cylinder is attached directly to the mast linkage and is mechanically pulled or pushed to drive the cage cylinder as a master slave system. Is this a correct assumption?

Yes

Which cylinder is moving in which direction?

When the knuckle extends the cage retracts.

I also item 19 a check and item an over center or counterbalance valve in the system. Depending on how these cylinders are attached to frame you could possibly have an internal circuit leak that would allow these two cylinders to move in unison causing the basket to tip or the directional control valve connected to the knuckle cylinder could also allow that cylinder to drift again depending on how the cylinder is connected. The check and over center valve should prevent the cage cylinder from moving if they are working properly.

Ok, thanks, that's on the edge of my hydraulic understanding, let me research that a bit to see if I understand it.

I would be very surprised if a pump suction leak would only effect this one function and none of the others.

The only reason I made that guess is because the manual says that the way to purge air from the system is to cycle the cage cylinder. So I assume it's designed for any air in the system to end up at the cage cylinder. In addition to the motion I describe, I also get slight shuddering of the cage when the leveling system is operating, which is another sign of air in the cage cylinder. After I bleed it, that shuddering goes away for awhile.

 

[BeezFun]

To help visualize things, here's the lift:



The knuckle cylinder is behind where my finger is pointing:



Here's the knuckle cylinder:



And here's the cage cylinder, I manually rolled the cage forward to make it more visible:

 

[oldnslo]

Beez
Great job on the pictures they certainly do help.

Can the knuckle cylinder move without the main lift mast moving. Suspect no from pictures but would appreciate confirmation on this concept.
In looking at cage cylinder it could drift or extend if seals were bad but looks like load would trying to retract cylinder. Is this also correct?

does the cage feel spongy or bouncy after drifting out of position? If yes this would point towards air in the cage cylinder

shuddering can also be caused by the PO check and or the over center valve which could be another symptom or side effect of air somehow getting into the system

 

[dstig1]

Didn't you post this exact same thread a few months (or more) back? It sure sounds familiar to me. Not that I have anything helpful to add, just wondering why it wasn't resolved last time or what has changed.