How to find a suction leak

[oldnslo]

Beez,
In reviewing schematic again and thinking about this further I am still puzzled on where the external pressure is coming from to extend the cage cylinder.

While manually extending with ratchet strap you could potentially pressurize the rod of both cylinders if the over centervalve is opened. This would then allow a slightly larger volume of oil to pressurized since the knuckle cylinder is in the fully extend position while you were doing this. Pressurizing the rod ends should keep the cage from extending but crazy thought if the seals on the knuckle cylinder are leaking slightly could you have also induced some pressure into the cap end of the knuckle cylinder? If yes then this pressure would extend the cage cylinder since would have free flow through the PO check and if high enough to over come the over center valve setting. Any trapped air would also act like an accumulator to increase potential flow.

Problem is this really doesn't work when you tried retracting the cage cylinder with the ratchet strap.

A bad seal on the cage cylinder and I would think you could extend the cylinder fully with the ratchet strap.

Back to thinking we need gauges in the system. Since closed system be nice to have one in both the rod and cap end lines so can observe what is happening. Problem how to do this with introducing more air or in a location where the air cab be purged easily or easier.

Can you safely operate the cage tilt cylinder fully with the mast fully raised? I.e. knuckle cylinder fully retracted. Just curious is this would help remove any potential trapped air in cap end since it should be forced out to the cage cylinder.

 

[BeezFun]

Beez,
In reviewing schematic again and thinking about this further I am still puzzled on where the external pressure is coming from to extend the cage cylinder.

While manually extending with ratchet strap you could potentially pressurize the rod of both cylinders if the over centervalve is opened. This would then allow a slightly larger volume of oil to pressurized since the knuckle cylinder is in the fully extend position while you were doing this. Pressurizing the rod ends should keep the cage from extending but crazy thought if the seals on the knuckle cylinder are leaking slightly could you have also induced some pressure into the cap end of the knuckle cylinder? If yes then this pressure would extend the cage cylinder since would have free flow through the PO check and if high enough to over come the over center valve setting. Any trapped air would also act like an accumulator to increase potential flow.

Problem is this really doesn't work when you tried retracting the cage cylinder with the ratchet strap.

A bad seal on the cage cylinder and I would think you could extend the cylinder fully with the ratchet strap.

Back to thinking we need gauges in the system. Since closed system be nice to have one in both the rod and cap end lines so can observe what is happening. Problem how to do this with introducing more air or in a location where the air cab be purged easily or easier.

I'm going to watch a few videos on how over center valves work. It's been awhile since I had a hydraulics class and the only thing I remember is how basic PO check valves work. I also have to keep looking for the 145 bar reliefs.

Can you safely operate the cage tilt cylinder fully with the mast fully raised? I.e. knuckle cylinder fully retracted. Just curious is this would help remove any potential trapped air in cap end since it should be forced out to the cage cylinder.

I can't do it with the mast raised but that's what I do to purge it on the ground. Here's the air purge instructions from the manual. The green button they refer to turns on the hydraulic pump- it's a safety thing so you can't operate the lift without keeping this spring loaded button depressed.

 

[BeezFun]

Beez,
Couple of more questions:
...
3 ) same with two relief valves set at 145 Bar. Are they separate line bodies or in a common valve body. Not sure if these are in play but trying to gain a fuller picture of how this is plumbed.

I found the 145 bar reliefs, they are both attached to the valve body that manually operates the leveling cylinder. My pencil is pointing to one of them, the other one is on the opposite side of the valve:


I also discovered that the knuckle cylinder and cage cylinder are T'd together in front of the manual valve. So that means when I manually apply pressure to the cage cylinder, it's also applying the same pressure to the knuckle cylinder. Of course it has no effect because the knuckle cylinder is too small and poorly positioned to have any effect on the lift mast. My pencil is pointing to the T that connects the lines from the cage cylinder and knuckle cylinder:

 

[oldnslo]

Excellent job tracing the lines and finding the work port reliefs. You are learning more about this machine day by day.

 

[Jofang]

Forget air in the system. The hydraulic oil pump blasts the oil through the system so fast that any air is pushed back to the reservoir before it has chance to rise and settle anywhere. I note that cylinders 1 and 2 both have pilot operated check valves, so any oil to and from these cylinders has to go via the PO check valves. These PO check valves are there to lock the cylinders in the desired position, so if the cage is tilting these valves are allowing oil to pass when they shouldn't. PO check valves are cheap to replace so this would be my first port of call. The valve/s may have a weak spring or a worn seat and no longer work as they should.

 

[BeezFun]

For everyone still keeping score, I called tech support at Nifty Lift, surprised to get a guy who really seems to know his stuff. The guy said a bad load valve on the cage cylinder fits all my symptoms. He said if the system were sucking so much air that the problem returns after a few hours, I would be seeing problems with some or all of the cylinders on the lift being spongy or jerky. I asked him why the bleed process makes the problem go away for a few hours, he said he didn't know but it's possible that forcing that cylinder to the far ends of it's stroke in both directions is causing the short term fix rather than it actually purging any air. Normally that cylinder spends much of its life between fully and halfway retracted. It seldom moves into the extended position because that only happens if the lift is at it's full height straight up.

A replacement load valve is $115 and the local dealer has it in stock. I'm talking with oldnslo about any testing I can do to confirm it's bad before I replace it. But that's a modest enough amount of money I'm willing to see if it solves the problem.

 

[BeezFun]

Forget air in the system. The hydraulic oil pump blasts the oil through the system so fast that any air is pushed back to the reservoir before it has chance to rise and settle anywhere. I note that cylinders 1 and 2 both have pilot operated check valves, so any oil to and from these cylinders has to go via the PO check valves. These PO check valves are there to lock the cylinders in the desired position, so if the cage is tilting these valves are allowing oil to pass when they shouldn't. PO check valves are cheap to replace so this would be my first port of call. The valve/s may have a weak spring or a worn seat and no longer work as they should.

Thanks for the input, do you have any reason to suspect the check valve rather than the over center valve (also called a load valve in the parts list)?

 

[Jofang]

Once the hydraulic cylinder is in the desired position the PO valve circuit de-energises allowing the PO valve to close, which locks the hydraulic oil inside the cylinder thereby locking the hydraulic cylinder in place. This is a safety feature used on countless hydraulic circuits, so unless you've literally got oil dripping on the floor the hydraulic cylinder cannot move until the PO valve is re-energised to release the oil in the cylinder.

 

[Jofang]

You mention the load valve could be a possible problem, but I don't know the age of the machine or the condition of the oil and filtration system. The spools in your load valves are machine lapped to a tolerance of several micron and should give good service for many years, however, the debris in dirty oil will cause wear on these lapped tolerances and you could get slight weeping past the spools, but this is only remotely possible. I've seen 40 year old valves still in pristine condition, so I doubt if this is your problem. Your oil filtration will be somewhere between 5 and 15 micron and for longevity of the system it's essential that you have the correct oil filter/s and that you change regularly.

 

[Jofang]

Beware of the difference of terminology between the USA and Europe. I think in the USA the load valve and PO check valve may be the same thing. In Europe we use the terms PO check for the cylinder valve and directional control valve for the spool valve. The information I've given you is correct but beware of terminology.