Cage leveling on aerial lift

[BeezFun]

I have this aerial lift.


I'm trying to figure out how the hydraulics that control the operator cage leveling work. As the cage is raised or lowered by the boom, there is a small hydraulic cylinder directly under the cage that extends or retracts to keep the cage level. The operation is hydraulic, there's no electronics or wires involved. The service manual gives vague instructions how to rebuild the cylinder but no explanation how it works. I'm assuming it must be sensing the relative pressure between the two cylinders that control the position of the lift arms because that gives you an indirect indication of the relative angle of the two lift arms.



Here's the part of the hydrauilc diagram that shows the leveling cylinder that lives under the cage:

Parts 19 and 20 are listed as check valve and overcenter valve respectively. Can anybody get me pointed in the right direction on how this thing works. It's not functioning quite properly but I don't want to get into that until I have some idea how it works.

 

[oldnslo]

Beez
Is there some form of mechanical linkage tying the bucket level to boom?

I see nothing in the portion of schematic you posted that would control synchronized movement. What does the rest of the circuit look like that is connected to these cylinders?

I suspect over center valve is a counterbalance valve but terminology can be confusing.

 

[Big Barn]

Sorry I don’t have the answer but I realize that’s a certified setup but too scary for me…

 

[4570Man]

I would guess there’s a second cylinder in there somewhere that’s not powered. As the arm moved it moves the non powered cylinder and squirts the fluid in the basket cylinder and makes it move.

 

[ptsg]

That looks like the same setup as the hydraulic self leveling loaders for tractors we have here.

Basically a smaller cylinder runs on top of the lift cylinder that's tied directly to the curl/dump cylinder, or in your case, the basket cylinder. There is also a cross over relief valve, because something has to give when the dump cylinder is either fully retracted or fully extended. The lift will probably have an extra safety valve of some sort.

Once the boom lifts, the small cylinder will push fluid from the rod end to the extending port of the basket leveling cylinder, and vice versa when going down. Doing it this way, it always maintains the position compared to a self leveling valve which will lose position over time.

It requires some decent math to figure exactly the size of the small cylinder that works with the lift, so it keeps the basket level across all positions.

It's also a common setup on telehandlers.

Here is a picture to show the small cylinder on top of the lift cylinder:

 

[LD1]

MAke and model of lift, better pictures, and perhaps a link to the COMPETE hydraulic schematic would be helpful

 

[BeezFun]

Thanks for all the replies, here's some more info. It's a Nifty Lift TM34T.
Hydrauilc circuit is attached.
Here's pictures of the leveling cylinder with the cage rolled forward so you can see it:

 

[BeezFun]

That looks like the same setup as the hydraulic self leveling loaders for tractors we have here.

Basically a smaller cylinder runs on top of the lift cylinder that's tied directly to the curl/dump cylinder, or in your case, the basket cylinder. There is also a cross over relief valve, because something has to give when the dump cylinder is either fully retracted or fully extended. The lift will probably have an extra safety valve of some sort.

Once the boom lifts, the small cylinder will push fluid from the rod end to the extending port of the basket leveling cylinder, and vice versa when going down. Doing it this way, it always maintains the position compared to a self leveling valve which will lose position over time.

It requires some decent math to figure exactly the size of the small cylinder that works with the lift, so it keeps the basket level across all positions.

It's also a common setup on telehandlers.

Here is a picture to show the small cylinder on top of the lift cylinder:

View attachment 714710

I think you've got the answer but I can't quite understand how it works on my lift. You're saying that on your tractor that small cylinder above the boom cylinder has its input and output connected to the curl cylinder, so when the boom cylinder raises, the small cylinder is extended and forces fluid into the curl cylinder to extend in just the right proportion to keep the bucket level. That makes sense.
So when I look at my lift, there are 3 cylinders involved, I'll call them the upper arm lift cylinder, lower arm lift cylinder and cage leveling cylinder. If either of those lift arm cylinders moves, it causes the basket to go out of level unless the leveling cylinder moves the right amount in the right direction. So how are both lift arm cylinders "connected" to the leveling cylinder and what does it mean to be connected. It almost seems like I need one more cylinder to mimic the behavior of your tractor.

 

[ptsg]

I think you've got the answer but I can't quite understand how it works on my lift. You're saying that on your tractor that small cylinder above the boom cylinder has its input and output connected to the curl cylinder, so when the boom cylinder raises, the small cylinder is extended and forces fluid into the curl cylinder to extend in just the right proportion to keep the bucket level. That makes sense.
So when I look at my lift, there are 3 cylinders involved, I'll call them the upper arm lift cylinder, lower arm lift cylinder and cage leveling cylinder. If either of those lift arm cylinders moves, it causes the basket to go out of level unless the leveling cylinder moves the right amount in the right direction. So how are both lift arm cylinders "connected" to the leveling cylinder and what does it mean to be connected. It almost seems like I need one more cylinder to mimic the behavior of your tractor.

I was checking the parts manual for you lift to get a better idea. It still works as I said before though.

The middle cylinder, the one mounted on what they call the knuckle and I believe you're calling it the upper arm lift cylinder, is the slave cylinder. It would be the equivalent of the smaller cylinder on the loader I showed before.

That cylinder is not actually actuated, instead it works exactly as your first paragraph of the quoted post, by forcing the fluid to extract or retract the basket cylinder as they are both connected to each other.

The parallel linkage on the lower arm, causes both arms to lift at once with the use of a single cylinder, the one on the bottom.

 

[LD1]

I agree it looks like the "knuckle" cylinder is controlling the basket cylinder when the boom is raises and lowered.

It also looks like there is a valve tied to the system so you can manually control basket tilt. Which would make sense cause every lift I have been on has been that way. So it's not a "closed" system. If you do need to remove/rebuild a cylinder.....you have manual control over basket til to get it back to level and purge air out. Then it's down to automatic operation as you raise and lower