JLG Boom Lift hydraulic "slack"

[DavidL]

I was using the 600A yesterday. Drove it to the worksite about 1/2 mile. Then, I rose the main boom, and telescoped the fly boom almost to full extension.

The lowest tower main Boom Assembly then rocked forward causing the whole upper section to drop suddenly.
Thankfully I was secured in the basket and had a hand on the cage.

I then retracted the fly boom, which caused the balance to change and the Boom Assy then rocked back to lowered position which was another catapult ride in the basket.

Got the basket down, checked the hydraulic fluid. I then moved the tower telescoped cylinder up / down which seemed to work normally.

The sensation was like there was no hydraulic fluid in the tower telescope cylinder so when the upper section reached it's tip point, there was no fluid to control the tower cylinder. Once the fluid compressed, the upper section stopped it's fall. It was slightly dampened stop. ie: not metal on metal like the cylinder topping out or in.

Any ideas? Could the fluid have left the cylinder during storage? Is it good practice to cycle all cylinders before raising? It had been a couple of months sitting before being used yesterday.

 

[DavidL]

No feedback?

 

[willie59]

The first thing I would do is have a helper repeat that process from the ground controls while you check the machine components operation to determine where that movement is happening for certain. It could be the slew ring is worn and nothing to do with the tower cylinder. You have to find out for sure what's causing the movement you described.

 

[DavidL]

Where's the slew ring?
Thanx...believe it or not, forgot that I could operate from the ground...first place you go blind is in the eyes.

 

[willie59]

The slew ring is the large diameter bearing that the upper rotates on. If that bearing gets excessive wear it will cause a rocking motion that you described when the load shifts by telescoping out. I've worked on those type of machines for years and I've never seen air in a cylinder that causes what you described. Not saying it could happen, just saying I've never seen it.

 

[DavidL]

I don't think it's the slew ring then because that wouldn't "fix itself". The unit currently doesn't have the issue, once I cycled the hydraulics. The lowest main Tower rocked maybe 30 degrees, which translated into the basket dropping about 20 feet. So it would have to be a realllllly worn out bearing to have caused that. And then all was good again once I brought the main tower up / down one time.
I am wondering if because the unit wasn't on level ground, if I sucked up air from the hydraulic tank while extending the top boom out. I'll have to run it from the lower station to try to duplicate the issue and monitor the fluid level while on the ground. With all collapsed and lowered, the fluid is in the middle of the level gauge.

 

[Vetech63]

Not sure on this model, but some sections are cable operated. Remove the rear cover on the back of your boom and make sure your cables and pulleys are not coming apart.

 

[92U 3406]

We used to have a bunch of 600A's. No cables on ours.

So you lifted the tower boom first and then extended the primary boom out horizontal? Then the tower boom dropped (moved towards the counterweight)? When you retracted the primary boom the tower boom raised (moved towards the platform)?

 

[Vetech63]

We used to have a bunch of 600A's. No cables on ours.

So you lifted the tower boom first and then extended the primary boom out horizontal? Then the tower boom dropped (moved towards the counterweight)? When you retracted the primary boom the tower boom raised (moved towards the platform)?

Yeah, I'm not sure I'm understanding what's being said either. Is the main boom dropping? Or is the inner boom sections sliding back into the boom?

 

[DavidL]

Was looking for a diagram of the lift parts so we are all using the same part names...
The lower tower is down ("G)". This is the lowest, largest cylinder.
The Main Boom was angled upwards 45 degrees or so (cylinder "A").
I extended the fly boom ("B" - linear) that holds the jib / basket.
Once the fly boom got to the "tip" point, the lower tower cylinder ("G") allowed the entire top assembly (A,B,C,D,E,F) to rock forward, causing the basket to drop.

My theory is that there wasn't fluid in the lower tower cylinder (G) so when the weight of the fly boom / basket was extended, the G cylinder expanded, allowing the upper section to drop as there was no fluid to stop the cylinder from expanding on it's own.

Hope that helps the explanation. Thanx

 

[DavidL]

Ok, So let's assume the main tower cylinder leaked down in the three months since the machine was last used...is that something that is expected or an "Oring" that is leaking down? If so, what's best guess of what might be leaking down - if this is abnormal? I gotta assume it's abnormal...this could have killed someone or damaged property when it dropped. I was just lucky with the circumstance that it was just a panty stainer and nothing more.

 

[Tradesman]

I'm anything but a safety Nazi I can routinely be found walking beam lines or on roof edges without safety equipment just for sh!ts and giggles, BUT!! I don' screw with aerial equipment. I run a crane close to half of my time and if something unexplained happened to my crane or boom lift while I was operating it, I wouldn't make another pick until I was sure what happened and was convinced it wouldn't happen again, or failing that it was checked over by a qualified person and they where confident it was safe to operate. There aren't many close calls with aerial equipment more often tragedies.
Play safe