one way dump won't go back down

[maddog]

hey folks:

I have a small dump truck that worked fine until about a year ago. I can raise the bed fine but it won't come back down without heavy weight. It is a one way pump, simple hook up, 1 line, vent screen on other end of cylinder. Nothing is bent or out of whack, it just quit wanting to fall back down. I even raised it and took off the line completely(I used supports in case it fell, safety 1st) it still stuck in place. I also changed the fluid and checked the pump over, everything works fine with the pump(fairly new). Could a seal or something have come lose inside the cylinder? The trouble must be in the cylinder, I would imagine, especially sense I disconnected the pump completely. I'm going to tear into it and check everything but wanted to see if anyone had any ideas to share?

This is just a simple electric pump, one way action, on a small truck, Isuzu NPR, average load would be 3- 3.5 ton. Thanks

 

[Delmer]

Is this a scissors lift with a regular cylinder? (not an extendable three stage cylinder) Are all the joints greased up on the pivots for the dump bed and the scissors? If the joints are all smooth, then you'll definitely find your problem in the cylinder, maybe a worn gland and/or piston that is letting the cylinder bend ever so slightly when fully extended?

 

[maddog]

Yes Delmer it's a scissor lift regular cylinder. Everything is greased and free, I'm a firm believer in grease and oil . Thanks for the reply

 

[The Peej]

I would check the pivots again. It's very common for them to freeze up

 

[87silvert]

^^^ this

 

[Former Wrench]

not seeing the truck, it is tough to make a comment; but I would look at the joints and if there is any issue with the stops. It could be it is extending too far and locking up. Good luck.

 

[kshansen]

Just as a test try stopping about 3/4 of the way up and see if it lowers smooth and as fast as normal.

Has anything been changed on the dump body? Any change that adds weight or redistributes it could cause body to go over center when raised all the way.

Does the lift cylinder have adjustable packings on the ram? Were they adjusted? Heil cylinders we use can be very sluggish coming down if packings are even a little too tight.

One question I should have started with, did this happen over a period of time or did you dump the load one time and five minutes later on the next lift it was acting up? How often is this truck used in the dump mode, does it sit parked for weeks on end?

Just noticed your location Adirondack Mtns. Is this a problem that came up with the cold weather up there? Could there be ice in the cylinder?

Years ago I was trying to understand a no start problem with a Cummins engine in an off highway Mack truck, problem turned out to be an ice blockage in the exhaust!

 

[Delmer]

Any chance you got water/dirt/rust into the empty end of the cylinder? One more thing you could try is to tap or pry the bottom of the cylinder at the gland end with a 2x4 when it's stuck. That might knock it loose if the cylinder is bent at the extended position.

I'd be tempted to add a solenoid valve and hook up the cylinder for power down. The down stroke will be faster than up because of the displacement of the rod, and it's on a truck so you don't have to worry about the battery dying. Smaller, older, worn, less frequently used lifts tend to stick so why not eliminate that problem?

 

[sawz5]

I would check the pivot points on the bed, they are probably rusted and binding.

 

[maddog]

Kshansen; it sticks at any angle, all the way. Nothing adjustable that I've noticed, nothing has changed, just doesn't like returning. I no longer live in the north(down in TN now) but cold never affected it before. 1st thing I did was replace all oil, including disconnecting the lines and letting everything drain.

Former Wrench; it's a very simple setup, one way dump on an Isuzu NPR, scissor lift. I'm wondering if it might have extended to far and caused something inside to break?

The Peej; all pivots a free and clear regular grease and regular use.

Delmer; foreign material is a possibility but the filter did seem very clean. The filter reminds me of the ones on oil furnaces small metal screens very tight, hard to even blow air through. Nothing is bent and it sticks at all angles even when it's an inch from being all the way down. I could add power down, BUT the ole saying comes to mind " don't fix it if it aint broke". It's always worked fine never an issue, I replaced the pump a few years back and nothing as ever bothered until this. I think I'm going with your previous advice and tear the cylinder apart, something must have let lose(??)'

Thank you everyone for the thoughts, I will keep you posted as to what has happened, I'll try pictures but for some reason I can never get them to post.

 

[51kw]

Remove the cylinder and use something else to raise and lower it. If it is free, it is in the cylinder. If not, then you need to look further.

 

[kshansen]

If you have a safe way to hold up the body, over head crane?, while removing the cylinder and then try lowering it with the cylinder out of the picture. This would prove if the cylinder was the problem. Then again if you remove the cylinder if the breather/vent is off the one end and the hose is off the bottom it should move pretty easy, maybe a good tap with a large rubber mallet. Also could try hooking an air hose to it, just be careful as air can make a cylinder move very fast and if there is any oil in it it can come out under pressure and make a big mess.

Don't know the size of your cylinder but just an example if it has a 3 inch diameter bore that gives approx. 7 square inches of area on the piston so if you put 100 psi air in it that ram will be moving with 700 pounds force. And also as air compresses and if the cylinder does have a problem and a tight spot it can build up to that 700 pounds force and then jump forward very fast so use caution if you do try to move it with air.

 

[maddog]

I have the bed up and blocked, unhooked the cylinder and it still took a 10ton porta power to close it, very sticky. Now my problem is getting the cylinder to come apart everything is rusted tighter then a _______. Might just be time to get a new cylinder

 

[Delmer]

https://www.heavyequipmentforums.co...on-end-cap-removal-question&highlight=peening

This is the best method to get a stuck gland loose if that's what you're dealing with. Might not apply to your cylinder? Pics of the cylinder will get you some good advice if you're stuck.

So are the seals swollen, or is the cylinder rusted up? can't wait to see the result.

 

[old-iron-habit]

If not already done check the hydraulic line going to the cylinder to insure the rubber is not failing on the inner layer and plugging the hose when trying to lower.

 

[maddog]

Unhooked everything that held the cylinder in place, 4 hrs later with many blows from a sledge hammer and the porta power pushing, plus having to cut into the rod holding bracket it finally fell free :cussing That pup was really rusted up in there(NY winters), it's a 98 and has probably never been taken apart. I gave up on the cylinder IMO it just wasn't worth fighting with any longer. Went to TSC just to see what they had for cylinders, lucked out they had what I needed hidden way back on a bottom shelve :woohoo I have to fabricate up one of my brackets but no real big deal, easy job. I may still try to get the old cylinder apart just to see what happened, but for the time being it's been put in the corner. The cylinder is definitely the issue even with it completely unhooked it was/is still a bear to retract.

 

[maddog]

New cylinder on everything works again :woohoo , thanks everyone for all the help.

 

[fergy]

Bent rod ?

 

[fergy]

Yes two words and I got one wrong
Stupid iPad

 

[maddog]

Rod was straight, just quit wanting to retract. I figure something must have come lose inside.