Bucket Truck Blues

Acablegypsy · Nov 10, 2010

I realize this is an unusaul topic on this forum but web info on bucket trucks is amazingly scarce from what I've seen.

I have a 1988 versalift telescoping boom that is powered from an Onan 4kw genset with a front mounted external gear

pump. (common setup in the day) The pump is 2250 psi and 2gpm. The boom used to work perfectly and then the truck sat

for 4 years. Recently I decided to get it running again. When it first started up everything seemed to work ok but a little

slow. The fluid was low so I got some standard tractor oil and topped it off. At the time I didn't realize my boom uses iso

32. When I tried running it again it worked for about 5 minutes and then started moving very slow and then stopped

completely. At this point the oil was very foamy. After letting it sit a while the boom worked again for less than 5 min.

Discovering my error, I decided to drain the tank and fill with 32 and put a new filter on. After this the boom would raise

about 6 feet off the cradle but then wouldn't come back down. The in/out function was the same and it wouldn't come back

in.

I took off the pump and found that both gears were chewed up on the side toward the shaft. I cleaned it up a little and

put it back on and then no functions even tried to work, no hose bouncing, no whining from pump. I later realized the

housing/head fits on the pump body both ways and that the pump body fits both ways on the flange. The hoses remained

connected to the head large hose on input small hose on output but i may have mounted the pump body upside down. Will this

cause the pump to run backwards? To make matters worse I had put the steel check ball into the wrong(input) side of the

pump.
Right after this, I noticed a new large leak dripping from the raise/lower cylinder in the knuckle over the pedestal where the

oil was pooling up in the front of the bed. Each boom testing session described was 5 minutes or less, with this last one

being only about 20 seconds. So did I blow some seals out or something?

I bought a new pump for it and tried it today. Now it will go up but not down again. Because I fear I wrecked the first

pump somehow, I only tried the new one for a few seconds. There is a safety valve to prevent the boom from falling on

pressure loss, but it seems a brand new pump should have enough pressure to pass it.

I got a hydraulic schematic from the manufacturer and it shows a line coming right off the tank to the pump and the return

line going through the filter, a ball valve and into the tank. On my setup, the big hose comes right off the filter to the in

side of the pump then the out side goes through a relief valve and then into the valve bank. This is how its always been and

used to work great. How can this be?

So someone please help me out. Any info at all will better my understanding of this problem. Thanks for reading this whole

thing.

mrmikey · Nov 10, 2010

There is a safety valve to prevent the boom from falling on
pressure loss, but it seems a brand new pump should have enough pressure to pass it.

Called a counterbalance valve, it won't allow fluid to flow out the port on the cylinder unless there's pressure to force it open. Used on manlifts is one application, to prevent freefall should a hose break on the boom.

I got a hydraulic schematic from the manufacturer and it shows a line coming right off the tank to the pump and the return line going through the filter, a ball valve and into the tank.

Sounds right, only thing to watch for is that you don't close the valve with the pump running. If you do, depending on the pressure rating of the valve, you'll either blow up the valve or the pump. The valve is more than likely there to allow changing hyd oil without losing any out the tank, if the oil isn't above the tank I'd lose it or at the minumum put a cable tie around it so there's no way to clsoe it accidentally.

On my setup, the big hose comes right off the filter to the in side of the pump then the out side goes through a relief valve and then into the valve bank. This is how its always been and used to work great. How can this be?

Not a good idea to put a filter of any kind on a suction line. You have to think, you've only got 14.7 psi forcing the oil into the pump so it won't take much to restrict flow. I'd say a strainer maybe but even then.... any tank I've build I leave the suction protruding up 1/2 to 1" into the tank so it won't pick up any big pieces should they get into the system.
Hard to diagnose long distance

but your description of foamy points to cavitation or air being sucked into the system. The cavitation would be caused by a restriction to the pump, it doesn't take a long time to ruin a pump with cavitation. It won't happen in a few minutes so there's no fear you ruined the pump but don't run it for any length of time. You should be able to hear it if it is......Mike

Rocwin · Nov 11, 2010

We are in the bucket truck industry so we see issues like this quite a bit, especially on neglected equipment.

As mentioned by mrmikey the safety valve is called a counter balance valve, or holding valve in the industry. After 4 years of sitting it may not be working correctly, they tend to wear and age poorly and are frequent problem areas on bucket trucks. Versalift used to use a lot of Sun Hydraulics holding valves (CBCA-LHN type) but at your vintage its hard to say what is on there, easily identifiable with a photo. There should be an adjustment on the valve, a set screw or something which you can turn in and out, this can be used to get the boom down in an emergency or adjust if the cylinder chatters or drifts.

The filter set up seems right, the two ball valves should be tied open (by law here) as to prevent the entire system from destroying itself if vibrated closed while driving. The filter should only be a strainer in that position but you should have a spin on filter element on the unit as well, and hopefully that has been replaced. The foamy oil does look like you have some restriction so I would start by removing and cleaning the strainer, return filter, and check your tank and breather cap.

mrmikey · Nov 11, 2010

There should be an adjustment on the valve, a set screw or something which you can turn in and out, this can be used to get the boom down in an emergency or adjust if the cylinder chatters or drifts.

Like Rocwin said, you can get it down in a pinch by backing out the adjustment.
Be advised, quite a few years ago our local power corp had a failed hydraulic system on a bucket truck with a man in it. The mechanic was doing just that, backing out the adjustment to allow gravity to lower the bucket. He inadvertantly (sp) removed the complete cartridge instead of backing the valve out which allowed the oil to come out the port dropping the boom with a fatal result......Mike