hydraulic pump stalls when descending

[megavolt512]

Hey guys!

I finally got most of my Versalift running nicely. All hydraulics smooth. However I've noticed that when descending (the main cylinder), the hydraulic pump really seems to put a lot of load on the engine - and it descents pretty jerky too.

None of the other controls seem to even slow down the engine driving the hydraulic pump. But descending nearly stalls it!

Any idea why this would be??? I would think the load/pressure would be greatest when raising the bucket with my heavy self in it. But the hydraulics just breeze through that task

[Muldo]

It's hard to say without seeing an actual hydraulic schematic for the unit, but if it's done properly it should be using a counterbalance valve to hold the load (you) up. For safety reasons a good system will never rely on the control valve to retain a load.

The counterbalance valve may be set too high. When lifting this isn't an issue as they have a freeflow check in that direction. (The unit isn't going to 'fall up'.) But on lowering the pump not only has to still hold up the load, but also has to overcome the pilot ratio in the counterbalance valve to make it open. This is why the pump will actually see a higher load when lowering than lifting.

Again, just assumptions without seeing the correct schematic for the machine...but that's the best I can muster off the top of my head.

-Mike

[megavolt512]

Hi Mike, thanks for your informative reply. It does have a counterbalance (holding) valve. It also has a flow control valve (you can see it just to the left of the holding valve in the schematic).

I've adjusted the flow-control, but it still seems to have a high load on the pump. I have not adjusted the holding valve, but the procedure in the manual says to extend the boom out all the way horizontally with a person in the bucket. You then adjust until the cylinder just holds this load without drifting down.

Does it sound like the holding valve is too high?

John

[Muldo]

It could be. Are you able to get part numbers off of the load control valve?

The procedure described in the manual is a good way to field-set a counterbalance valve. Need to ensure that this is done with slightly greater than expected load on the valve... So if you're normally lifting a person and some stuff, set the valve with a person, some stuff, and a little more stuff.

The counterbalance valve is essentially a lockout/relief. If the pressure induced on it by the load (the boom, basket, people, supplies) is higher than the setting, the valve will open and drift down. That's why they say to set with the boom all the way out and someone in the basket. To put a higher load on it than it would see under normal operation. This ensures that it won't drift down unless it's overloaded.

Where CBV's get fun is in the pilot ratio. If you have a valve with a low pilot ratio, and it's set too high to begin with, the pressure the pump has to make to get it to open increases.

To make matters worse, the pilot pressure required also has to overcome back pressure in the return line at a 1:1 ratio. The way the flow control is plumbed between the CBV and the control valve, and in a 'meter-out' (controls flow on descent, not raise) the pressure drop over that flow control is directly additive...so things start to pile up, so to speak.

All these factors are why your machine appears to be 'laboriously driving' the boom down, rather than just controlling it's descent.

[Muldo]

Also, start with the flow control screwed in almost all the way...like all the way in then back off a quarter or half turn. Then set the CBV. Once your CBV is set, then back out the flow control to a safe but productive speed.It's possible that the cylinder is trying to descend faster than the pump can get it oil.

Edited June 13, 2016 by Muldo

[Muldo]

Gahhhh....the more I look at that schematic the less I like it. We usually spec control valves that have work ports open to tank in neutral when we're using things like CBV's and PO checks. This keeps pressure from getting trapped and causing erratic behavior. It also means you may have to engage the 'lower' function in order to even set the holding valve. Could be a fun ride for whoever is in the basket at the time.

I have to assume yours isn't the only machine on the planet with this plumbing setup, which means that it's out there and working elsewhere on other folks' machines. Everything I'm saying is based in general hydraulics, not machine specific. I suggest if a slight fiddle with the holding valve and flow control still don't work, to get someone at the manufacturer on the phone. It's definitely not setup how I would spec a system.

-Mike