Hydrolic Valves Open Center

[Wern]

I purchased l in Oct.06 through eBay a 3 Point Backhoe from KPX Tractor. It had metric connection and I did not want to tie the backhoe into the NH TC33 hydraulics system. (did not know where and how anyway).
So I purchased a 21 GPM PTO pump from Prince Model HC-PTO-1A. I had the metric connectors replaced with quick connectors and finally managed to hook it all up.
Everything run smoothly, but whenever I operated a valve, the tractor stalled. At this time I did not know anything about center open / closed system. I reported my problem to the supplier, but could get absolutely no help from the KPX. After numerous calls to KPX they finally promised me a new valve set. Again, numerous calls until I received in May a new valve set, but it was not exactly the same type as the original. Two damper valves where missing.
Anyhow, I replaced the set but had more or less the same problem. The tractor nearly stalled and the oil got very hot very quickly.
I took the original valve set to a hydraulics shop and he said that this was a center closed valve set which will not work on my tractor.
Can anybody tell me from the attachment if the schemata shows an open or closed center valve set. I think it is a center open, but would like to get this verified.
Obviously it does not work with the continuous flow Prince Pump.
Could this valve set be hooked up anywhere on the tractor that it would work ?
Or could the valve set be modified into a center open system by drilling a hole in the endplate ?
Advise would be very appreciated. This problem, with no support from the supplier is driving me insane !

 

[shaley]

The schematic shows an open center system. I base that on the line that is shown going from the supply just before the check valve and thru the valve center section of all the valves.

 

[kennyd]

Wern, First, Welcome to TBN

This question will get more views in the "hydraulics" forum, but I will try to help you...

If your valve was CLOSED center, Then the tractor would stall as soon as the PTO was engaged, not when you operate a lever. You NEED an OPEN center valve to work with the PTO pump as it continually pumps fluid whenever it's turning as opposed to a variable displacement pump.

Once again, you NEED a OPEN center valve to operate the machine from either the PTO pump or the tractors hydraulic system.

I am not good at reading the schematics, but you can verify OPEN center by using air pressure on the inlet and see if its comes out of the outlet with no levers being operated.

1. How big is the reservoir tank on the backhoe? With a 21 GPM pump you will need a fairly large tank.

2. Does the tractor run with the PTO engaged?

3. How hot is the fluid with the tractor just running with the PTO engaged?

 

[shaley]

Did you add a hydraulic reservoir tank along with the pump or did one come with the hoe?

 

[Wern]

Thanks guys for the welcome and quick reply. Too bad that I discovered this forum only now, I could have saved myself a lot of time and frustrations.

I am glad that most of your answers confirm my belief on this subject.
The oil reservoir came with the backhoe and has about 41/2 Gal capacity, which is probably a bit small for the pump, but I don't think it is my problem.
The tractor runs fine with the PTO engaged, although I have to check the oil temperature in this mode, but I am quite sure that the oil is not getting very hot. So it is works like open center in this idle mode.

The moment I operate any valve the tractor almost stalls. I can barely lift the boom, the engine is bogged down to about 700 RPM. It seems as if the oil returns via the relief valve. That is when the oil gets very hot.
As I am describing this problem I am now convinced that this is what's happening, because when I fooled around with the pressure relief setting things got smoother. (I regretted later that I messed with this setting since I don't know anymore at what pressure it is left on).

The hydraulics shop checked the valve set and confirms that the oil is against dead end when a valve is operated. And this problem is on both of my valve sets!

If my assumption, that the oil returns via the pressure relief valve, is correct, then I wonder if or how this could be fixed. The valve set are made in China and it took me well over a half a year to get a replacement from KPX with the same fault!

Attached are some pictures of the hook up. Initially I had an adjustable Flow Contoller in the circuit, but I removed it to eliminate additional problems I may have introduced.

My biggest worry is that my wife would sell the backhoe to another poor guy and leave him with the problems I had to deal with i.e. KPX tractors.

 

[jinman]

If my assumption, that the oil returns via the pressure relief valve, is correct, then I wonder if or how this could be fixed.

I think KennyD has really "nailed it" with his description of how things work and what to check. I'm not a hydraulic expert at all, but on my tractor's remote valves, you can hear the tractor really loading up when I feather the operation of the remote valve. I think the valve is blocking the flow, but not letting much go to the cylinder. So most of the flow has to go through the pressure relief valve as you said it seemed to be doing with your system. Do you have a variable pressure relief valve? Is it possibly set too high? That seems to be the way your control valve also works.

I don't know if other backhoe control valves work this way when you feather the controls, but if they do, then the 4-1/2 gallon reservoir is less than half the size you'd need to keep the oil from overheating. I'd sure use an IR thermometer or something similar to measure the temperature of your oil. If it's much over 180 degress F, I'd be concerned about heat. Temperatures less than that are normal.

 

[dynasim]

I think the pump capacity is to high for both your tractor and backhoe. The tractor can't handle the hp required for full flow at max pressure. The moment you bump the handle, the valve becomes a closed center valve, and all the flow must bypass through the safety valve, adding 20KW(30 HP) of heat to the oil. That will heat it up fast!

Because you are likely at you hp limit, you will bog down the tractor. Once the tractor bogs down, you don't get the flow or the pressure required for the backhoe.

There is something to the concept that you can go too big.

That is my thought.

Chris

 

[dynasim]

For grins, I did the capacity and heat calculations.

A 21 gpm pump at 2500 psi will consume about 31 hp.

At 31 hp, a dead headed system containing 4.5 gallons of oil, with about 150 lbs of steel containing it(the pump, pipes, and container), will heat up at about 3/4 degree F per second, or 45 F per minute until it gets hot enough to transfer the heat to the air. That is fast.

Your pump is to big. You could lower the relief valve setting, though, and get by.

Chris

 

[Wern]

I have again attached the schemata marked up as I interpret how it should function (ValveSetFunction.pdf. As I mentioned before, I did make adjustments on the relief valve and things got smoother. But according to the specs the relief valve should be set at 16 MPa which is about 2320 PSI.
Changing this setting might be a work around, but I have the feeling that the valve set would not be working as designed. If I am wrong, in other words, if this is the way it should work, then the supplier or OEM should have made these adjustment accordingly, prior to shipping.
As I said, I bought this backhoe a year ago new at KPX Tractor. I have not been able to make it work since. The problem is that there is absolutely no help from the supplier.
I never ever got a reply to any inquiry. A second valve set was shipped to me, after numerous calls and promises, half a year after delivery of the backhoe. I found others in this forum complaining about the same thing.

According to dynasim it seems that I have a too large pump. This is possible, that's why I put at first a flow controller into the circuit, but it did not make any difference. (See attachment HydrConnWM1.pdf). It was just an other restriction heating up the oil.

I am a complete greenhorn in hydraulics and hesitate to disagree with you but I just can not imagine why a too big pump would make the oil to work against a dead end. These pumps are not cheap and I am reluctant to spend the money into a smaller pump unless it is obvious that this is the problem.

I think these valve sets have a fundamental problem which leads me to an other idea. Earlier I had to take a valve stem out because it was sticky. I found some machining debris in there which made it run smooth again after cleaning and blowing it out. The valve stems look like my sketch attached ValveStem.pdf. If these valve stem are not symmetrical and were inserted the wrong way, some ports may not match up and closing the return.
If anyone thinks this is a possibility I take it apart again and check this out before I buy an other pump or change the setting on the relief valve.

 

[kennyd]

I think the problem is in the valve...
The "pump to big" theory has merit, but I don't think the tractor would stall immediately if a lever was just cracked open as you have stated.

This is a lesson to all of us about buying Chinese stuff on EBay though...Save a little now but pay a lot later!

 

[kennyd]

Another thought, to eliminate the "pump to big" theory, try running the tractor at a lower RPM, that will slow the flow down.

 

[shaley]

I think now you need to do some testing. Remove the hoses from the circuit you show in your highlited schematic and find out if in fact oil will flow when the spool is shifted.

 

[dynasim]

Given that the relief pressure is 2320, I can almost guarantee that the problem is to much flow through the pump.

If a positive displacement pump does not have enough drive torque to pop the safety valve, it will drag the motor down. A 33 hp driver will not drive this pump at full pressure. Since the pressure capability is directly related to the available torque for a given capacity pump, and you are at peak torque at or near the operating RPM, it is a spiralling problem.

However, after the motor drags down to low RPM, you will get some back flow through the pump, and the motor will stay running, but you won't be able to do anything.

To test out your system, I would hook the valve system to the tractor's hydraulics for a bit. I would bet that your system will work as designed.
EDIT: I assumed(possibly incorrectly) that you had a rear hydraulic remote to tie into. You may need to use your FEL's curl circuit.

To use you system as-is, I would significantly lower your safety valve pressure(to say 1700 psi). This would give your tractor a decent operating margin, and you could slow the engine to get the pump capacity to something reasonable(10-12 gpm). Note that the lower pressure will limit your bh's lift capacity, but not its speed.

Best Regards
Chris

 

[dynasim]

I just looked at you flow controller schematic. Because this is a positive displacement pump, the flow can't be "controlled". The flow is going to be essentially dependent only upon the RPM. For a flow controller to work, there must be an alternate path for the oil to bypass.

Chris

 

[JerryG]

I think that everything Chris has said is correct. The valve and the relief combined together can't handle the flow from that pump. The least expensive way to get the hoe to work would be to plumb it into the tractors hydraulic system.

 

[Wern]

I have tried to run the engine at lower RPM to reduce flow, but the engine will stall right away.
Maybe rather than tying the BH into the tractors hydraulic system would be, as dynasim explains, to hook up the flow controller properly, that is to connect the bypass port of the controller to the oil tank,. This way I should be able to reduce flow and test the "pump too big" theory. I am afraid that I might contaminate the tractors hydraulic system with the rudimentary BH oil system (as mentioned earlier, I had debris in the valve set)
I will have to go back to the equipment hook it up again and do more testing. I'll be back, hopefully with facts which leads to resolve the problem.
Thank you all.

 

[kennyd]

to hook up the flow controller properly, that is to connect the bypass port of the controller to the oil tank,. This way I should be able to reduce flow and test the "pump too big" theory. I am afraid that I might contaminate the tractors hydraulic system with the rudimentary BH oil system (as mentioned earlier, I had debris in the valve set)

Wern, The bypass from the divider should go to the tank on the backhoe, not the tractor-so no fear of cross-contamination

 

[shaley]

I can turn my PTO pump by hand without much effort until a valve is actuated, then you are doing work so the effort is greater. If the actuated valve is the stablizer down, the effort is very little. This might be a way to test the circuit with little trouble. I don't think the releif in the BH circuit comes into play until a circuit is overloaded such as the boom trying to lift a tree stump out of the ground that is too heavy.

 

[jinman]

To test out your system, I would hook the valve system to the tractor's hydraulics for a bit. I would bet that your system will work as designed.
EDIT: I assumed(possibly incorrectly) that you had a rear hydraulic remote to tie into. You may need to use your FEL's curl circuit.

Both great ideas... It would be worth the expense of getting the hoses to the FEL curl circuit and show how the system would work.

 

[dynasim]

The only way to make that pump work with that tractor is to lower the relief valve setting. Installing the flow controller properly will not lower the torque/hp required to drive the pump if the pump outlet pressure is held constant.

Diesels are essentially constant torque machines. Lowering the speed will not help with the stalling issue because the tractor just doesn't have the torque to drive the pump at the relief valve pressure(at any rpm).

For a given pump, torque requirement and pump outlet pressure are directly related.

Hope this helps.
Chris