Chinese backhoe hydraulic pressure issue

[brettski]

I purchased a backhoe, FEL, and hole auger new from a factory in China this past spring. Yes, it was and is a risk and I knew that going in. I am pretty comfortable with wrenches and mechanics, so I let the price make the final decision. In the end, it played out about as well as I expected. All 3 items had some issues. The only issue that I cannot definitively resolve is with the backhoe.
It is an LW7 knockoff, if not the exact model since so many of the Chinese factories build and market their own version of the LW7. It has an on board, PTO driven pump and tank. Right from the beginning, it had low power. I could not curl the bucket thru firm clay loam at a 6" deep bite. If I stretch out the boom and dipper to full horizontal extension, it struggles to pull it back up.
I removed the hose on the dipper and put a 5000 psi gauge on it. The dead head pressure bumps 1300 psi, then precipitously drops to 1000 psi in about 4 - 5 seconds and holds there. I can hear the rush of the pressure release bypass occurring. I adjusted the pressure relief valve, 1/2 turn clockwise; no change. Another 1/2 turn, no change. About 2 total turns and it was buried; no change. I disassembled the PRV all the way down to the check valve ball bearing. It was clean. I cleaned and blew everything out anyway and reassembled. No change. I disassembled and added a couple of small stainless steel washers to the top of the PRV spring to really push it down. No change. I removed the test washers, re-assembled, and gave up.
What next? How does one analyze these findings? Does any of this point to the pump as the culprit? I can find no ID on the pump, but I was told by the factory that it produces 2600 PSI and 8 gpm. These specs seem fair, but how do I know that it is actually working as designed?

 

[CJONE]

Sounds like the pump may have a internal relief or you are missing a relief somewhere. I would back the relief that you messed with back off or when you find the problem you may get another! I see it all the time. Start testing each item from the pump out with a flow/pressure tester until you find the problem. CJ

 

[schmalts]

I am no Hydro expert by any means but I have a Chinese hoe as well and sold a couple of them to offset the cost of the one I bought. I can tell you it was a common problem to find chips of metal in the PR valve when new. I would take the PR apart again and look it over. A small pump usually will just make it slow, but not weak. I never seen a pump on one with an internal pressure relief, so I doubt that is it. Usually the pumps have a number on them such as CBN-E310 or something similar. The last 2 digits is the amount of volume per rev.
Also look to make sure no other cylinder is creeping or moving on it's own indicating a valve is stuck from the neutral position robbing power from the intended movement. Most times the pumps were so undersized that two motions could not be made at the same time or one would just flop dead.

 

[dfkrug]

I have found debris (rust) in a Chinese RV before. Also, look for improperly installed
piston seals. This causes high bypass flow under pressure, perhaps enough to give
you the 1300psi you see on the gauge.

 

[CCWKen]

Could be a bad case seal in the pump itself. It may be rolling, cut or pinched. Or it could be some debris between the plates. 1300psi is pretty low to start with. Sounds like an internal leak that won't show on the exterior.

 

[brettski]

Thanks for the replies.
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If I am hearing the rush of fluid at the controller when the pressure dead heads, does that not tell me that the PRV has opened?
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or, try this one....
Why does the dead head pressure NOT change a bit, regardless of what depth I set the PRV adjustment screw? This includes inserting the 2 washers to collapse the PRV spring even further. The rush of fluid at the controller occurs right when the dead head happens, yielding the same reading of 1300 PSI at every test.
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I should add that these tests were all taken at 540 PTO and 1600 RPM at the tractor. I realize that adjusting the RPM a few hundred here or there should have no effect on the ultimate pressure, but it will have an effect on the volume of fluid pumped. Please correct me if I am wrong on that statement. Given this assumption, when I do adjust the RPM up or down, I do see corresponding "quickness" of the hydraulic cylinders as they react faster with higher RPM. Does this tell me that the pump is less likely the source of the problem since it is increasing volume as PTO RPM increases?

 

[CCWKen]

I don't recall you mentioning the type of pump. (piston, vane or gear) Volume output of the pump may increase with RPM but not necessarily pressure if there's too much clearance between the gears or the gears and the case. These are symptoms of a worn out pump. Or poor assembly if it's a new pump. You've already demonstrated that you can't increase the pump pressure. I'm not sure what you mean by the sound "rush of fluid" but a dead-headed pump should scream! Like stepping on a cat's tail. What you may be hearing is pump cavitation as the pressure bleeds back to the input side and not raising the PRV.

That's just my view of it. But's it's the first place I would look.

 

[brettski]

I don't recall you mentioning the type of pump. (piston, vane or gear) Volume output of the pump may increase with RPM but not necessarily pressure if there's too much clearance between the gears or the gears and the case. These are symptoms of a worn out pump. Or poor assembly if it's a new pump.

I have no idea on the construction or drive train within the pump; sorry.

I'm not sure what you mean by the sound "rush of fluid" but a dead-headed pump should scream! Like stepping on a cat's tail. What you may be hearing is pump cavitation as the pressure bleeds back to the input side and not raising the PRV.

Interesting point; let's explore a bit more.
No it is not a scream or squeal or high pitched noise. It is more of a sound like..well...maybe...like a pump straining. It sounds more mechanical in a fluid kind of way. I have a short vid clip. Can I post a vid clip on this forum? EDIT; see video link, below.
So, for the sake of argument, let's say that the sound that I am hearing IS pump cavitation. What explanatory path does that lead me down?

[video]http://vid43.photobucket.com/albums/e373/brettski69/8-27_8-28-16%20004_wmv2.mp4[/video]

 

[CCWKen]

Photobucket or my wireless must be getting hit a lot at the moment. I can't see or hear more than half-second pieces at a time between one-second pauses. It's like burp, burp, burp so I can't make any determination from that.

Probably nothing to do but disassemble the pump and check for damage and/or wear. A few things to check first. Make sure the input hose isn't pinched, kinked or collapsed and the oil is free flowing from the tank. Is there a suction filter on the pump input side? Strainer in the tank output side? Check those. As simple as it sounds, does the tank have enough oil? While the engine is running above idle, see if you can squeeze the input hose. You should not be able to collapse the hose by hand.

If you're sure that the pump is getting the oil, about all I can suggest is the exploratory operation on the pump. Don't buy a new pump yet. It may be nothing more than a bad seal. If you don't feel up to the mechanical task, a hydraulic shop should be able to test/fix the pump.

 

[CJONE]

Thanks for the replies.
-
If I am hearing the rush of fluid at the controller when the pressure dead heads, does that not tell me that the PRV has opened?
-

Why does the dead head pressure NOT change a bit, regardless of what depth I set the PRV adjustment screw? This includes inserting the 2 washers to collapse the PRV spring even further. The rush of fluid at the controller occurs right when the dead head happens, yielding the same reading of 1300 PSI at every test.
-
This is why I am suggesting it is something else and also why I suggested using a flow/pressure tester. Any good hydraulic shop should have one and most will either loan or rent them reasonable. Otherwise you are basically guessing. CJ