Hydraulic Pressure Question, please help the rookie!

[gravinus]

Hello to everyone in the forum!


Due to the fact that I am too poor to buy a nice skidder at the moment, I bought an old L-785 that had been sitting for 2+ years due to an engine issue and was involved in a good old fashioned barn fire too (big mistake!)... I’ve got the machine running and the hydraulic oil was very very milky, so I drained all 22 gallons and refilled. It is still a bit cloudy, but doesn’t look nearly as bad as it was. Now when I raise the boom cylinders the charge pressure warning light comes on and it gets worse as the machine warms up. I replaced the warning sensor (preset at 50psi) and it appears it is a real low pressure condition as it is still alarming. I installed a pressure gauge on the bucket cylinder and when the stroke is max’ed out the pressure can reach 2,200psi and you can hear the control valve begin to bypass. I took the gear pump apart and the gears looked perfect, although there was probably .010” of wear in one side of the aluminum casing.

Could this be enough wear to cause a low flow condition in the pump? The machine is supposed to have around 2,500hrs on it, although I’ve come to trust nothing the seller told me about the condition of the machine at this point. I’m on the fence on if I should bite the bullet and buy a new $400 gear pump? The way my machine is designed, the gear pump feeds the control valve, fluid is then filtered, goes into a 110psi check valve, excess goes back to the sump while the constant 110psi fluid feeds the hydro drives (charge switch is located on the feed to the hydro drives). I would hate to install the gear pump and be back in the same spot again, but it would sure be sweet if that worked too! Any advice would be GREATLY GREATLY appreciated! I don’t have much experience with this type of equipment and thoughts/ideas would help!

Thanks A Lot!

 

[gravinus]

Forgot to also mention that I've cleaned the prefilter several times and the canister filter twice. Initially, there was a bunch of white snot clogging the prefilter, but they are all clean now.
Thanks!

 

[BillG]

Actually .010 wear is considerable when you take into consideration the leakage at say 2200 psi. If it were mine I would reassemble the unit, hook up an external filter and tank and do a thorough job of cleaning the system. This way when you replace the pump you have a good clean system. That said as far as the warning buzzer goes, have you checked the actual charge pressure to verify it is low. Just because you replaced the switch doesn't necessarily mean the new one was good (been through this too many times). The 110 psi pressure control could also be leaking (I am not familiar with what is used here, an orifice or a pressure control valve). Good luck, if you can borrow a portable flow tester and do a load test on the pump this would be a good solid test. Sometimes a dealer will rent special tools, try them.

 

[gravinus]

Thanks for the reply! I haven't actually checked the charge pressure, but my feeling is that it is actually low. I can notice the bucket hydraulics slow down as the machine warms up and the charge light only flickers when the boom is raising, which I'm assuming is putting the largest oil flow demand on the system? It uses a spring loaded check valve to create the 110psi charge pressure. I had hydraulic fluid tested and it came back great as far as particle counts and trace metals were concerned, but it did test positive for water. I will have to look further into external water filtering systems.

Thanks for the advise! I might have to try to find a flow meter, but if your saying .010" is considerable wear, it seems to make sense with the issues that I'm seeing.

 

[Bobcatdan]

Basic hyd principle, gpm equals speed, pressure equals power. Pumps do not built pressure, restinace in the system built pressure. Now bad pumps can equal low pressure if the pump has so much leakage it can't push enough fluid for the system to built pressure. Check what actual pressure and go from there. Also if you do replace the pump, check and set all pressure after. High relief pressure will take a pump out and if not caught will make short work of a new one. Hope I said something useful.

 

[gravinus]

Thanks for the reply! That does make sense! This seems more like a low flow condition (I think). The boom is slow to raise and the charge light will be on. Once the cylinder tops out, the pressure in that line (I have a gauge on the control valve outlet) will spike to 2200psi and you can hear the relief valve bypassing. Once this happens and there is not a major flow demand on the system the charge pressure light goes out again. It is only the raising of the boom that causes this light to come on, going down I think it has the weight of the boom/bucket to assist.

Would a low flow condition make the pump more or less suspect?

Thanks again! I appreciate all the input!

 

[Bobcatdan]

Low full normally equals bad pump. That said certain system, something could have such bad internal leakage that low flow could result in low flow. Bobcat recommands when checking flow to do a direct pump test before condeming a pump. Hook the flowtester right to the pump outlet, just watch max pressure since there is no relief. I know in hyd school they gave examples where there was low flow at the aux couplers, but when tested at the pump, the pump was in fact good.

 

[NCPIPELINE]

All good info here.