Hydraulic Pump Woes

[samoht]

Hello,

I have been tracking down an issue with the 3pt on a Ford 1500 that I recently purchased. The threads here have been incredibly helpful (many thanks to JC for his well documented 3pt overhaul.) Initial issue was that 3pt was down and unresponsive. That turned out to be a control valve that some previous owner had broken thru overheating the relief (which was stuck open.) At that point, the 3pt reacted beautifully to compressed air blown in the inlet to the control valve. When I connected the pump discharge line to the control valve inlet...nothing. At this point, I figured there was a problem on the pump side.

On the pump side, there was the typical VERY clogged inlet strainer. When I gave this guy a good bath in a parts washer. The pump came alive and would make plenty of flow with the discharge disconnected. I then connect the discharge and the 3pt reacts to requested position, but very slowly. It lifts, but only if there is no implement and no me standing on it (~2000 rpm.) Once it gets to position, it does hold position well, so I figure the cylinder seal and check valve are in fair to good shape. I take apart the relief valve and all looks well.

So, I tear into the pump. Sealing surfaces between gears and aluminum housing (both sidewall and outer cylindrical surface) of the pump look good (no detectable ridges or scoring with fingernail.) Seals all "look" good.

Finally, HERE IS MY QUESTION. There is play between the sealing surfaces of the gears. The gears still fit nicely on their shafts, but the driving gear can be turn a few degrees, clockwise and counterclockwise, before engaging and turning the driven gear. Is there still hope for this hydraulic pump, or am I out a whopping $1500 for a new one (it hurts to write that)?

Thanks for the advice.

 

[JC-jetro]

Howdy Sam and welcome to TBN Blue

let me understand it, so you're saying, gear to pump inner case tolerance is good, but there is a bit play initially between driven and driving gear. Am I right? The tolerance between gear and case is more important here as the oil doe snot get squeezed between the gears. The pressure is caused between gear and the case. if you look at the 4 bushing in the pumps you see soft friction bearing. lube for that is done with the internal seals. you can also see passage way in the bushing to direct some flow for shaft lubrication. A tear there would reduce the pump efficiency. I suspect that your main culprit be the input shaft seal. Did you take that out during pump tear down. That seal is about $10. On mine that was the culprit. It appears to me that it sucked air in rather than pushing fluid out. I did not see any fluid rise in my crankcase then.


Let us know.
JC,

ps. Look at the operation of external gear pump operation below.

Hydraulic pump - Wikipedia, the free encyclopedia

 

[samoht]

JC,

You have it. There is "slop" between the driven and driving gear so that you can turn the driving gear a few degrees without engaging the driven gear. The seals all looked nice, both the orings and the input shaft seal.

My thought is that, as the gear teeth come together and squeeze the oil out, forcing it into the discharge port, that the slop in the gearing would result in a gap between the driven and driving gear and the hydraulic fluid would squeeze thru this gap.

I talked to a couple NH techs. They are of the opinion that it won't make pressure, but they were both surprised that the housing was in good shape.

I will rebuild the pump this week. If it turns out not to work, what is another $50 on top of $1500. If it does work, it would make my month.

Thanks.

 

[JC-jetro]

JC,

You have it. There is "slop" between the driven and driving gear so that you can turn the driving gear a few degrees without engaging the driven gear. The seals all looked nice, both the orings and the input shaft seal.

My thought is that, as the gear teeth come together and squeeze the oil out, forcing it into the discharge port, that the slop in the gearing would result in a gap between the driven and driving gear and the hydraulic fluid would squeeze thru this gap.

I talked to a couple NH techs. They are of the opinion that it won't make pressure, but they were both surprised that the housing was in good shape.

I will rebuild the pump this week. If it turns out not to work, what is another $50 on top of $1500. If it does work, it would make my month.

Thanks.

Sam,

Oil can not be squeezed as it is incompressible same as water and anyother liquid. What happens is that oil trapped between the gear in the inlet volute is pushed forward between the gear and the case. tight tolerance and oil film coefficient would keep it pretty tight. when the oil gets to the outlet housing then the only option is left for the oil to exit the outlet port. At this time the volume of oil that gets pushed has to fight off the relief pressure spring that might have stiffness let say equal to 2000 psi. end result is that you get certain gpm at some given rpm matching up to 2000 psi relief setting. Look at the pic below, oil does not get squeezed. I hope your overhaul solves your problem. take a small punch, dremmel tool or what have to leave witness mark as you disassemble to make sure 4 bushing can go to original spots. Gears come together on the outlet and not on inlet side.

JC,


I also had a link for more on gear pump on my previous comment.

 

[Jerry/MT]

Sam,

Oil can not be squeezed as it is incompressible same as water and anyother liquid. What happens is that oil trapped between the gear in the inlet volute is pushed forward between the gear and the case. tight tolerance and oil film coefficient would keep it pretty tight. when the oil gets to the outlet housing then the only option is left for the oil to exit the outlet port. At this time the volume of oil that gets pushed has to fight off the relief pressure spring that might have stiffness let say equal to 2000 psi. end result is that you get certain gpm at some given rpm matching up to 2000 psi relief setting. Look at the pic below, oil does not get squeezed. I hope your overhaul solves your problem. take a small punch, dremmel tool or what have to leave witness mark as you disassemble to make sure 4 bushing can go to original spots. Gears come together on the outlet and not on inlet side.

JC,


I also had a link for more on gear pump on my previous comment.

What JC is trying to get across is that a gear pumps works by adding incremental velocity to the fluid, thereby increasing it's dynamic head(V^2/2). It doesn't "squeeze" anything.

 

[samoht]

I understand about the not squeezing and incompressibility of hydraulic fluid. Maybe push was a better word. Problem with this pump is the previous owner did not need the three point as he was using it for raking. It went dry for years, so the between gear tolerance is way out.

If you look at the picture, the teeth of the purple and blue gears fit quite snug. I have been asking around at work and a few service techs at NH dealers. Looks like everyone agrees that it won't make pressure if the gears have a lot of slop. New pump is ready for pickup on Thursday.

 

[samoht]

Jerry, I think your description better fits centrifugal pumps, not positive displacement pumps (gear pumps.) Positive displacement pumps take a fixed amount of fluid from one port and force it into another port.

 

[Jerry/MT]

Jerry, I think your description better fits centrifugal pumps, not positive displacement pumps (gear pumps.) Positive displacement pumps take a fixed amount of fluid from one port and force it into another port.

And just how does the pressure rise occur?

 

[barbqranch]

If the fluid can leak between the meshed gears, that will reduce your pressure. Just like a leak beside one of the gears. If that is enough for you problem, I don't know. How are the bearings that support the two gears? If they are worn, then that could produce some slop.

 

[JC-jetro]

Sam,

All you can do at this time to overhaul the pump, clean the inlet screen, put brand new fluid and give it a whirl. I don't know how the farmer "can let it run dry" it were absolute dry then he would have destroyed the transmission already. Transmission and rear diffy use common sump. The inlet elevation of pump intake is below normal hyd level in transmission. Running it dry can be only achieved in two ways. no oil in transmission and rear diffy or plugged screen leading to low oil transfer. A gear type oil with no lubrication will self destruct in a matter of very short time due to abrasion and much heat. I think you might have some minor flow since the gear to inner case is tight and no major sign of abrasion.

JC,