Gear pump rebuild questions

[etpm]

Way back in 1979 I worked in a lumber mill as a machinist and one of my jobs was to rebuild gear pumps. This old guy who was their head machinist told me what had to be done. I had to flatten the end plates of the pumps by machining them There would be wear from the ends of the gears in the end plates and this would allow hydraulic fluid to flow past. So, after I machined away the pockets worn by the gears away I would check that the plates were flat and then reassemble the pump using new shaft seals.
Now I think the gear pump on my mid 1970s Case 580CK needs this same service. This is because when the oil is cold the tractor develops over 2500 psi but when the oil is warm the tractor is lucky to maintain 1000 PSI.
When I did the work on those gear pumps way back when I never checked the clearance between the gear teeth and the gear pockets. I know this clearance must be small but I don't know how much wear I can get away with.
Though just retired I still own a complete machine shop so I can do any machining needed.
So, can anyone here give me any guidance regarding gear pump rebuilding on this old machine?
Thanks,
Eric

 

[Hay Dude]

Can you get a Case backhoe work shop manual? Or find out brand of pump and get clearance?
Hopefully not an aluminum housing..

 

[ROUSTABOUT]

Have you checked your by pass, maybe the spring is weak and need a shim. Maybe stretch the spring. It's been bypassing a long time. I was putting together a 79 model log skidder today that we had apart rebuilding the transmission. Didn't mess with hydraulics. Didn't need to.

 

[oldnslo]

Gear teeth tips to gear case clearance should be minimal but I don’t know exact dimension. Do have the equipment to check the gear case diameter? If yes I would compare the inlet to pressure side. The wear will typically be on the inlet side and would also potentially have a lip on either edge. If plain sleeve bearings also check them for roundness. Again wear will on the inlet side since pressure is forcing the gears towards the inlet.

 

[etpm]

Have you checked your by pass, maybe the spring is weak and need a shim. Maybe stretch the spring. It's been bypassing a long time. I was putting together a 79 model log skidder today that we had apart rebuilding the transmission. Didn't mess with hydraulics. Didn't need to.

I don't think it's the bypass because the pressure is high until the oil warms up. I suppose the arm oil could also make the spring get weaker. My experience with springs though tells me that this would not happen.

 

[etpm]

Gear teeth tips to gear case clearance should be minimal but I don’t know exact dimension. Do have the equipment to check the gear case diameter? If yes I would compare the inlet to pressure side. The wear will typically be on the inlet side and would also potentially have a lip on either edge. If plain sleeve bearings also check them for roundness. Again wear will on the inlet side since pressure is forcing the gears towards the inlet.

Yes, I have the equipment to check the gear case diameter. To 50 millionths of an inch easily, 20 millionths with a little more trouble. I'm not bragging, it's just that in my shop I used to do some very fussy work. I can check for roundness too. I guess if a visible lip has formed the pump case is too worn. I didn't know that about the inlet side wearing first. Thanks. Your explanation makes perfect sense. I can see the gears being pushed from the pressure. I can even see how a lip would be worn into the case.
Thanks Again,
Eric

 

[oldnslo]

Eric
Usually any gear case wear is evident by looking at the gear case bore. If gear teeth have been touching or rubbing that portion of the bore will be polished a different color and or also be scored. Typical wear area is approximately 30 degrees off center line between inlet at outlet.
Since you are a machinist I am sure you will figure this out

 

[etpm]

Thanks oldnslo, I'll look at the bores. Any idea on gear tip to bore clearance?
Eric

 

[oldnslo]

Eric
I do not know what the tip clearance should be but a suggestion. Measure the gears OD across the tips and then measure the pressure side bores of the gear case. The difference should provide a decent idea on what they are on that pump.

my guess is around . 002 inch but that is pure guesstimate on my part.

 

[Alan W.]

Some gear pumps have bronze wear plates that fit into the center section against the gears.
Have rebuilt a lot of Commercial Shearing pumps like this.
If there are no wear plates and the end plates are wore substantially you will need new gears also.
Until you get it tore down its all speculation.

 

[IH3444]

Gear Pump - Working Principle, What Is Backlash?

As the gears rotate they separate on the intake side of the pump, creating a void and suction which is filled by fluid. The fluid is carried by the gears to

mechanicalportal.com

The gear backlash on my 1971 IH3444 Industrial TLB were pass 0.020 of an inch, so I had to also replace the gears. $150 for two gears back in 1987. After warm up oil can be squeezed backwards through too much clearance on gears backlash. I did all mentioned here also. Flatten plates, re-shimmed pressure relief. The clearance around the housing that supported the gears was acceptable. Had to purchase gears from IH dealer. No one else had them.
Push gears together clean and dry in direction of pumping. Take a feeler gauge and measure clearance between gears on back side. I think 0.008 was minimal acceptable, mine were shot at 0.020.

 

[LD1]

Most people dont rebuild pumps anymore because the machining cost and labor dont make it very cost effective. And most of the pumps people are dealing with are <$500...so they just replace.

However you are in a unique situation with the time and ability to do the job and not pay a shop ~$100/hr to do said work.

I would try and contact the manufacture....or even a case industrial dealer and see if they can dust off an old book somewhere because I am sure there is a spec.

I do agree with oldnslo that if practical, disassemble and find a spot that isnt worn and measure it and the gear and use that as a baseline.

Two clearances to worry about, which is gear to case as well as gear to end plate.

My gut says 0.002" is too tight for a pump. I was thinking more like 0.005" or a tad more. 0.002 is knocking on engine main bearing clearance tolerances.

Curious.....does RPM have an effect on your pressures? Usually a big symptom of a worn pump is inability to make pressure at lower RPM.

Let us know what you decide to do or what you find when you take it apart