Gear pump rebuild questions

etpm · Mar 12, 2022

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 · Mar 12, 2022

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

ROUSTABOUT · Mar 12, 2022

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 · Mar 12, 2022

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 · Mar 12, 2022

ROUSTABOUT said:
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 · Mar 12, 2022

oldnslo said:
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 · Mar 13, 2022

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 · Mar 13, 2022

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

oldnslo · Mar 13, 2022

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. · Mar 13, 2022

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.