I'm in the process of sketching out a gear pump to provide a gallon per minute of cutting oil to a gun drill. It'll have 1.5" OD gears and run at about
1000rpm and generate approx 100-200 psi max, with a total running time (in my life) of no more than 500 hours. Materials will be EN1A for the body and either EN1A or EN8 for the gears because that's what I've got.
The question is what sort of clearances should I aim for?
I'm assuming that the radial clearance can be set to zero on the grounds that there will be plenty of oil washed past the metal and careful running in should produce a reasonable fit if I don't let it seize in the first few hours.
What should I do with axial clearance though? Would a radial oil groove on the pump face help to ensure that the sides of the gears and the journals don't run dry when run with effectively zero clearance? Or should I allow for a fraction of a thou's clearance and no oil groove and hope that leakage won't be a problem?
I would buy one.Gear pumps are very cheap especially the Italian ones.The big problem will be getting one with a small enough output.I would work on 1440 rpm as well to use a standard motor with standard control gear.
Hmm, you're taking the fun out of life... looks like I could pick up a similar sized Rexroth pump for about £60 to my door. On the other hand, the reason for aiming at 1000 rpm was that I had intended to drive it from the change gears of the lathe, thus avoiding the need for another motor.
Tricky call, make up pulleys+flanges for a bought in pump or build one that I won't get upset about if it accidentally pumps the odd bit of swarf.
I think I'll carry on making one just for the fun of it.
What about using a car engine oil pump already designed for the job? I don't know what flow rates they provide v rpm though. Something I've never actually looked into as regards engine design. I'm sure you could test it easily enough though.
A nice cheap compact pump is the one out of the Ford OHC Pinto engine (Capri, Sierra etc). AOP101 in their catalogue. Only about ten or fifteen quid at trade price I think. Maybe two or three times that to a punter. It drives off a hexagonal shaft but you could no doubt modify that. I may even have an old one knocking about in the workshop somewhere and I'm building a Pinto in a few weeks time so there'll be an old one out of that for sure if you want to have a play. I definitely have dozens of old Ford CVH pumps, mostly in good condition internally still. Those would be numbers AOP1128 and AOP1130. They fit over the nose of the crankshaft so there's a fair sized hole through the middle but again you could modify it. Anyway you might find something in their catalogue which looks as though it can fit your machine and I can buy them through my trade contacts. You'll also get a pressure relief valve built in which you can tinker with or block off as suits you.
Clearly big engine = higher flow rate so you might need something out of a truck engine designed for high flow at low rpm rather than a car engine revving to 6000.
As to clearances if you build your own I think they'll need to be larger than you think especially if you're running similar metals against each other. Usually car pumps have steel gears in an aluminium body. The odd thou here and there isn't going to hurt much anyway.
The other alternative and probably easier to fit is an electric pump such as a car fuel injection pump. I think an average one shifts about 2 to 3 litres/min at 45 psi but that might be lower with a more viscous fluid. Cutting oil isn't very thick though. You can use more than one of course and you can buy high flow ones designed for race engines. I use one in my parts washing tank but it's well buggered with all the unfiltered gritty paraffin that's gone through it over the years and barely delivers more than a dribble now. Time I fitted another one and maybe even some filtration if I can be arsed. I did have an engine oil filter in the system once but it blocked up every five minutes so I ditched it.
Anyway, have a look here. All the info you could want.
Whether its 1440 or 1000rpm the pump won't care, most pumps these days are rated for that reason, as so much per revolution. If you're only looking for those sorts of pressures a car oil pump would do the job. Peanuts to buy. A BMC mini would be a good candidate, easy to mount, drive and cut down if the capacity is too great. BTDT.
For what it's worth; commercial high pressure hydraulic gear pumps do not set up axial clearance as such. They have loose plates in the housing that are pressurised by output oil, holding them against the sides of the gear. Radial clearances are tight as you suggest; Pumps actually use the gears to to "machine" the final working clearances.
Of course these are high pressure pumps 2000psi+ - whether its appropriate for 100psi is another question....
The gear pump on my Deckel (for coolant, not oil) also *was* a gear pump. It's known that they fail (after many years) because of tiny chips being recirculated. Anyhow, it had 0.1mm clearance and no longer worked. Except if I sucked on the coolant-tube. :-)
I have some used Volvo truck hydraulic pumps here for a similar project. No clue why they were swapped. Very tight fit, run very smoothly though. Two casing halves with built in seals, gear clearance almost zero. Dirk
The significance of that btw is the differential expansion rates of steel and ally. Hot oil in a car engine is going to be anywhere between 80 and 130 degrees C. Call it 100 anyway. For a pump that's 4 inches in diameter the ally will expand by 4 thou more than the steel gears at that temperature so even if the clearance was zero cold it can't be less than 2 thou radially when hot. There'll be a similar albeit smaller effect axially assuming the gears are thinner than they are wide.
If you're using a steel body and steel gears you won't have that differential and you're dealing with cold fluid anyway which is why I doubt if a thou or so of clearance can hurt anything.