Supporting > Engine Ancillaries
Gear pump
petertha:
On my next model engine I want to pump & scavenge oil like a FS engine. On this bench tester I used conventional involute brass gears ~9.6mm OD, M0.8 x 10 tooth x 8mm thickness AliExpress specials. I had to make axles to fit the 5mm gear bore & 3mm ID bearings. I gave it a test run with my cordless drill on some hydraulic oil, shockingly it actually works. No performance numbers yet but it outputs a steady stream & ~6" head (the length of my lines right now). I have PTFE gaskets between the gear plates & sealed (3x6x2.5) bearings.
petertha:
An issue I'm having is some oil is bypassing out the shaft hole. It didn't seem like much with the oil but when I ran it with a low viscosity solvent to flush it out, quite a lot more fluid was leaking out the shaft hole. I think it makes a path from the high pressure port down the face of the gasket plate & along the shaft, presumably through the bearing shields. It would be nice to use bearings for the running aspect but I'm unsure of how to make a seal. On another attempt I fitted some little O-rings but it seemed like with the slightest of squeeze I added a lot of friction to turning the shaft. The drill would easily overcome it but I could no longer hand turn it. I cant think of a good place to put a low friction but sealing O-ring
Here is a sketch of another version, slightly different gear. Any ideas of how you may have prevented leaks? Maybe...
a) counterbore the bearing recess on inside, shaft terminates within the end plate, no shaft hole exit at all
b) a dust cover type plate over the bearing area (yet more fiddly tapped holes)
c) replace the bearing with a bronze bushing on the plate & make a recess groove for O-ring
Roger B:
Some thoughts:
Most engines will have the oil pump in the sump (oil pan) so leakage is not really a problem unless it is enough to reduce the pump efficiency.
I have made some gear water pumps where I have used a relatively long Oilite bush for the drive shaft with a small lip seal and the idler gear runs on a fixed shaft. The one in the pictures has a 6mm shaft.
If the body and gears are of different materials thermal expansion may be a problem. I had to increase the clearance of the Delrin gears to stop it seizing at working temperature.
If you use a lip seal on a ground shaft (silver steel/drill rod) give the shaft a quick polish with some very fine abrasive cloth to remove the sharp grinding marks.
Smaller lip seals are available, I have also used 4mm ones. A possible source of small quantities seems to be the model boat suppliers although you will need to check the material type. The industrial supplies will happily sell you a packet of 100 seals ::)
astroud:
I would suggest that you check the recess or groove you made for the O rings if the fitting resulted in being too tight to turn. Grooves correctly sized according to the manufacturer's technical guide should result in a barely noticeable difference. At higher pressures fluid pressure leakage will slightly deform the ring to effect a seal.
gbritnell:
Peter,
What is happening is the gears take the oil around to the discharge hole but as they pass the discharge hole there is still oil between the teeth. The smallest clearance is right when the tooth of one gear is directly in the recess of the adjacent gear so a small bleed channel needs to be cut or ground from that mesh point back to the discharge port. It can be done with a small burrito in a hand grinder.
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