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.

[AVTUR]

I am sure that this has already been said but gear pumps have to leak, slightly. If they don't the oil trapped the teeth, as gbritnell says, will produce very high pressure. This in turn fatigues the teeth. A running clearance, say 0.010", between the gear face and end plate will surfice.

AVTUR

[petertha]

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.

Thanks George, can you elaborate on what you mean by bleed channel? Maybe a modification to the vertical slot profile? Hopefully this sketch depicts what I suspect is going on, but I well be mistaken. As the gear turns, the packets of fluid are increasing in pressure as they approach the discharge slot (red path) & then out the discharge nipple. I'm showing a thin, kind of combination rub/wear plate & body seal gasket part made from PTFE sheet. I'm not convinced that has to be there if both the gear faces & outer metal plates are well machined but that's what I'm using at present. Anyway, 'some' clearance will be present between the gear face & the rub sheet, ~0.001" per side or less. I think high pressure fluid is able to migrate between the discharge area or lower quadrant of gear & make its way through to the shafts (blue) & laterally out. Using bearings, the ID & OD are sealed tight to shaft & plate respectively, so I surmised fluid coming through the bearing shields?

This sketch is a different module/gear size but exact same principle as machined one. I'm showing holes in the end plates where there would have been bearings but maybe I should use bushings with a better engineered seal ring there. My built pump had 3mm thick end plates but the bearings were 2.5mm thick, so there was enough room to fit a small O-ring in there with TLAR squish. But it was a bit of afterthought so I think that contributed to high friction. I think I ultimately want the O-ring/seal to be held stationary by its OD & the shaft running inside its ID with only just enough seal friction? having trouble visualizing that with bearings but it could be done with bushings.

I suppose the pump would ultimately be bolted to the engine on its rear face so there would be no shaft leakage there, but only on the front/nipple side. When I ran low viscosity fluid which leaked more, I think I could see fluid exiting both shaft ends about equally, hence my theory about leak fluid migrating via gear face. Not a lot but discernable.

[petertha]

Just a side note while I have the pics handy. A quick way of making finicky gaskets if you have a 3D printer to make a template. Starting with smooth MDF or similar scrap sacrificial board, spray some 3M adhesive, then  the gasket sheet, multiple layers at same time if you like. Then the 3DP template. Now its one happy glued sandwich that will not shift around when you then drill the passage holes & cut the profile with Exacto. Then soak it with some mild thinner to dissolve the glue & separate everything. More intricate shapes cutout shapes could be a bit more challenging but its actually quite controlled when one has a hard template to guide the blade. Mine was 3mm PLA, no special settings.

[john mills]

Hi
when i worked were we were reconditioning engines  30 a day on the line i looked after the others did vw engines
the trick was to reduce end clearance so the end plates were ground flate  the vw pumps the housing and gear assembly
was ground flate to reduce clearance  .i have a gear pump with 2" gears and the end plates are flate
I would think in these miniature pumps you would want flate plates and just enough clearance for the pump to turn
no channels for leakage  .full size engine gear pumps have large teeth for there size but want clearances to a minimum
often the end plate would be ground flate when rebuilding and replace shafts and gears.
john

[petertha]

Hi John. That's kind of what this next iteration was about. If the middle pump housing width was say 0.002" wider than the gear width, that would equal 0.001" gap either side of gear face assuming it was positioned centered. A radial gap of 0.001" between gear OD & housing bore was what I was aiming for but it didn't work out quite that precise in real life I suspect making my own step down axles & maybe QC on the cheapo brass gears. Anyway, from what I gather, efficiency goes to crap with much more gap. I think I discovered this first hand when I tried to correct the gear OD, spinning it in my Dremel hand tool against 1000# backed paper. The teeth are so thin on this little gear I think I removed more.

When I made the PTFE gasket out of some 0.005" thick stock, I was amazed at how slick the gear rotated on the surface; the face of gear against the PTFE. So that's what gave me the idea to use an easily replaceable, sacrificial 'wear sheet' on either side of pump housing that served double duty as a fluid seal between plate & body. Easier than re-making the plates if the brass gear wore a ring into the plate's inner surface. Even though there should be no axial thrust on the shaft, it might end up riding on one plate or another? Now, choosing aluminum for end plates may not have been so smart over say steel, but there again, which part do i want to preferentially wear? these gears were cheap & might work in the end, but if I had to cut my own custom size?

Here is what I was trying to say about O-ring seal within a bronze bushing, doing away with the bearing races

[Mike R]

You should be able to get away with only one seal on the input shaft if you cap the non drive end and the ends of the idler shafts. 
As for o-rings as shaft seals, I'd be reading the Parker O-ring handbook for guidance on that.  My initial reaction to what you have drawn:
You have both radial and axial surfaces as moving contact.  Probably better to limit to just one of those.  I'd pick to seal on the OD of the shaft, and trap the o-ring in a groove - results in the lowest surface speed difference between the o-ring and the sealing surface.

[gbritnell]

Peter,
Here's the drawing for the oil pump for my flathead engine. I did an overlay in color to show the gears in the pump cavity and the bleed port. The bleed port is .047 from the centerline of the pump body. You can see how it allows the residual oil to be evacuated just before the gears come into full mesh.

[steamer]

Hi Peter,

Thanks George!   

I did mine a little different, the input shaft runs in the bronze but I have a ball bearing behind the driving gear to support the overhung gear loads.
The input gears are keyed to the shaft.   The output or idler gears are free spinning.

The idler shaft is stationary and a light press fit in the housing and the idler gears ( steel) spin on the shaft.  The gears are 1144SP while the shaft is hardened 0-1 tool steel pins.   

I ran my pump at 4000 rpm for 3 hours by mounting it in the mill and running it immersed in oil. and it appeared to survive just fine.

I don't have any seals on the pump, other than close fits.   George, my output ports are large enough to allow bleed.

Flow in the little bucket seemed quite vigorous, and as I'm spraying on the rod ends , I don't need a lot of pressure.

https://www.youtube.com/shorts/2zqZayxDflo

Dave

[petertha]

Peter,
Here's the drawing for the oil pump for my flathead engine. I did an overlay in color to show the gears in the pump cavity and the bleed port. The bleed port is .047 from the centerline of the pump body. You can see how it allows the residual oil to be evacuated just before the gears come into full mesh.

Thanks for this. Lots of details to study & understand.

- is it possible to share pics of your pump disassembled that show these features (or PM if that's preferable)

- it looks like your gears are 0.463" wide & the bore cavity is 0.464" deep, so 0.001" total face clearance with the cover plate on? Is the general idea here that the gears have just an oil film between the face & adjacent surface?

- so the bleed extends 0.094" beyond the gear face? Is this somehow derived by the volume of a tooth or how teeth partially block the fluid port, or...?

- I think 12T x 32DP gear gives 0.4375" nominal OD. The cavity bore is 0.439" so that would be a very slim radial gap, 0.00075" if I did the math right. Do you lap them in or just very careful machining to get this kind of fit? I know cutting low tooth count gears can get technical with correction factors & I'm unsure what model engineers may be doing or not doing. I have a M0.8 gear cutter set I suspect is quite similar to the IMP equivalent, the last cutter in the series is for 12-13T. My assumption is with a 12T gear you still start out with the calculated nominal blank OD & cut depth by the book? i.e. they have built in whatever corrections into the tooth shape so we don't have to diameter & DOC adjustments from nominal?

I've read that pump efficiency (both rate & pressure) goes to crap very dramatically at this scale. I was hoping for 0.001" radial clearance & in hindsight that was wishful thinking. These el-cheapo 0.8M brass gears I bought vary by more than on diameter. But when I made tight fitting shafts on the 4mm hole & clocked the assembly they are not running true. So next step is to make my own gears & control that aspect better.

[petertha]

Hi Peter..... I did mine a little different...

Hi Dave. FYI I pulled out the sizing spreadsheet you & I were bouncing back & forth, what a year or more ago?    Better late than never. Because I thought I'd try some off the shelf (AliExpress) brass gears to start & they come in limited sizes, I just plugged those numbers in to give be a ballpark estimate. Anyway my tester pump got a 'visually encouraging' stream with ISO-32 hydraulic oil turning it with my cordless drill. But it was a gong show trying to hold the pump with one hand, hoses in another, drill with a third... spillage etc. I figured water would be easier to clean up. Well that was disappointing. I was sure I reassembled incorrectly. Very anemic flow & it no longer pulled fluid the length of my 6" hoses. Then I thought, hmmm ~40cp oil vs 1cp water viscosity. I did some Gemini Q&A & (assuming its not hallucinating again) sure enough, makes a big difference. I don't think we factored that originally because gear pump was intended for oil. I've actually had Gemini/ChatGPT write some spreadsheets through some of these formula based exercises. Its clunky (and always questionable) but also kind of cool. Some of this information is just not readily accessible so I view it as a more efficient form of Googling. I'll keep plugging away & share anything of interest.