Gear pump

[Roger B]

The gears are commercial Delrin 12T MOD1 with the bosses turned off to give a width of 9mm. The flow is more than adequate for water cooling a couple of my engines.

I didn’t initially think much about the clearances but the first one seized due to the expansion of the plastic. The bores of the pump are around 0.05mm over size and the gears were a bit under the nominal so I would guess there is a clearance of around 0.1mm when cold which will reduce to almost nothing at 100°C. The initial version had almost zero face clearance so I increased this to 0.1mm which seems to work.

This is the first trial some years ago with my 500 rpm battery drill:

<a href="https://www.youtube.com/watch?v=KFRFxUtCWEE" target="_blank">http://www.youtube.com/watch?v=KFRFxUtCWEE</a>

I am currently building a three gear oil pump for my Junkers CLM based on the same concept with commercial steel gears, an aluminium body and a brass thrust/port plate. As it hasn’t been run yet I can’t comment on the success or not

[jcge]

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

I wanted to revisit this because I ended up adding spare PTFE gasket sheets between the middle cavity lock & end plates to alleviate what I thought was slight friction somewhere in the system. I still don't have a definitive answer.

I'm not sure that you'll need to address the tooth/housing pitting issue at such a small scale due to the expected greater leakage, and limited running time. However for visualisation purposes, here's an example of a small spot face relief on the side plate in the tooth mesh zone of a cam housing scavenge pump of the Porsche 917 (typ 912 engine).

Regards
John

[petertha]

Thanks John. I think that feature is what GeorgeB was getting at with his V8 gear pump if I interpret drawing correctly. I'm unclear about sizing though. Is the intent to encompass a certain proportion of intersecting teeth area from perspective of looking at the gear face on? Or is recess depth dimension important if it relates to gear width? Thicker gear = more pump volume so more 'reservoir' required?

[steamer]

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

I wanted to revisit this because I ended up adding spare PTFE gasket sheets between the middle cavity lock & end plates to alleviate what I thought was slight friction somewhere in the system. I still don't have a definitive answer.

I'm not sure that you'll need to address the tooth/housing pitting issue at such a small scale due to the expected greater leakage, and limited running time. However for visualisation purposes, here's an example of a small spot face relief on the side plate in the tooth mesh zone of a cam housing scavenge pump of the Porsche 917 (typ 912 engine).

Regards
John

thanks for the shot John!

[jcge]

Not sure I can give any meaningful guidance with regard to sizing the relief in the meshing zone...it obviously must not extend beyond the gear pair center line, but it seems that backlash between the gear teeth will also provide for relief of the pressure spike at point of meshing at the center line.

https://www.modelenginemaker.com/index.php/topic,11357.msg265561.html#msg265561

Refer to Section 2.3,  Fig 26 & Fig 27 relating to pressure spikes.

At such small scale in these model engines, and given the limited run time I really wouldn't be too concerned about it.

[petertha]

I added a 0.004" shim to the pump body sandwich so effectively another 0.002" clearance per side of gear face assuming they self center while running. For one thing it runs very smooth now when I tighten the bolts hard. So its obviously is a bit tight axially somewhere in the assembly. I can tell no difference in pump performance with extra clearance, mind you this is just visual observation until I rig something up to better quantify things & ISO-32 oil. It readily self primes from a height of 6" (the length of my hoses) & drains a 30 ml cup in about 10 secs of 'slow Milwaukee drill' speed whatever rpm that is. So I think it would be satisfactory for an engine sump pump as-is. Maybe elevated pressure (lubrication pump) will be more challenging, that will be next. Another little observation when I was re-assembling. When I pushed the gear axle into the blind bearing hole it promptly ejected out of the hole under hydraulic pressure. These pumps make great fidget toys. One day when I find time I'll figure out a way to post videos on YouTube or similar.

[jcge]

See also section 4.2.1 Relief Groove of the previous reference
Regards
John

[petertha]

(VW Beetle oil pump, YouTube video). What do you suppose the ring groove with a single feeder is about?

[jcge]

The VW pump you've shown is sealed with a flat plate cover and paper gasket, and not an oring as the peripheral groove might suggest.

I believe the short radial groove connects the peripheral groove to the suction side (low pressure side) of the pump and is to provide for the return of leakage from the periphery of high pressure oil against the cover plate.

Note also the pressure relief spot face similar to that posted previously

Regards
John