So, I just printed out this battery holder off Thingaverse. Unfortunately, it doesn't work very well, so I'm interested in redesigning it. There's a number of issues but the most annoying one is that the batteries keep getting stuck and not gravity feeding like they are supposed to, forcing you to shake the holder or reach up the output slot to bump the batteries until they feed out again.

The reason is obviously that the holder is just a box and that the cylindrical batteries jam up against each other at the bottom where the downward motion suddenly becomes horizontal.

A quick perusal of things like soda vending machines and ammo feed mechanisms shows that a v-shaped feed mechanism seems to be the standard way of doing this correctly.

However, I have no idea what sort of convergence angle and other considerations need to be made for designing such a device. I could made a tall, slowly converging angle, but that makes the storage box a lot taller and lower capacity than is idea. Too sharp of a convergence angle and I'll have the same jamming issues found in the existing design.

So, I assume that there is some sort of literature out there for designing feed systems for cylindrical objects. Obviously, things like cylinder density, surface roughness and other factors will affect this, but I assume that there's at least some sort of rough rule of thumb out there for what the optimal shape should be - e.g.: 45 degree convergence angle + some factor * the cylinder W/L ratio, etc.

I know I can figure this out with Edisonian trial and error, but I really don't feel like burning up a bunch of PLA and time if there's a decent standard rule of thumb I can use as a starting point for experimentation.

  • $\begingroup$ dont burn pla. just experiment with paper and cardboard. cereal boxes should still be ok (plastic bottles have lost a lot of craft usage though) $\endgroup$
    – Abel
    Dec 20, 2021 at 0:51
  • $\begingroup$ coolest one ive seen involved rungs of rails. cylinders that didnt meet the necessary orientation fell through to the next set of rungs oriented perpendicular. $\endgroup$
    – Abel
    Dec 20, 2021 at 0:54

1 Answer 1


This is more a problem of 3D-printing than engineering, I think. If you skim thru the comments on that Thingiverse page, you'll see that people have designed and posted internal baffles to ensure that AAA batteries properly dispense. Try that first.
And, of course, since this is extrusion printing, make sure to clean out any support material, whiskers, etc.

If you really wanted to do an engineering analysis, you'd have to determine the static coefficient of friction between battery cases and make sure the angle of inclination of the bottom level is sufficient that every battery can overcome said friction and roll to the end. Not really worth it :-) .


Then there's always the hoary problem of batteries randomly forming a stable "arch" in this open design (similar to sand jamming in an hourglass). If the interior were a series of ramps, the overall design would be a lot more reliable.

  • $\begingroup$ I hear you. Thing is that this is less about the battery holders in particular (I can always just shake them to dislodge the batteries) but building my ME design skills. More specifically, I'd like to move beyond just eyeballing a design and sticking in baffles and whatnot to solve an issue that has probably been well solved in the material handling industry. Making systems for gravity feed is something people have been making for centuries and there has to be some general design rules out there but I have no idea where to start looking. $\endgroup$
    – DanHeidel
    Dec 5, 2018 at 18:58

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