Will a 3/16" bore spur gear fit on a 0.2" diameter motor shaft or will I need some sort of coupler?

Question

I'm trying to make an automated cabinet door with a spur gear and gear rack. The motor specs say 0.2" shaft diameter and the spur gear I ordered says 3/16" bore hole. Will these work together or will I need some sort of coupler? The shaft is basically 0.0125" too big or 1/80th of an inch. I'm hoping I can just press fit this on.

Answer 1

The first thing to do is measure the shaft in case whoever wrote the "motor specs" rounded the size. 0.2 seem a somewhat unusual size to me. If inch measurements are being used I would expect 3/16 (0.1875) to be more common, or if it is really 0.2, I would expect it to be specified as at least 0.200 and probably with a tolerance appended to that.

A 0.0125 interference is far too much for a press fit in that diameter, it is nearly 6% of the diameter.

You don't say how much torque you are transmitting, 3/16" is a little small to be using a key, and you don't say if your gear has a hub for a setscrew. A press fit would be common in that size, but the trick might be to get a suitable bore size. For your finished gear reaming will probably be the most practical way to size the hole, but you have the problem of getting the proper size reamer, a #8 drill size reamer (0.1990') might be worth a try

For that shaft size an interference of about one thousandth of an inch would be about right, and so the tolerances on each part would have to be a few tenths. If the torque is fairly low, and adhesive bond, for example Locktight® retaining compound,  might be useful as you could use a relatively loose tolerance slip fit, possibly even drilling out the gear with #8 drill (much cheaper and possibly easier to get than a reamer) might work. As I mentioned  above #8 is nominally 0.1990 so it would sound like a reasonable interference fit, but drilling is not the most accurate process, oversize holes are not uncommon., so I wouldn't count on a proper press fit drilling.

Note that the section on fits in the Machinery's Handbook is quite long, so one might assume it is a relatively complex subject.