Need help finding the pitch of a pinion gear

[pterrus]

My project to turn an RC truck into a robot is coming along nicely (http://imgur.com/533aEgH).  There's a major problem though that in retrospect I should have anticipated: the toy drive motor no longer has enough torque to drive the added weight.

My plan is to buy a good gearmotor from servocity or something to replace the old motor: (http://imgur.com/7FcxN51).  I'd prefer to leave the old gearbox intact and just swap out the motor because that's clearly way less work, but the pinion gear appears to be permanently attached to the old motor shaft and I'd need a replacement.  So I need the pitch.  I've read that pitch is equal to the number of teeth that would be on a one inch gear, but this gear: (http://imgur.com/70ZALxD), which is just under an inch in diameter, has 34 teeth... it seems to be a nonstandard pitch?

I am an electrical guy and kind of clueless about gears... any advice on what to do in this situation?

[DiegoChavez]

what you have to do is find a caliper and measure the exact diameter of your gear and then make the math. they sell calipers for $25 at radioshack if you don't live near one the same prices are in amazon.com.

[bdeuell]

I suspect the problem you are having is you are not measuring the correct diameter. With your equation you would need to measure the pitch diameter which is somewhere between the root and the tip of the gear teeth.

If you are measuring the outside of the gear you need to use: diametral pitch=(number of teeth + 2)/(outside dia)

I recommend you measure the largest gear you have that has the same pitch (this will minimize he effect of error in your measurement). better yet measure several gears and make sure your numbers check.

Also they could be metric gears which are measured in module (circular pitch) which is equal to (pi)/(diametral pitch). Keep in mind you will also need to convert units to metric.

[jkerns]

I suspect that pressing the gear you have off the shaft may be the easier solution.

Cut a slot in a bit of metal to fit behind the gear to give you something to pry against.

[pterrus]

Thanks so much for the replies everyone.

I suspect that pressing the gear you have off the shaft may be the easier solution.

Cut a slot in a bit of metal to fit behind the gear to give you something to pry against.

Let's say I succeed in doing this.  Now I need to find a motor with the right size output shaft to press it on to, right?  It certainly wouldn't work with the 4mm D-shaft output motors I was looking at.  Any recommendations on what motor to buy and how to attach the gear?

[jkerns]

With a little care you could drill the hole out in the old gear. But!!!! If you get the hole too far off center the wobble will make it bind / slip.  It's not impossible - you just need to work very carefully - and I suspect it's one of those "nothing to lose" situations.

[bdeuell]

If you are trying to machine a larger bore in your pinion gear and press fit it onto your new motor it will need to be very precise. The tolerance on a interference fit for a shaft that small is probably less than a thousandth. If you use a drill bit to enlarge the hole there is a big chance it will be too large and will slip on the shaft or will be too small and you might break the shaft or gear trying to press it together. Brass is soft tho so you might get lucky and be able to machine the hole on the small side and still be able to assemble it. Typically the bore would be reamed to size, I would use a one thou undersized ream. You could try and glue the gear on but I wouldn't count on that lasting too long.

[jkerns]

I've had good luck with plastic gears.

Brass is not as flexible and would need to be fixed in place with epoxy.

[bdeuell]

you could always heat your gear up before you press it on to help reduce the press fit force required, might make enough of a diferance but i wouldnt count on it. This can help when disassembling as well if you dont care about saving your existing motor.

If you do use epoxy a rough surface will be tour friend, you could scuff the surfaces with some sand paper before gluing.