Gear Boxes

We’re gearing the chippy and drill together for our drive, taking the output from the chippy gear, passing it through a larger gear to drive another gear on the drill drive shaft. All gears will be brass. Problem is we’ve never worked with gears before and I’m not sure how the basics work. Are the gears supposed to turn on a fixed shaft or do you attatch them to a rotating shaft and support that shaft with bearings? If they do slip on a fixed shaft (this would be ideal), what kind of fit do you design for? Do you have to worry about wear? What about thermal expansion? Will the gears bind on the shaft after spinning for a while and getting hot?

Any help would be greatly appreciated.

Keith Sevcik
#920

On a side note, what happens when you burn up a motor? Do the windings actually melt or short or something? Or is it that the magnets are demagnetized from the excessive heat? Do you always have to replace the motor or is it somewhat functional?

…Can only answer the side note about the motors burning up.

IF longwind =0 goto :NoBS

When a motor spins from voltage applied, the motion induces a counter EMF… in other words, it’s also acting as a generator and producing a voltage of opposite polarity to the applied voltage. This makes the voltage drop across the motor effectively less, the resistance of the motor APPEARS higher than the reading that an ohm meter would give across the terminals of a motor ( out of circuit), this in turn limits the ammount of current flowing through the coils. As the motor is loaded, as in dragging a weight, the rotation slows down, the counter emf decreases so, there is a rise in current to maintain speed. The more load, the slower the motor is trying to go the less EMF so the higher the current. This might continue until the motor reaches the worst case condition of Stall.

:NoBS

At Stall, the motor is carrying more current than it was designed for and generates a LOT of heat that will burn off the wire insulation on the coil causing a short which may not disable the motor but damage the effective torque the motor can generate. the excessive current will eventually burn the winding as if it were a fuse and destroy the motor.
Bottom line, once a motor is damaged, replace it.

Good luck on the gear questions.

Best Wishes

Steve Alaniz

Are the gears supposed to turn on a fixed shaft or do you attatch them to a rotating shaft and support that shaft with bearings?

The answer is BOTH. Which approach you use depends on the rotational speeds and loads of your gear train. It also depends on the sizes of gears and shafts involved. It can be kind of hard finding ball bearings to mount your 32 pitch 16 tooth gear on a 1/4" dia shaft. Though a journal bearing might work well. Especially if it is made from the gear itself.

You also asked about galling and heating. Neither should be a problem in a properly designed set-up. But it is the proper design you’re probably asking about. Fits also depend on the diameters of the shafts involved.

I think your post ID’d you as a college student. If so check with an ME prof if you haven’t gotten that far in your own course work. If you can find a book called Mechanical Engineering Design by Shigley there is an excellent discussion about this sort of thing and a table of “fits” in the back. For this application I think you want a “running” fit.

BTW galling can mostly be prevented by using different materials for the parts of the joint. ie a brass gear on a steel shaft. The important thing is to have different hardnesses in the pieces that are moving relative to one another. Though proper lubrication doesn’t hurt either

Whatever you do, don’t put an aluminum gear on an aluminum shaft and expect it to turn.

*Originally posted by Keithicus *
**We’re gearing the chippy and drill together for our drive, taking the output from the chippy gear, passing it through a larger gear to drive another gear on the drill drive shaft. All gears will be brass. Problem is we’ve never worked with gears before and I’m not sure how the basics work. Are the gears supposed to turn on a fixed shaft or do you attatch them to a rotating shaft and support that shaft with bearings? If they do slip on a fixed shaft (this would be ideal), what kind of fit do you design for? Do you have to worry about wear? What about thermal expansion? Will the gears bind on the shaft after spinning for a while and getting hot?

Any help would be greatly appreciated.

Keith Sevcik
#920

On a side note, what happens when you burn up a motor? Do the windings actually melt or short or something? Or is it that the magnets are demagnetized from the excessive heat? Do you always have to replace the motor or is it somewhat functional? **
:cool: Hey Keith. We at Team 27 are also building a Taco/Hole shooter/Shifter setup. We have the Taco geared to an idler which transfers to an output -i call it the pto shaft- the drill motor is geared the same way. We are using Stainless Steel gears and shafts. The idler shafts are fixed, and the gears are bushed, they run freely on the shaft. The Engineer spaced the gears using the pitch dia. + .001/.002 backlash. I have the whole system modeled in AutoCad 14 in 3D, if anyone is interested. Sure would rather use a Chevy 471 though, instead of these torque challenged electric motors!:smiley: