Re: Re: Re: Gearing Question relating to a math problem????

Posted by Nolan on August 09, 2001 at 03:47:20 from (209.48.190.220):

In Reply to: Re: Re: Gearing Question relating to a math problem???? posted by Alberta Mike on August 08, 2001 at 11:57:31:

As a general rule of thumb, yes larger diameter gears are easier to transmit power through, all other things being equal.

There are of course other factors that come into play with larger gears that can promptly negate that. Polar inertia, increased mass, etc.

The effect of parasitic waste with smaller gears as compared to larger gears follows an exponential curve. Bicycle gears are fairly far out on that curve for the power provided by human legs, so the effect of the different diameters is negligible, other then as ratio changes. This is why bicycle gears are the size they are.

It would not be so neglible if those same sized bicycle gears were being used to drive a tractor. Then they would be too small for the forces on them, and the side loading ratio would be high (skipping the gears simply breaking under the power of a tractor engine). The tractor would be wasting a lot of power grinding the gear against its bearing.

Put those bicycle gears in a little toy slot car, and they would become too large. Their power drain by virtue of their mass would be extreme to the tiny little electric motor. A factor that is negligible when being pumped by your legs.

So, even though larger gear sets transmit power more efficiently then smaller ones, there are other factors at play that make the determination of what is large and small for a given application.

You can observe this when opening a door. The handle is almost always on the opposite side of the hinge. Push the handle (or knob) and the door swings open easily. Push in the middle of the door, and it no longer swings so easily. Though you are putting the same force against the door. Push close to the hinge, and you might not even get the door to swing open, though you might rip it off the hinges.

Now does it matter very much if the door knob is at the very extreme of the door away from the hinge, or that it's a few inches in? No, it doesn't matter. A few inches difference out that far are negligible. But if the knob was right up close to the hinge, then a few inches difference would make a great deal of difference.

There's a few more factors coming into play with that door example, but it gets the essentials across.

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