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

Posted by Nolan on August 08, 2001 at 09:57:50 from (209.48.190.220):

In Reply to: Gearing Question relating to a math problem???? posted by Alberta Mike on August 07, 2001 at 14:13:01:

The smaller gear unit is always less efficient then the larger unit. Has to do with the efficiencies of power transfer. Bear with me, and I'll explain and give you an example.

Whenever you put power across a gear (or a sprocket) the power is tangental to the axis of rotation. The gear can rotate, so the force across it causes rotation. The force is always a side force, the reaction (rotation) is the torque.

Take a look at your bicycle example. Lets start the bicycle from a standstill. The pedals are at the 3 and 6 o'clock positions. You stand on the pedal, providing the force. The shaft rotates (torque), transmitting a force to the chain, and away you go.

Now when you first put your weight on the pedal, all the force was straight down. If the bicycle couldn't move, there would be no rotational action. If things were weak, the pedal assembly would break off straight down under the force of your weight.

That's the key point.

Take a normal bicycle crank, and because of the leverage arm, it's going to rotate. Lots of the force from your weight goes into rotating the crankshaft.

Now lets put a tiny crankshaft on there. One with say 1/4" throw. You won't be able to make that bicycle move, even if you had legs like Charles Atlas. The reason is because percentage wise most of the force is now going into the bearing, not into creating rotational force.

So with that little tiny bicycle crankshaft, we lose all our power into side forces (pushing down in the case of a bicycle), and don't get it into the rotation. Push hard enough, and you'll push the crankshaft right off the bottom of the bicycle frame.

There is a ratio of how much power gets transferred into rotational torque vs side thrust against the bearings, and it has to do with the length of the arm.

Aha! So little gear sets lose more power in side loads then large gear sets do, *of the same ratio*!

Put too much load against a little set of gears, and they'll rip right out of their bearing races. Sound like what happens to a lot of car transmissions when the owner hops up the engine? Yep, that's the problem. The gears are physically too small a diameter for the now increased power being applied across them.

This is why heavy machinery uses much larger diameter gears then light weight equipment does. Even when they have the same ratio.

This exact subject is covered in engineering courses btw.

Follow Ups:

• Re: Re: Gearing Question relating to a math problem???? Ray,IN 19:39:41 08/08/01 (0)
• Re: Re: Gearing Question relating to a math problem???? Alberta Mike 11:57:31 08/08/01 (2)
  • Re: Re: Re: Gearing Question relating to a math problem???? Nolan 03:47:20 08/09/01 (1)
    • Re: Re: Re: Re: Gearing Question relating to a math problem???? bob 19:57:16 08/10/01 (0)

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