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Participant

Join Date: Oct 2012
Posts: 2

Gear Torque

10/18/2012 4:05 AM

hi, i'm trying to work out how much torque is exerted by a gear. I am trying to convert a mechanical clock so that the main axle turns a lever as well as the minute hand. I need it to be able to exert 0.5NM but I have no idea how to check this.

Any help would be appreciated. Thanks.

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3
Guru

Join Date: May 2007
Location: Harlow England
Posts: 15211
#1

Re: gear torque

10/18/2012 4:57 AM

Attach a lever of known length with a small container at the end of it into which you can add weights (sand, grains of rice or something similar).
Add weight until the mechanism stalls and can no longer turn.
Multiply the weight by the length and you have the stall torque.
Measure the weight and length is suitable units to give kg metres. gram centimeters ounce inches or whatever unit takes your fancy (foot/pounds is what I relate too, but you won't get many of them from a clock).

Del

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2
Power-User

Join Date: Sep 2008
Location: Eldoret, Kenya
Posts: 139
#2

Re: gear torque

10/18/2012 6:04 AM

You could use a spring:

F = k Δx

First step is to find the spring's constant, k [Units N/m]. Use known weights of different weights to stretch the spring at least 10 different times. For each instance, measure the displacement Δx. Divide each weight [N] by the corresponding Δx [m]. Do the average and you have your k.

Next set up as follows:

Let τ = F y

Where F = k Δx

Then τ = k Δx y

Since:

k is known

Δx is measured

y is measured

Solve for τ

Hope this helps!!

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#3

Re: gear torque

10/18/2012 8:03 AM

P.S.

The spring should have a sufficiently high value of k so that the gear "struggles." At this point the Δx will correspond to the maximum (or near maximum) Τ for the gear.

For practical purposes, I suggest, this gear should be rated to provide torque that is ~20% below this calculated value.

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#4

Re: Gear Torque

10/18/2012 9:20 AM

Are you building a time bomb?

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#5

Re: Gear Torque

10/18/2012 10:50 AM

Can't talk... sweating too much... hands trembling...now was it the red wire or the blue wire?

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Participant

Join Date: Oct 2012
Posts: 2
#6

Re: Gear Torque

10/18/2012 12:25 PM

Thanks but is there no way of theoretically working it out? Surely if know that the gear takes 60 min to make one revolution and I know its radius...

I've already destroyed one innocent timer (unless i work out a way to recoil a mainspring)

Would using this gear to open a lever cause the timer to slow down? Surely if force is being exerted the speed will decrease?

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#7

Re: Gear Torque

10/18/2012 1:13 PM

Sure, there is probably some really complicated theoretical way of simulating it or calculating the wrong answer... but you'd need to know the torque generated by the prime mover.
You can't just start with a sack full of unknowns and expect to pluck an answer out of thin air. Maths isn't a panacea for all ills.

Regarding the 'will it slow it down' question, I'd think the escapement will either work or not work, and that will keep time... dunno though as it's not my field of expertise.
If you really want to know, measure it!!!!.

If you can't attach a lever, stick a thread onto the shaft with a spot of superglue and see how much weight it can lift, that's gotta be quicker, easier and more accurate than any amount of calculation.
Alternatively you could purchase the KrisDelTM book of random answers, which be about as much use as a calculation.
Del

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#8

Re: Gear Torque

10/18/2012 9:00 PM

Surely if force is being exerted the speed will decrease?

It is true that the Velocity Ratio and Torque Ratio of a gear system are inversely proportional. Think of the mountain bike. When the chain is on the largest diameter gear, the torque required per revolution is smaller than when its on the smallest diameter gear. And vice versa for speed.

Surely if know that the gear takes 60 min to make one revolution and I know its radius...

With this info you have the period , but NOT the torque?

Thanks but is there no way of theoretically working it out?

No.

Torque is a vector quantity like weight. Is their a way of theoretically calculating the weight of a bag of sand?

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Location: Moncks Corner, South Carolina, USA
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#9

Re: Gear Torque

10/23/2012 1:39 PM

Is this a Schrodinger timer? When you open the case will you find the timer dead or is it still alive? Could the formulas you are seeking be found in advanced spring theory? Pun intended!

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