# torque question

Discussion in 'Physics' started by tracecom, Sep 25, 2014.

1. ### tracecom Thread Starter AAC Fanatic!

Apr 16, 2010
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Suppose a motor stalls at 20kg.cm and its shaft is .25" in diameter. What is the maximum load that can be exerted on a cord wound directly on the shaft? (Be nice; remember I am a marketing guy.)

2. ### studiot AAC Fanatic!

Nov 9, 2007
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Well I assume that the cord is wound round securely at one and and the other hangs down, supporting a pan.

Then weights are added until it stalls and the question is what weight stalls it?

So the shaft has diameter 0.25 inches = 6.35mm = .00635m
So the radius is half this or .003175m.

The cord hangs down with tension T equal to the weight in the pan.

This develops a torque of T x radius which we are told stalls at 20 kgcm = 0.2 kgm

So max T x R = 0.2

T = 0.2/0.003175 = 63 kg force, or 63 x g Newtons force = 618N.

So your motor shaft will wind up a load of just under 63kg mass.

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3. ### tracecom Thread Starter AAC Fanatic!

Apr 16, 2010
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Thanks for the answer and for showing the math to get there.

4. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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The tangential force is the torque divided by the radius. .125"=.3175cm. 20/.3175=62.9 Kg.

Eric

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5. ### studiot AAC Fanatic!

Nov 9, 2007
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The only real thing is that kilograms force are a non standard unit.

In the ISO metric system the kilogram is a unit of mass and the unit of force that corresponds is the newton.

So working is kgcm is definitely likely to lead to trouble.

since you had a 1/4 inch shaft and are in America, why not keep it imperial and use poundsinches poundsfeet?

6. ### tracecom Thread Starter AAC Fanatic!

Apr 16, 2010
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I converted 63 kg to 139 pounds, which tells me what I need to know, which is that a 100 pound test cord would be suitable.

7. ### studiot AAC Fanatic!

Nov 9, 2007
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Glad it is sorted for you.

I assume you know that if you wrapped the cord several times around the shaft the anchoring force needed at the fixed end would be a lot less than the lifting force available at the free end?