Calculating Torque on a Pulley 3

[Stanfi]

It's been a while since I worked any statics or physics problems. I have a 3000lb weith that is going to be suspended by a pulley that is 6" in diameter. How do I calculate the toruqe in inch-lbs on that pulley?

Thanks

 

[MintJulep]

Zero.

By definition a pulley cannot exert or resist any torque.

 

[Stanfi]

Ok, how about how much torque is being applied to this pulley?

 

[desertfox]

Hi Stanfi

Torque is force * radius

therefore a force acting on the 3" radius would create a torque of 3000*3 = 9000lbin

regards desertfox

 

[MintJulep]

Still zero.

You have a weight on a cable going around the pulley. What is at the other end of the cable?

If nothing, then the weight will fall.

If an equal weight, then the two weights will balance and there will be no torque, only vertical force equal to 2W.

If the cable is fixed to an "immovable" object then the forces will balance and there is still no torque on the pulley, only vertical force equal to 2W (or some horizontal component if the attachent is not directly below the pulley radius).

If the other end of the cable is attached to the pulley itself then the pulley will rotate until the attachment point is vertically below the pulley's axis of rotation. Still no torque, only vertical force equal to W.

 

[ARenko]

MintJulep is right. Are you sure it's a pulley you're talking about? Why do you think you need to calculate a torque? Can you describe the application better?

 

[codeeng]

It like asking how much torque is transmitted on a chariot wheel - nothing.

 

[tlee123]

MintJulep has properly answered the original poster's question.

However, if the pulley is undergoing angular acceleration, there is a non-zero torque.

Also, all real pulleys have frictional torque when the pulley is rotating about its axis.

The original poster mentioned "statics" and "suspended" so I guess zero is the right answer.

 

[Cockroach]

I think buddy just wants to hear 3000 lbf X 3.0 in = 9000 lbf in or about 750 ft lbf. This would be the moment experienced by the six inch pulley and rope tension.

Of course what has been mentioned is also very true, pulley systems do not exhibit resistance, therefore no torsion. This is just confuses StanFi.

Thanks to DesertFox for being direct.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[MintJulep]

I don't intend for this to be condescending, but if StanFi is confused by the above discussion then perhaps he shouldn't be designing anything that has 3,000 pounds hanging on the end of a rope.

 

[desertfox]

Hi Cockroach

I saw the posters plight the same way as you thankyou for your comment.

regards desertfox

 

[MintJulep]

I've been burned too many times by assuming too much into unclear material. I don't do that anymore. I answer the question that was asked.

 

[zekeman]

Stanfli,
I'm sure you don't have a clue from the answers given.If you want the answer you must give a description of how this weight is suspended from the pulley. Is the weight attached to a string that is wrapped around the pulley?
If so, the torque on the pulley is the tension in the string times 3. The tension is determined by applying Newton's second law to the system of a pulley and suspended mass as follows:

W-T=W/g*a
T*r=torque=I*a/r

Where
T= tension
W= weight
a= acceleration of mass
a/r= angular acceleration of pulley
I =moment of inertia of pulley