Calculating Power Needed for Torque & Revolutions

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Discussion Overview

The discussion revolves around calculating the power required for tightening a screw, given varying torque values and the number of revolutions. Participants explore the relationship between torque, angular velocity, and power, considering both instantaneous and average power calculations. The conversation includes aspects of theoretical understanding and practical application.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose using the formula P = Tω for instantaneous power, while others suggest integrating torque over the angle for average power calculations.
  • There is a discussion about whether torque depends on the speed of rotation, with some arguing it does not, as torque is based on force applied, while others note that power does depend on speed.
  • Participants question how to handle non-constant torque, with suggestions of numerical integration if only data sets are available.
  • One participant mentions that if torque is constant, work can be calculated simply as Work = Tf*θf, assuming initial torque is zero.
  • Another participant emphasizes that power varies with rotational velocity, even if torque remains constant.
  • There is mention of needing to find maximum power if both torque and angular velocity are not constant, suggesting a graphical approach.

Areas of Agreement / Disagreement

Participants express differing views on the dependency of torque on rotational speed and how to approach calculations with non-constant torque. The discussion remains unresolved regarding the best method for calculating power in these scenarios.

Contextual Notes

Some limitations include the need for clear definitions of torque as a function of angle and the assumptions made about the constancy of torque during the operation. The discussion also highlights the potential complexity of integrating torque values over time or angle.

uselesspotus
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Homework Statement


Lets say I am tightening a screw and I have multiple torque values with their respective number of revolution. How should I go on about calculating the power required?

Also let's assume we know the time it takes to tighten the screw.

2. The attempt at a solution
I know Power = T*dθ/dt = Tω = 2*pi*n*T, where T = torque, ω = angular velocity, n = number of revolution/sec
Since the torque is not constant I am not sure how to approach this problem. Is there any other information i need in order to calculate the power?
 
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Hi uselesspotus, Welcome to Physics Forums.

Power is measure of the rate at which energy is being delivered (Watts, or Joules per second). So either you want the instantaneous power at some instant in time, or perhaps the average power delivered over the entire operation.

If the former, then you have your P = Tω formula. If it's the average power you want then if you have the torque as a function of θ, T(θ), you can integrate the torque over the angle and divide by the total time.
$$P_{avg} = \frac{1}{\Delta t} \int_{\theta_i}^{\theta_f} T(\theta) d \theta$$
 
gneill said:
Hi uselesspotus, Welcome to Physics Forums.

Power is measure of the rate at which energy is being delivered (Watts, or Joules per second). So either you want the instantaneous power at some instant in time, or perhaps the average power delivered over the entire operation.

If the former, then you have your P = Tω formula. If it's the average power you want then if you have the torque as a function of θ, T(θ), you can integrate the torque over the angle and divide by the total time.
$$P_{avg} = \frac{1}{\Delta t} \int_{\theta_i}^{\theta_f} T(\theta) d \theta$$

Thnx I get it now. Also does torque depend on how fast you rotate something? I would think it does not since it only depends on force. For example, using a hand instead of a machine.

This means work would just be Work = Tff (assume initial torque was zero in previous power calculation) since work is ∫Pdt
 
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uselesspotus said:
Thnx I get it now. Also does torque depend on how fast you rotate something? I would think it does not since it only depends on force. For example, using a hand instead of a machine.
Just look at how torque is calculated. No mention of speed in there. What it is that applies the force is irrelevant too.
This means work would just be Work = Tff (assume initial torque was zero in previous power calculation) since work is ∫Pdt
Sure, if the torque has a constant value for all values of θ.
 
gneill said:
Sure, if the torque has a constant value for all values of θ.
How would the equation change if the torque wasn't constant? Wouldn't the work still be, after integrating, W = P*tf-P*ti And because the initial time = 0 W = P*tf where P is Pavg (the equation u provided in earlier post)...I believe I am thinking of this wrong way.

Going back to the Pavg equation, I would have to do numerical integration if I just have the data set not the function itself right?
 
Last edited:
uselesspotus said:
How would the equation change if the torque wasn't constant? Wouldn't the work still be, after integrating, W = P*tf-P*ti And because the initial time = 0 W = P*tf where P is Pavg (the equation u provided in earlier post)...I believe I am thinking of this wrong way.

Going back to the Pavg equation, I would have to do numerical integration if I just have the data set not the function itself right?

If you have a data set with constant torque values that cover a number of revolutions each then you can break the problem up accordingly; Presumably you'll have time intervals to correspond with torques and angular displacements. On the other hand, if you have torque as some function of the angle then you can integrate symbolically and plug in the limits.
 
I'd have to ask why you need to know the power "required"? In any case, as mentioned, torque does not depend on speed or velocity of any kind, power does. For example, if you are twisting at constant torque and varying speed then the power will vary linearly with the rotational velocity.
power = work X time; work = force X distance; power = force X distance / time, power = force X (distance / time), so power = force X velocity
 
Power = torque * angular velocity

If neither torque or angular velocity are constant you need to plot a graph of the product and find where it's a maximum. That will give you the max peak power.
 

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