Figuring out the torque required to rotate a dome

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Discussion Overview

The discussion revolves around determining the torque required to rotate a heavy dome, specifically in the context of a motorized observatory design. Participants explore the factors influencing torque, such as moment of inertia, friction, and the mechanical setup of the dome's support system.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant seeks to calculate the minimum torque needed for a dome rotating in place, emphasizing the role of moment of inertia.
  • Another participant requests specific details about the dome's size, weight, rate, and acceleration to provide a more accurate assessment.
  • Some participants suggest that if friction is negligible and time is not a constraint, even a very small motor could suffice for rotation.
  • Concerns are raised about the impact of friction on the required torque, with suggestions to measure the force needed to initiate movement.
  • Participants discuss the importance of knowing the moment of inertia to estimate the torque necessary for movement, while also considering the torque needed to overcome friction.
  • One participant highlights the need for engineering effort to achieve low friction in the dome's support system.
  • Details about the intended mechanical setup are shared, including the use of wheels and a platform for the motor, though clarity on the support mechanism is sought.
  • A participant shares an example of an astronomical dome, providing insights into its size, weight, and motor configuration, suggesting that the original poster's dome could potentially use smaller motors.

Areas of Agreement / Disagreement

Participants express varying views on the significance of friction and moment of inertia in determining the required torque. There is no consensus on the exact torque needed or the best approach to calculate it, indicating ongoing debate and uncertainty.

Contextual Notes

Participants note that the analysis may be premature without a well-defined structural solution for the dome's support system. The discussion also highlights the dependence on specific parameters such as size and weight, which remain unspecified.

doopa
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TL;DR
I'm trying to calculate the minimum torque needed to rotate a fairly heavy dome and I'm a bit lost on how to approach this issue.
Hello,

Assuming the dome will not spiral away using a single motor and is set up in a manner such that it would rotate in place, I want to find the minimum torque needed to rotate a fairly heavy dome if a motor was located on the dome's inner face.

I understand that this is an issue that deals with the moment of inertia, but I'm not entirely sure how to apply it to this specific problem. Any input on the issue is much appreciated.
 
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We need specifics: size, weight, rate and acceleration.
 
If there is no friction and you can take an arbitrarilly long time then the world's smallest motor will work.
Clearly these are two things that need to be specified (and you need to know/calculate the moment of inertia of the shell...that is straightforward with help)
 
hutchphd said:
If there is no friction and you can take an arbitrarilly long time then the world's smallest motor will work.
Clearly these are two things that need to be specified (and you need to know/calculate the moment of inertia of the shell...that is straightforward with help)
Just to be clear, @hutchphd said two things. Friction is the second thing.
 
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Welcome!
A heavy dome rotating about a vertical axis like the one in the picture?

e-elt.jpg
 
Usually it's all about friction. Once the dome is moving it usually takes less torque than what is required to get it moving. Is it possible to just measure it? Can you push on the dome and measure the force needed to get it moving?
 
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Lnewqban said:
Welcome!
A heavy dome rotating about a vertical axis like the one in the picture?

View attachment 305357

Yes! It is actually quite the coincidence that you brought up an image of an observatory because that is what I intend on building.
 
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How big? How heavy? Can you just rotate it by hand, or will it be so large that it will definitely require external power to rotate?

Please share as many details as you can.
 
hutchphd said:
If there is no friction and you can take an arbitrarilly long time then the world's smallest motor will work.
Clearly these are two things that need to be specified (and you need to know/calculate the moment of inertia of the shell...that is straightforward with help)
I see, so figuring out the moment of inertia will give an idea on the torque necessary to move the dome itself, assuming friction is negligible. And then after finding that, I would just add the torque required to overcome friction to my previous result, correct?
 
  • #10
doopa said:
I see, so figuring out the moment of inertia will give an idea on the torque necessary to move the dome itself, assuming friction is negligible. And then after finding that, I would just add the torque required to overcome friction to my previous result, correct?
No. Your MOI is negligible, and bearing friction will dominate. I have a crystal ball in front of me that tells me that...
 
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  • #11
How is the dome supported. It could be low friction but not without a lot of engineering effort. Then any additional force (torque) will cause the dome to (angularly) accelerate
 
  • #12
doopa said:
Yes! It is actually quite the coincidence that you brought up an image of an observatory because that is what I intend on building.
I never would have guessed! :biggrin::wink:
 
  • #13
hutchphd said:
How is the dome supported. It could be low friction but not without a lot of engineering effort. Then any additional force (torque) will cause the dome to (angularly) accelerate
The plan is to rest the dome on top of an open cylinder and create a platform on the top of the cylinder so that a motor could rotate the dome. I don't have an idea on the exact material I will use for the platform, but I have a feeling it will end up being made of wood or some 3D printed material.
 
  • #14
doopa said:
The plan is to rest the dome on top of an open cylinder and create a platform on the top of the cylinder so that a motor could rotate the dome. I don't have an idea on the exact material I will use for the platform, but I have a feeling it will end up being made of wood or some 3D printed material.
I didn't quite understand this. Will you have some type of bearings or wheels to support the weight of the dome and allow it to rotate?
 
  • #15
phyzguy said:
I didn't quite understand this. Will you have some type of bearings or wheels to support the weight of the dome and allow it to rotate?
There will be wheels attached to the motor. The wheels themselves will be on the platform and that allow the dome to rotate. Sorry if I wasn't clear on that.
 
  • #16
doopa said:
Yes! It is actually quite the coincidence that you brought up an image of an observatory because that is what I intend on building.
Practically, you could use almost any torque, because your rotational velocity can be very small in this case.
The inertia of that big mass should be considered for starting and precisely stopping the rotation.
The friction of all the driving and guide wheels, electrical tracks and transmission mechanisms, under windy conditions, should be also considered.

It seems to me that this analysis may be a little early in this project, if structural solutions have not been engineered yet.
 
  • #17
Well, I can give you an idea from an astronomical dome that I'm familiar with. The dome is about 10 meters in diameter and covered with metal sheeting. It probably weighs several tons, but I don't really know how much it weighs. It is supported on about 100 wheels that run in a track around the perimeter of the dome. Four of these wheels have 3/4 HP motors that drive the rotation of the dome, spaced 90 degrees apart around the dome. Your dome is probably smaller and lighter so could use smaller motors.
 
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