Calculating Inertia at Output: Motor with 140gcm2 Rotor Inertia & 71:1 Gearhead

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Discussion Overview

The discussion revolves around calculating the output inertia of a motor with a specified rotor inertia and gearhead ratio. Participants explore the implications of the rotor's moment of inertia in relation to the gear ratio, focusing on theoretical and mathematical aspects of the problem.

Discussion Character

  • Mathematical reasoning
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant presents a formula for calculating output inertia as 140*(1/71)², questioning its validity.
  • Another participant seeks clarification on the original question, suggesting that the rotor's moment of inertia is a fixed characteristic and comparing it to the mass of a car in relation to its engine.
  • A third participant provides a mathematical approach, defining the motor mass moment of inertia as J and the gear ratio as r, leading to the conclusion that the apparent moment of inertia at the output shaft can be expressed as Jout = J/r², assuming the inertia of the gears is negligible.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the question and the appropriate method for calculating output inertia. There is no consensus on the initial formula proposed, and the discussion remains unresolved regarding the best approach to the calculation.

Contextual Notes

The discussion includes assumptions about the neglect of gear inertia and the fixed nature of rotor inertia, which may affect the calculations presented.

zoom1
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I have a motor with rotor inertia of 140gcm2 with 71:1 gearhead.
Considering this configuration what would my inertia be at the output ?

140*(1/71)2 ??
 
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Thanks for the post! Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
 
Let me make a try.
By rotor inertia you mean the rotor's moment of inertia, I suppose. This is a characteristic of the rotor and does not change. It sounds like asking how much the mass of a car would change depeding on its engine (or gearbox ;) ).

What/why are you asking such a question?
 
Let the motor mass moment of inertia be J, and let r be the gear ratio, such that
theta-out = r*theta-motor
If the inertia of the gears is neglected, the system kinetic energy is
T = (1/2) J * theta-motor-dot^2
= (1/2) J * (theta-out-dot)^2/r^2
So the apparent MMOI, as seen at the output shaft is
Jout = J/r^2
 

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