How do I model the motor in this free body diagram problem?

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SUMMARY

This discussion focuses on modeling a motor within a free body diagram problem involving a pivoted shaft and magnetic bearing. The magnetic bearing has a radial stiffness of 2600 N/mm, while the motor has a radial stiffness of 1200 N/mm. The user seeks clarification on how to accurately model the motor, noting that it behaves like a spring with a negative force when off-axis. The goal is to determine the reaction forces and deflections at the pivot, bearing, and motor when a force is applied to the shaft.

PREREQUISITES
  • Understanding of free body diagrams
  • Knowledge of mechanical stiffness and spring models
  • Familiarity with magnetic bearings and their properties
  • Basic principles of equilibrium in mechanical systems
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  • Research modeling techniques for motors in mechanical systems
  • Learn about the behavior of magnetic bearings in dynamic applications
  • Explore advanced free body diagram analysis methods
  • Investigate the effects of radial stiffness on system stability
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Mechanical engineers, students studying dynamics, and professionals involved in the design and analysis of motor systems and bearings will benefit from this discussion.

ghobbs
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Hi

I have a problem in understanding the attached free body diagram problem.

I have a shaft that is pivoted at the top. Below the pivot is a magnetic bearing with a radial stiffness of 2600N/mm. I've modeled this as a springs in parallel. Below the bearing is a motor with a radial stiffness of 1200N/mm.

The problem is that I don't know how to model the motor. I've previously thought of it as springs in parallel, but that's not correct. The motor is in equilibrium when on axis, but as soon as it moves off axis it is radially attracted. It's almost like it's a spring with a negative force.

I'm trying to produce a free body diagram for this system. When I apply a force F at the bottom of the shaft I want to know the reaction forces and deflections at the pivot, bearing and motor.

Any thoughts? Thanks.
 

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Is this a vertical shaft motor with only one bearing?

Your figure does not show very much. Please add a bit of detail, both in the figure and in words.
 
There is a lower bearing, but at this stage it's not assembled. I'm not sure what detail I've missed out. I guess the idea of a free body diagram is to keep it simple.

The pivot at the top is fixed, so the shaft is only able to pivot about the pivot. There is a magnetic bearing which I've modeled as springs in parallel. There is a motor which I'm not sure how to model. The shaft can be regarded as infinitely stiff.

Please let me know what detail I've missed out.

Any thoughts anyone? Thanks.
 

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