Question about a dynamic equation of a crane model

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Discussion Overview

The discussion revolves around the dynamic equation of a portal crane model, focusing on the analysis of tension in the rope and spring, the velocity of the load, and the conditions under which oscillations occur. Participants explore the implications of dynamic forces and oscillatory behavior in the context of crane operation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses uncertainty about the correctness of their dynamic equation and the application factor for the crane.
  • Another participant suggests that the frequency of oscillation for the spring and rope must be considered separately, noting potential constructive and destructive interference between the two.
  • Static tension in the rope is proposed to be calculated as ML X G, while static tension in the spring is described as the sum of the masses times G.
  • A participant mentions that when the crane moves, the tensions will increase due to the acceleration of the loads, leading to oscillations.
  • Clarification is sought regarding whether the crane is lifting, translating, or both, with confirmation that the crane is lifting the load.
  • One participant indicates that they have identified a single frequency peak for the spring and suggests that the rigidity of the steel rope may allow neglecting its frequency contribution.
  • There is a question about whether the lifting velocity of the crane is equal to the difference in velocities of the load and crane body.
  • Another participant states that the tension in a rigid rope would equal ML X G at steady lifting speed, but additional tension arises from the load's acceleration.
  • A participant inquires about the conditions under which the spring begins to oscillate, proposing that it may occur when the rope tension equals the initial spring force.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the dynamics of the crane model, with no consensus reached on the specific conditions for oscillation or the relationship between the lifting velocities. Multiple competing views remain on the interpretation of the forces involved.

Contextual Notes

Participants reference specific assumptions about the rigidity of the rope and the nature of oscillations, but these assumptions are not universally accepted or clarified, leaving some aspects unresolved.

Iqbal94
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Hi guys,

I have a question on the fundamental dynamic equation as I am not sure if my equation is correct. I am doing an analysis on a portal crane to find its application factor, somehow I couldn't get the desired value for the velocity of each masses as I couldn't find the tension of the rope. The portal crane model is simplified to be (picture).

http://[ATTACH=full]200070[/ATTACH]
 

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Since it's a dynamic situation you have to get an idea of the frequency of oscillation at the spring and the rope separately (two sine waves). Probably the two won't be in sync so there will be constructive and destructive interaction. So the load in both the spring and the rope will change over time. There are available equations for doing this calculation. The static tension in the rope is ML X G. The static tension in the spring is the sum of the masses X G. When you move the crane the tensions will increase (acceleration of the loads, which you'll have to know), then the "springiness" will start the loads oscillating.

Your sketch isn't very clear. Is the crane lifting the load, translating the load, or both?
 
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OldYat47 said:
Since it's a dynamic situation you have to get an idea of the frequency of oscillation at the spring and the rope separately (two sine waves). Probably the two won't be in sync so there will be constructive and destructive interaction. So the load in both the spring and the rope will change over time. There are available equations for doing this calculation. The static tension in the rope is ML X G. The static tension in the spring is the sum of the masses X G. When you move the crane the tensions will increase (acceleration of the loads, which you'll have to know), then the "springiness" will start the loads oscillating.

Your sketch isn't very clear. Is the crane lifting the load, translating the load, or both?
The crane is lifting the load. Yes, I have the frequency of oscillation at the spring. From the tests and analysis that I've done, I found out that there is only 1 frequency peek which is for the spring. I used steel rope/wire during the tests and I assume due to its rigidity, the frequency due to load on the spring can be neglected. Anyway, I would like to confirm either the lifting velocity of the crane is equal to the difference of velocity of load(x'load) and crane body(x'm1)

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or

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The tension on a reasonably "rigid" rope would be equal to ML X G. You have the velocity of the lift correct. That's at steady lifting speed. If the load starts out static then there's addition tension in the wire rope due to acceleration of the load. There will be a fluctuating component equal to the oscillation rate and magnitude of the spring. That's because the load accelerates and decelerates with the extension and retraction of the spring.
 
Hi, thanks for your explanation. I have one more question, is it possible to know when the spring will start to oscillate? Is it right after the load is lifted, or is there any circumstance that I need to take note? I'm not sure if my idea is right. But I think the spring starts to oscillate (the load as well), when the rope tension is equal to the initial spring force?
 

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