When do reactive moments appear?

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SUMMARY

This discussion centers on the concept of reactive moments in journal bearings, specifically addressing when these moments can be considered zero. The participants highlight that in two-dimensional scenarios, moment reactions are typically present unless specific conditions, such as additional supports, are met. The conversation emphasizes the importance of visualizing the physical behavior of structures rather than strictly adhering to rules, noting that certain supports can simplify analysis by excluding reactions. The distinction between translational and rotational motion in shafts is also clarified, underscoring that moment reactions may not exist in certain configurations.

PREREQUISITES
  • Understanding of journal bearings and their mechanics
  • Familiarity with two-dimensional static equilibrium principles
  • Knowledge of force and moment reactions in structural analysis
  • Basic concepts of translational and rotational motion in engineering
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  • Research the principles of static equilibrium in three-dimensional structures
  • Study the effects of different support types on moment reactions
  • Examine case studies involving journal bearings and reactive moments
  • Learn about advanced structural analysis techniques, such as finite element analysis (FEA)
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Mechanical engineers, structural analysts, and students studying dynamics and mechanics of materials will benefit from this discussion, particularly those focusing on the behavior of journal bearings and reactive moments in engineering applications.

arestes
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Hello:
I have some questions regarding what books say about reactive moments when dealing with journal bearings and such.
For example:http://[ATTACH=full]200072[/ATTACH] [url=http://postimage.org/][ATTACH=full]200073[/ATTACH]

What I'm looking for is an explanation as to why the moments can be set to zero. We never do that in 2 dimensions!
I would be grateful for any enlightening thoughts.
Regards
 

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If you're talking about like in (7) there for example, why would there be a reaction on the shaft if it is free to spin, neglecting friction, if there's no load?
 
Reactions happen when motion is restricted. Imagine all these things are made of spaghetti, what would break and what would stay intact and in which directions?
 
I understand force and moment reactions must appear. ( but I'm *only* talking about moment reactions).

My point is that... I don't see why SOMETIMES they appear (the text says when other supports are present they don't appear). and why this has to be so tricky.
In 2D, if we have two supports, say two hinges, force reactions always are considered and if there's not enough data, it's statically indeterminate. Here we can set *moment* ractions to zero if there are other supports... I don't understand that.

I also understand that if there's no load they shouldn't appear. But in a general structure? sometimes we don't even know if its going to be nonzero.
 
Try to visualize what is going on in the specific situation rather than try to follow some set of rules. Sometimes in 2d problems one support is modeled as a roller to exclude a reaction in the horizontal direction, because certain problems have more unknowns than equations and you'd need to introduce equations for the stiffness of the material to solve it. They simplify it that way before you get to material property stuff.

Look at the difference between 6 and 7. The square shaft is for translational motion and there is no reaction in the y direction. It is free to move back and forth in the y direction and happens to have no reaction in y. The circular shaft is free to rotate about the y-axis and happens to not have any moment reaction about the y axis. Noticing the pattern?
 
If you have hinges free to rotate there are no moment reactions
 
Set to 0 sounds misleading because there is no reaction in the first place
 

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