When do reactive moments appear?

[arestes]

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

 

[devecseri]

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?

 

[devecseri]

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?

 

[arestes]

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.

 

[devecseri]

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?

 

[devecseri]

If you have hinges free to rotate there are no moment reactions

 

[devecseri]

Set to 0 sounds misleading because there is no reaction in the first place