Free Body Diagram Help for Counterweight Pivot Moment

AI Thread Summary
The discussion focuses on calculating the moment at a pivot point due to a counterweight connected to an L-shaped link. Two free body diagrams (FBD) are considered, with the second diagram being recommended for accurate calculations. It is clarified that the moment at the pivot changes with the length of the second link, and the correct approach involves using the top two arrows from FBD 2 to compute the moment. The user seeks confirmation on whether to multiply the weight by the distance directly or to break it into components for calculation. The consensus is that either method can be used, depending on which is more convenient for the user.

Which free body diagram is correct

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    3
  • Poll closed .
VooDoo
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Hey guys,

I have a counterweight that connects to an L-shaped link that pivots about a point.

I need to work out the moment at the pin due to the counterweight, but I am stuck on the basics.

The first free body diagram I drew assumes that the weight of the counterweight acts entirely as a compressive force along the first link. The second assumes that this force is broken into x and y components.

I believe that the length of link 2 has an effect on the moment at the pivot point. I.e. as the length of link 2 is increased the moment at the pivot point will change, is this correct?

I am stuck figuring out which FBD is correct.

Any help is appreciated!

Edit - Having trouble uploading the image, so I used imageshack

http://img511.imageshack.us/img511/8489/fbdp.jpg
 
Last edited by a moderator:
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Use FBD 2. And use only the top two arrows on FBD 2 to compute the summation of moment about the pivot point. You do not need the other four arrows on FBD 2, unless there is something else you need to do besides computing the moment about the pivot point.
 
I guess my question to you is why do you think that #1 is correct (which it is not)?
 
FredGarvin said:
I guess my question to you is why do you think that #1 is correct (which it is not)?
Thanks very much for the replies. To be honest, I originally had FBD 2 down, but then the thought of the link being in compression threw me off for some reason. But the logic behind FBD 2 kind of makes sense out.

Just for the sake of learning, I have drawn up an arbitrary arrangement and attached it. If I were to find the moment that this weight creates at the pivot point, would I be simply multiplying the weight by the distance X1 or would I be separating the weight into components parallel and perpendicular to the links and multiplying them by the appropriate distance?


Thankyou very much for the help, it is much appreciated!
 

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Regarding your question in post 4, you can do it either way you prefer, whichever is easiest or readily available.
 
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