# Thread: Stress-based design of a homogeneous rigid bar pendulum

1. ## Stress-based design of a homogeneous rigid bar pendulum

Hi everybody
I'm trying to design an industrial pendulum mechanism, but first of all I should solve a simplified example.
As we know the equations for a rigid bar are: ( length = l, m= mass , theta= anfle of pendulum with respect to horizontal line )

(Extracted from Dynamics book)
R(r) = force along the bar applied on the pivot = (5/2) *mg*sin(theta)
R(t) = Force perpendicular to the bar direction applied on the pivot =(1/4) * mg *cos(theta)
So the maximum load is at theta = 90 which R(r)=(5/2)mg and R(t)=0 so R(total)=R(r)=(5/2)mg

the maximum value of bending momentum is at theta = 0 ( beginning of movement)
all data are verified by ADAMS/View software

Question:
How can I design the pendulum? Is it enough to extract the stresses just at angles which forces or momentums are maximum?
It could be true but simulating in Visual Nastran shows that the maximum Von Mises stress is at angle of 27 degrees.
I'm quite confused because nothing is maximum at this angle (after checking the Adams view software results/figures) . I wanna solve it manually but I can't check it at each angle. What do you suggest?  Reply With Quote

2. Originally Posted by ehsan_kabiri_33 but I can't check it at each angle.
Hi, welcome to the forum, but why can't you check it at each angle, say, one-degree?

Maybe use a Spreadsheet to calculate each degree. Once you have the result for one-degree, just copy the Cell(s) and change the angle to two-degrees. Make as many calculation Cell(s) copies as the included swing-angle requires.

You could of course automate the process within the Spreadsheet if you are familiar enough with the Spreadsheet functions. That would require an advanced knowledge of the Spreadsheet, but just copying single Cells is an easier approach.

{edit}
I can't recall ever having done anything with pendulums, but wouldn't the max stress be at Vertical which I assume is at max acceleration and centripetal?
{/edit}  Reply With Quote

3. "Question:
How can I design the pendulum? Is it enough to extract the stresses just at angles which forces or momentums are maximum?
It could be true but simulating in Visual Nastran shows that the maximum Von Mises stress is at angle of 27 degrees.
I'm quite confused because nothing is maximum at this angle (after checking the Adams view software results/figures) . I wanna solve it manually but I can't check it at each angle. What do you suggest?"

Your maximum stress at any angle is the SUM of the flexural stress, Mc/I due to the transverse acceleration plus the centripetal stress R(t)/area. It should be pretty easy to get the angle of max stress analytically.  Reply With Quote

4. Is this a homework assignment for school?  Reply With Quote

5. Certainly sounds it.
Where could you ever get stresses that are significant when gravity is the only force?  Reply With Quote

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