Simple static load problem giving me hell (sketch provided)

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In summary, the conversation discusses a design evaluation problem involving the calculation of loads at certain pins using software and hand calculations. The solutions offered by the software do not match the hand calculations, leading to confusion and questioning of the accuracy of the software's results. Further discussion and calculations are done to try and resolve the discrepancy.
  • #1
FreeBodyDiag
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Homework Statement



This is not actually a homework problem but a design evaluation problem I have encountered.
I am trying to solve for all the loads at the pins and have used WorkingModel to try and give me some quick answers. However, the solutions offered by the software do not make sense to me/can't verify with hand calcs.

The image below shows the values of Fx, Fy and F as calculated by the software at each pin. I have added the co-ordinates of where the pin joints are and where the load acts (in mm). +X is to the right, +Y is to up. The load acting on this system is a 10kgF acting directly down as shown. Each grid square is 50mm wide. The mass of the elements is set to zero.

[PLAIN]http://img833.imageshack.us/img833/7528/forces.jpg

The unknowns are the reaction forces (Fx, Fy) at each of fixed pins 11 and 3 and the force acting on the pin at 13.

Homework Equations



Ʃ forces in X direction = 0
Ʃ forces in Y direction = 0
Ʃ moments about any point = 0
M=F.x
cos(θ)=A/H
sin(theta)=O/H
tan(θ)=O/A

The Attempt at a Solution



Unless I have severely overlooked something, I immediately dismiss the solutions offered by the software because a simple check of the equation:

Ʃ forces in X direction = 0
...reveals that this is NOT the case.

Ʃ forces in X direction = Fx@pin11 + Fx@pin3 + Fx@LOAD
= (-5.186) + (16.305) + (0)
≠ 0!

??

What is going on here? Am i going loco or is this solution just plain out wrong?

Thanks
 
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  • #2
(hand calc removed because of error)
 
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  • #3
I used a ruler on your image and took the torques about pin 11 and got,

- 45mm*16.5 + 78mm*10 = 0 ?

- 742.5 + 780 = 0 ? Close

The load tends to rotate the arm about pin 11 clockwise and the force of pin 3 on the arm, via the link, tend to rotate the arm counterclockwise.

Given the distortion of your image (doted squares measure on screen 30mmW X 22mmH) those numbers look close. What confuses me is why the force tables for pin 13 and 3 are not equal and opposite?
 

1. What is a simple static load problem?

A simple static load problem is a type of structural engineering problem that involves calculating the stresses and deformations of a stationary structure under a given load. This load can be in the form of forces, moments, or pressure applied to the structure.

2. What factors affect the calculation of a simple static load problem?

The factors that affect the calculation of a simple static load problem include the type and magnitude of the load, the material properties of the structure, and the geometry and boundary conditions of the structure.

3. How do you approach solving a simple static load problem?

The first step in solving a simple static load problem is to draw a free body diagram of the structure, showing all the external and internal forces acting on it. Then, using the principles of equilibrium and stress analysis, the forces and deformations can be calculated to determine if the structure is stable and able to withstand the load.

4. What are some common challenges in solving a simple static load problem?

Some common challenges in solving a simple static load problem include determining the correct boundary conditions, dealing with complex geometries or loads, and considering the effects of material nonlinearity or structural imperfections.

5. How can I check the accuracy of my solution for a simple static load problem?

To check the accuracy of your solution for a simple static load problem, you can compare it to known analytical solutions or use finite element analysis software. It is also important to perform a sensitivity analysis to see how changes in input parameters affect the results.

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