Machine Parts - Screws fastening a board calculation

[Femme_physics]

Homework Statement

Board AB is fixed to the wall through 6 screws as described in the drawing. Beams CD and DE are connected to the board by joints at points C, D, E. At point D acts a vertical force of 7500 [N].

Calculate:

A) The reaction forces at joints C, E.

B) Describe the external forces acting the beams CD, ED, on board AB.

C) Calculate the screws measurements for fastening the board.

http://img341.imageshack.us/img341/4181/rivets.jpg

The Attempt at a Solution

I left clause C out, wondering if so far so good

http://img854.imageshack.us/img854/851/fixxxxxx.jpg

 

[I like Serena]

Brings back the old days!

The reaction forces at C and E looks good! I'm afraid your drawing for part (B) is not quite right yet.
You have left out the horizontal forces which you shouldn't.
Btw, the direction of one of those vertical forces is the wrong way.

 

[Femme_physics]

How's that for whoopin' butts mechanicho-style

Old skool :)

http://img543.imageshack.us/img543/8567/oop1c.jpg

http://img254.imageshack.us/img254/1282/oop2.jpg

http://img248.imageshack.us/img248/2513/oop3.jpg

 

[I like Serena]

Looks familiar. :)
I'll check the numbers later.

The forces in part (B) look much better now... but they seem to be the forces that the board exerts on the beams rather than the forces that act on the board...

 

[Femme_physics]

You're right, I forgot, it's the same with the principle of the frames. When you look at a different beam the forces are equal but opposite.

So, consider all the forces drawn on the board equal but opposite (I'll redraw it later :) ). Right?

 

[I like Serena]

Right! :)

 

[Femme_physics]

Excellent! I'll tackle clause C then :)

Basically with screws our teacher gave us 6 cases to choose from... I think our case is number 2. Sounds right? (I don't know why the scan turned out so blotty and blurry... I'll go clean the scanner right now)

http://img18.imageshack.us/img18/7830/111111mv.jpg

http://img5.imageshack.us/img5/613/22222222v.jpg

http://img831.imageshack.us/img831/9716/3333pdx.jpg

 

[I like Serena]

Hmm, case 2 shows 4 horizontal boards that are entwined.
Do you have 4 boards?

 

[Femme_physics]

Our teacher picked case 2 for this other exercise: (which doesn't have 4 boards, only 2 as you can see)

http://img837.imageshack.us/img837/8774/exercisen.jpg I added translation for the cases I posted in the post before.

1) Force P is parallel to the screws axis and passes through the center of gravity (C) of the connection

2)
a) Force P acts in 90 degrees to the axis of the screws and passes through the center of gravity of the conection

b) Connection with no space between screw and drilled hole

3)
a) Torsion moment M acts in the plane of the conection
b) Screws are in difference distances from center of connection C

4)
Force P acts 90 degrees to the axis of the screws (does not pass through center of gravity C)

5) Force P (or moment M) acts parallel to the axis of the screws (does not pass through center of connection C)

6) Force P acts 90 degrees to the axis of the screws (does not pass through center of gravity C).

 

[I like Serena]

I checked your numbers... and they check out!
It looks like you still know how to do it!

And with the translation, case 2 does sound right!
Did you clean the scanner?

 

[Femme_physics]

My classmate claims it's case 6... I'm a bit conflicted myself. Any second thoughts?

Thanks for checking my numbers :)

And yep, it's all cleaned up and ready for mechano-scans

 

[I like Serena]

The downward force on the board is at point C.
At this point the force is at 90 degrees to the screws and passes through the center of gravity of the board.

The main horizontal force is at point E, trying to pull the board from the wall.
But both cases include this force.

So I'm going to stick with you. ;)
Case 2 it is.

 

[I like Serena]

And yep, it's all cleaned up and ready for mechano-scans

Can it also scan or?

 

[Femme_physics]

Can it also scan or?

Heh ;)

The downward force on the board is at point C.
At this point the force is at 90 degrees to the screws and passes through the center of gravity of the board.

The main horizontal force is at point E, trying to pull the board from the wall.
But both cases include this force.

So I'm going to stick with you. ;)
Case 2 it is.

Thanks!