# Bracket design stress formulae help

1. Mar 14, 2013

### jonnyjames1985

hello everyone.

I am in desperate need of help from some mechanical design/materials engineers/students. I am an Electrical/Electronic Engineering student currently completing a project and require assistance on some mechanical design issues. I have searched numerous websites and books looking for formulae relating to allowable sheer strengths in both a weld and the bracket itself. It is a rough design so some of the variables such as plate thickness, materials etc are changeable. I am not worried about the normal stress values as this bracket will support a filter that weighs around half a pound. I require a formula to calculate the maximum allowable force shown at the point shown in the attached diagram, with regards to the weld (transverse fillet?) and the materials properties (mild steel or similar around 0.2mm thick if the calculations allow). Any help would be greatly appreciated, im pulling my hair out at this :-)

Jonathan

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2. Mar 14, 2013

### pongo38

You need to consider (1) deflection (2) bending capacity (3) shear capacity; and the failure could be (A) in the horizontal plate (B) in the weld or (C) in the near vertical support. So the problem is a bit bigger than you thought. With 0.2 mm thick I would look at deflection first D=PL^3/3EI
If the combined throat thickness of the welds is at least equal to the thickness of the thinner steel plate, then you can probably not worry about it, provided the weld is achieved by an accredited welder, rather than an amateur. The shear case is complicated because in a rectangular section there is a significant reduction of bending strength due to the presence of shear force. It's too complicated to describe in a quick answer like this. Bending the edges of the horizontal plate down at 90 degrees to form an upsidedown U section would improve the design a lot.

3. Mar 14, 2013

### jonnyjames1985

Thanks for your reply, basically i'm not actually manufacturing the bracket, i'm just constructing the idea of it with an emphasis on the project development side, but i am requires to provide evidence of further learning. I was looking at maximum allowable force because this bracket has to overcome issues with an existing bracket that sheers when any accidental downward force is applied to it i.e a heavy technician leaning on it while changing a filter. Are you aware of anything along them lines?

4. Mar 14, 2013

### pongo38

The basic problem is that the geometry is not well-conditioned for bending, shear, or deflection. Your further learning would be enhanced if you had a good idea from relevant formulas how those three attributes depend on the thickness of the steel plate to some power.

5. Mar 14, 2013

### jonnyjames1985

The geometry suits the current installation which is on a vacuum pump, otherwise the design would be different. Formulae for calculating the thickness of metal required to cope with deflection bending and stress would be exactly the kind i need, sorry to be a pain, its obvious this is not my forte

6. Mar 17, 2013

### nvn

jonnyjames1985: As stated in your given problem, stress is the issue here, not deflection. Furthermore, in your particular problem, shear stress and weld stress do not appear to govern. For a plate thickness on the order of the value given in post 1, the governing stress is only the plate bending stress.

Therefore, the maximum allowable force (V) that can be applied at the point shown in your diagram is, V = 0.1667*b*(t^2)*Sta/L, where L = plate length (mm) (i.e., the distance from the weld to force V), b = plate width (mm), t = plate thickness (mm), and Sta = allowable normal stress = 167 MPa. Force V is in units of newtons (N). This assumes the plate will be subjected to relatively few stress cycles. If you instead want the plate to be subjected to a large number of stress cycles, you perhaps could change Sta to Sta = 125 MPa.

Last edited: Mar 17, 2013
7. Apr 1, 2013

### GT1

You can add a bend to your "shelf" and then connect it to the main body with screws, I usually prefer to avoid welding for connecting different parts like yours.
I think that the best (and cheapest) option is to redesign this part as a single piece sheet metal part.