# Deflection of a Cantilever with support

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1. Jan 1, 2016

### Firzan

I sketched out a slightly simplified model. It is a cantilever rod with two supporting rods of slightly larger diameter, connected by a plate. (Ignore the weight of the individual parts. For illustration purposes, I draw this to resemble a cantilever beam from the side view but in actual design, this drawing is actually the top view.)
My aim is to theoretically calculate the deflection/bending in the rods (more importantly, the two side/supporting rods) due to the force applied. The plate can be considered rigid.

I'll break down the question into two parts, so that I can understand the concept better:

1. If the three rods were of the same length (the middle rod is not extended and force is applied at the end of the three rods), how do I calculate the forces acting on each rod and then the deflection?

2. With the middle rod being longer than the supporting two rods. How should I approach calculating the deflection in each of the rods?

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2. Jan 1, 2016

### DTM

If you assume the plates are rigid and rigidly attached to the rods, they could not allow any deflection along the full length they are attached to. I think you need to rethink that assumption.

3. Jan 1, 2016

### DTM

After another look, I'm not sure I understand what your rigid plate looks like. I thought it was 500mm x 200mm extending to the support surface, but now I'm thinking it only exist in the plane that is 500mm away from your support surface. If this is the case, the plate constrains all three beams to have the same deflection at 500mm from the support?

4. Jan 2, 2016

### DTM

For #1. I'm pretty sure you can just add the 3 rods moment of inertia and use the total for your deflection equation y = F*L^3/(3*E*Itotal)

For #2. Start with #1 where L = 500. Then just think of the middle beam as starting at the L=500 location and add the additional deflection to the deflection from #1.

5. Jan 2, 2016

### Firzan

The plate connects the three rods/beams 500mm away from the support surface. So yes, since assumed to be rigid, the plate would constraint the three to have the same deflection.

6. Jan 2, 2016

### Firzan

What if the plate was not rigid? Say, it has a rectangular profile of 125mm*30mm. How would I then calculate the X-axis deflection on the rods?
Also, what other things should I take note of? (ie possible bending of the plate, etc?)