# Bending moment

1. Aug 16, 2006

### teng125

Calculate the bending moment in the centre of the axle??

may i know how to form the moment eqn??
F=300N

picture - http://files.filefront.com//;5410274;;/

pls help
thanx

2. Aug 16, 2006

### Meson

What do you think? Try to form the equation on your own and we will inspect it.

3. Aug 16, 2006

### teng125

i have try it many times but i don't know.
if in mechanics,this have to use the heaviside function,but this is machine design.therefore ,i don't know what to use.

i assume it is F(1) <x-175>^0 + F(1)<x-825^0>...
but i think this is incorrect

4. Aug 16, 2006

### Meson

Well, heaviside functions aren't needed here.

Step 1: Determine the value of $$F_{1}$$ using equilibrium condition along the vertical (y-axis).

Step 2: By considering half part of the rotating machine (cut machine into two at the centre), write down the bending moment equation of either side using $$\sum\vec{F}l=\vec{M_{b}}$$. It will be the same for the other half since it is symmetrical.

Last edited: Aug 16, 2006
5. Aug 16, 2006

### teng125

if i cut into the half,then i'll get F * 325mm = M(b) rite??am i correct??

if it is correct,in order to find limit stress = bending moment / W , where W= pi (d^3) / 64 right??
how can i find the diameter and which shaft should i consider??

6. Aug 16, 2006

### Meson

Your answer isn't correct. Try writing down the moment at C. Then we discuss the rest of the question.

Last edited: Aug 16, 2006
7. Aug 16, 2006

### teng125

then it should be M + (F1*325) - (1/2F * 500) =0 right??am i correct??

if it is correct,in order to find limit stress = bending moment / W , where W= pi (d^3) / 64 right??
how can i find the diameter and which shaft should i consider??

8. Aug 16, 2006

### Meson

Bingo.

How did you obtain the W? The correct axial section modulus W is $$W=\frac{\pi*d^3}{32}$$, assuming that the cross section of the axle is not hollow.

Setting $$\sigma_{all}=\frac{\sigma_{lim}}{n_{s}}=\frac{M_{b}}{W_{by}}$$ and solve for the diameter d as you have found $$M_{b}$$ in the first part, $$\sigma_{lim}$$ can be found in a strength data table, depending on the material given and the safety factor should be, well, leave the diameter in terms of $$n_{s}$$ if it isn't stated.

Last edited: Aug 16, 2006
9. Aug 16, 2006

### teng125

ok......thanx...pls help me in the other question in the engineering side...thanx