Flappy machine - Advise needed please!

**John McNamara** · 24-10-2013

Hi Jim

I have used this little program for several years.

You may find the following link useful.....
Straight beams with constant cross sections (Straight beam)

MITCalc is a small Easy to use tool for calculating beam strength and more importantly deflection. It should help you to understand how "floppy" a steel bar really is. Don't confuse strength and deflection. For most machine design deflection is the enemy, as you have found out your 20mm bars easily support the spindle they will never fail in bending. They do however sag and deflect under load.

Many years ago time ago I helped my uncle and a mate restore a 6 cylinder car engine with a cast iron block. we re bored and honed the cylinders to size. While he was testing the bore size he placed a micrometre across the bore longitudinally that is along the block and adjusted it until it just held by its own weight in the bore. He then placed his two index fingers on each side of the particular cylinder and pressed hard. The micrometer fell out. That was my first lesson about steel deflection. a heavy casting is not immune to it.

Get a free trial version of the "beam" module and try it out Only a few Euros if you like it. (I have no connection with the company.)

The program will give the deflection at various points along the length of the beam for a point load, as with a router. This is a static load calculation tool, meaning it does not allow for vibrating or moving loads. Software to do that is not for the feint hearted. My solution is to add a reasonable margin of safety.

Please refer to Beam (structure) - Wikipedia, the free encyclopedia
Note the different kinds of supports..... Use the fixed setting in MITCalc for firmly attached ends. Although steel fixings also flex a bit!

Experiment with different beam depths and widths. MITCalc will allow you enter your own RHS sections You can also enter your 20 and 25mm bar and compare that to other sections.

Regards
John