increasing the bending stiffness of steel pipe or tube?

More a thinking aloud query, crowd thinking

I have some reasonable seamless steel tubing which could be used for say, making a light drilling press and would do a very reasonable job but it would be nice to "make it as strong as posslble" and fill the void with something which would spread the bending moment across the tube wall if you tried to do something more stressful like light milling

A very good Concrete mix is the obvious answer but there are also various epoxy mixes out there

Anyone got any suggestions/knowledge?

Or putting a fabricated stiffener in with the mix, a piece of steel box section for example

Or packing it with steel rods

Edited By Ady1 on 05/06/2015 10:57:50

if you are going to the expense of putting stiffening inside the tube, why not get a stiffer tube in the first place - could be cheaper and better as all inside tubes etc would have to be bonded well to the inside of the column?
BobH

Depends on your tube, I suppose.

I just played with an online calculator. For a 500mm long steel tube with a 50kg load (a typical drill press needs about 35kg to push a decent sized drill into steel, but I won't calculate exaclty what force this translates to at the column!)

For different wall thicknesses these were the deflections:

Wall Deflection

0.5mm 4.1mm

1mm 2.1mm

1.5mm 1.5mm

2mm 1.1mm

3mm 0.8mm

4mm 0.6mm

5mm 0.5mm

6mm 0.5mm

25mm 0.3mm (solid bar)

Interesting how a 4mm wall only deflects twice as much as a solid bar.

A while back I was making a table from a slice through a windblown oak on the farm. To fix the few checks I plaigerised the idea of mixing epoxy with brass powder (to make a feature of the damage)

Extrapolating from that it might be cool to mix up a batch of epoxy and swarf to pack your tube - bound to have a load of that lying around somewhere and free! Although it'd probably benefit from a degreasing rinse first.

Find a tight fitting steel bar and drive that into the tube.

As you have found Neil it's the outer section that give the most stiffness, we used to have this demonstrated in classes years ago but I'm not sure they do it any more, my son is in his first year of an engineering course so I'll ask him if he's been shown with a demonstration.

As to making the tube stiffer, gluing a good fitting tube inside will add a lot of stiffness or a large diameter threaded rod tightened on to caps at each end may add a bit of stiffness.

Steve

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Indeed it is ... and even more interesting to see how much stiffer a given mass of material is, when you use it as [a larger diameter] tube compared with using it as solid bar.

For optimum stiffness to weight ratio, seek inspiration from Bird bones [popular with with flat panels, less common with tubes]

MichaelG.

Edited By Michael Gilligan on 06/06/2015 00:36:36

Preload with a Truss rod?

Martin.

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Neil,

There are two important details missing:

How & where supported ?

How & where loaded ?

MichaelG.

Do we have a diameter for this tube?

graham.

Do we have a diameter for this tube?

Just think of a number that would be useful for a model engineer instead of solid bar and can be easily obtained via steel piping

35mm with a 5mm wall

50mm with a 5mm wall

It's about what methods can be employed to improve the resistance to bending

There are two important details missing:

How & where supported ?

How & where loaded ?

Use the simple worst case scenario, support at one end, load at opposite end

(Like a drilling/milling press)

Edited By Ady1 on 06/06/2015 11:21:04

If you are going to be using the machine for manual milling, so separate X and Y movements rather than a combination of the two then you could increase the stiffness in just the X and Y plane by somehow welding a 'cross' cross section piece down the middle. (like your second example but rotated 45degrees to align with the axis of the machine). A round section gives the most efficient use of material as it has the same stiffness in all directions.

To make life easier and achieve a good compromise I would probably fill the tube with an epoxy loaded with a filler, metal particles or fibres?? I think this practise is used for modern machine tools to absorb vibration when cast iron is not used....

Hi Ady,

I find I get flexing in my cast iron milling machine depending on what type of cut I'm doing, so I'm not sure that pipe would be a wise material for a milling machine structure. If I had to use pipe I'd probably think about using triangular webs in the corner areas or maybe some kind of geodetic structure if you have lots of it.

With support at one end and load at the other I'd weld a fin or spine to the outside, under or over, or both. Or mill a slot top and bottom and pass a plate through, then weld it.

Does the structural support have to be internal?

Ed.

Whatever you put inside a tube will be less effective as a stiffener than a thicker-walled tube in the first place. If the extra material is not fixed to the tube with the same strength as the tube material, it will add very little to the stiffness. Really there is no way round this unless you add extra material outside the tube - effectively increasing its diameter.

Sorry - Tim

Hi Ady1

Some back of the envelope thoughts and musings re your drill press or machine......

I ran A computer program to test for deflection with a load of 35kg applied laterally at the unsupported end of a 500mm hollow steel tube, over a few different sizes and got:

1.28mm with a 35mm 5mm wall tube.
.97mm with a 35mm 12.5mm wall tube. (Note wall thickness 10mm hole in centre)
.39mm deflection with a 50mm x 5mm wall tube
.11mm deflection with a 75mm x 5mm wall tube
.04mm deflection with a 100mm x 5mm wall tube

To completely "fix" a tube (beam) to an unyielding support structure is difficult, there will most likely be some flexure at the fixed end connection, this would increase the deflections mentioned.

As the numbers show, Increasing the diameter has a much greater effect on the stiffness then making the walls thicker. When a beam is subjected to a load the stresses on the load side of the beam are in tension while the opposite side is in compression. The centre of a beam is sometimes called the neutral axis, the closer to the centre you get the less tension or compression forces are applied to the material, in the centre there is equilibrium = no stress. this partly explains why making a beam solid has less effect on stiffness than making it larger, thus moving the tension compression faces further away from each other. Adding material to the centre where the stresses are low is not an efficient use of material.

Filling with Concrete or Epoxy composite materials will increase the vibration damping but not materially increase the stiffness compared to steel they are both less stiff. Using them requires greatly increasing the thickness of the beam or structure as done by the big CNC machine builders.

You also have to consider the strength of a material as opposed to its deflection. Will the beam resist permanent bending damage or creep? will it resist fracture caused by a fluctuating load? (Just like when you bend a piece of wire back and forth to break it). When working with machinery that requires low deflection for accuracy strength normally looks after itself. It should however always be checked and considered.

Regards
John

Edited By John McNamara on 06/06/2015 17:37:00

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Ady,

My question was directed to Neil, and was specific to the calculated figures that he presented.

MichaelG.

Is the idea to increase the stiffness of an existing tube? if so I guess changing the dimensions or material of the tube is not an option...

If you check the variables in the formulae for beam bending a solid bar is stiffer than a hollow one as the second moment of area for a solid is more than that of a tube but obviously the solid will be heavier. If you fill a tube with another solid material it will add stiffness compared to that of the tube but the resultant 'composite' may or may not be stiffer than the parent tube material depending on what it is filled with. If you fill a toilet roll with concrete then you can bet it will be stiffer than the cardboard tube on its own.

Simple cantilever, supported 'rigidly' at one end, loaded at the other normal to the axis of the tube.

Neil