increasing the bending stiffness of steel pipe or tube?

[AjohnwParticipant @ajohnw51620]

Tubes crush when bent so concrete should help but unfortunately shrinks when it dries and doesn't take kindly to being bent much either – that's why rebars are fitted in structures but that would make filling difficult in this case.

You will find that stiffness goes up markedly with diameter for the same wall thickness. If I remember correctly the width preventing the deflection goes up at a cubic rate were as the other one doesn't so diameter has the biggest effect in this case.

I'm pretty sure you could find simple sums in Machinery's if you want to work it out – almost bound to be in imperial though. They do work correctly.

John

–

[Michael CheckleyParticipant @michaelcheckley34085]

Radius (or diameter) to the power 4. Hence a small increase makes a significant change.

Edited By Michael Checkley on 06/06/2015 20:43:44

[Ady1Participant @ady1]

Sounds to me like a 50x5mm pipe is the minimum I should consider for a more serious setup, which is a pity, I have rather a lot of 35×5.

Can still do a light drill press though so it's not all bad news

The 50×5 would be a lot easier for welding internal bracing as well

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ady1 on 06/06/2015 21:20:57:

… which is a pity, I have rather a lot of 35×5.

.

It may, or may not, be appropriate for the column of your proposed drill-press, but; a very good way of using the 35×5 would be to weld it in a cluster of three, so that the cross-section is like a clover leaf.

MichaelG.

[Nigel McBurney 1Participant @nigelmcburney1]

A drawn tube(seamless) with a similar type of tube push or force fitted inside is no doubt stiffer than a thick wall tube, Back in the 1950/1960s when we built motorcycles in the uk some scrambles bikes were fitted with twin wall tube handle bars,ie a tube within a tube and they were very strong,dont know how they made the tube.

[Gordon WParticipant @gordonw]

I think the twin wall handles started out as two thick tubes pushed together and then drawn out. Certainly some tubes were made like that. I don't think it was done for extra stiffness but for better shock resistance and to retain the original shape.

[Ian S CParticipant @iansc]

In the early 50s they may not have been able to get tube of a heavier gauge.

If the tube for the drill press were too light/Thin walled, I'd be worried about the tube kinking mainly at the attachment points. The material to use would be hollow bar, maybe a grade that could be Nitrided.

Ian S C

[MuzzerParticipant @muzzer]

In bending, half of the tube is in tension and half in compression, so concrete isn't going to help a great deal, especially if there is nothing stopping it simply moving along the tube on the compression side.

Similarly, filling the tube with resin etc and compressing it will simply increase the hoop stress on the circumference (and very marginally at that – think of the stress in the studding vs the minimal stress you would generate in the tube itself) but do nothing for the axial / bending stiffness itself, even if it is somehow firmly anchored to the inside wall of the tube. Making it into a pressure vessel won't make it stiff although ultimately it would bring it closer to yield in bending.

I can't imagine how you'd weld these stiffening members inside the tube or why you'd want to. I hear you say it's just a discussion but as pointed out, the stiffness is related to the second moment of area ie fourth power of radius, so surely the lesson is that if your application needs more stiffness, go for a thicker walled (or solid) pipe. If you are determined to use up your thin walled pipe, it's going to bend something rotten before it collapses. Are you stuck on a desert island?

Merry

[Michael GilliganParticipant @michaelgilligan61133]

Murray,

I'm surprised, and a little disappointed that you didn't endorse my clover-leaf.

… It does make a very stiff section.

MichaelG.

[John McNamaraParticipant @johnmcnamara74883]

On way of minimising the deflection of the 50mm tube would be to solidly fix both ends of the tube instead of a cantilevered beam (post) as is the norm with a drill press. The support would have to be well clear of the table to allow it to rotate around the shaft without hitting a support structure.

Using the same 500 mm length of 50mm diameter 5mm wall steel tube the deflection with a 35kg load applied in the centre 250mm from the ends would only be about .01mm.

However the design of the drill press has evolved over many years with thousands of design iterations to reach the present almost universal form. An interesting project to better it.

Regards John

[DrDaveParticipant @drdave]

I just did a quick check of a 50x5mm steel tube when filled with concrete. The maximum increase in stiffness (ignoring differences between the tension & compression sides) is only 10%. As mentioned above, the best way to significantly increase its stiffness would be to put it inside a bigger tube or get a solid bar.

Incidentally, a bar in the shape of a cross has the same stiffness in any direction. So a + and a x that would fit in the same tube would be equally stiff.

[Michael CheckleyParticipant @michaelcheckley34085]

This second statement is incorrect as the second moment of area of the cross (taking moments about the same plane) will change as the cross rotates about its centroid.

[Ian S CParticipant @iansc]

The drill press could be made like a black smith's wall drill, mine a 1"dia steel bar for the column, the top and bottom of these are bolted to the wall much as a lathe is bolted to a bench.

Ian S C

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 07/06/2015 13:45:44:

Murray,

I'm surprised, and a little disappointed that you didn't endorse my clover-leaf.

… It does make a very stiff section.

MichaelG.

He might struggle to make a table that moves up and down it easily and accurately

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 08/06/2015 15:19:12:

Posted by Michael Gilligan on 07/06/2015 13:45:44:

Murray,

I'm surprised, and a little disappointed that you didn't endorse my clover-leaf.

… It does make a very stiff section.

MichaelG.

He might struggle to make a table that moves up and down it easily and accurately

Neil

.

Neil,

1. That should be a trivial matter for a man with a shaper !!
2. I did say that it may, or may not , suit Ady's design for a drill stand
3. The original post was a general 'concept' question; the drill stand was introduced a 'ferinstance"
4. Ady says that he has a lot of 35×5 tube available
5. Murray's rejection of various 'internal stiffeners' was very valid, but my suggestion requires none.

MichaelG.

[DrDaveParticipant @drdave]

Posted by Michael Checkley on 07/06/2015 23:22:47:
This second statement is incorrect as the second moment of area of the cross (taking moments about the same plane) will change as the cross rotates about its centroid.

This is a common misconception, which is why I mentioned it. For a shape such as a cross with equal leg legs, the second moment of area is the same at any angle: do the math for a "+" and a "x" and you will see!

[Michael GilliganParticipant @michaelgilligan61133]

For info.

Whilst I am sure that users of sophisticated CAD software have such tools readily to hand … I was very pleased to find this plug-in for SketchUp. … I use v8, and the plug-in appears [within the idiosyncrasies of SketchUp] to work perfectly.

MichaelG.

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 08/06/2015 17:36:40:

Posted by Neil Wyatt on 08/06/2015 15:19:12:

Posted by Michael Gilligan on 07/06/2015 13:45:44:

Murray,

I'm surprised, and a little disappointed that you didn't endorse my clover-leaf.

… It does make a very stiff section.

MichaelG.

He might struggle to make a table that moves up and down it easily and accurately

Neil

.

Neil,

1. That should be a trivial matter for a man with a shaper !!
2. I did say that it may, or may not , suit Ady's design for a drill stand
3. The original post was a general 'concept' question; the drill stand was introduced a 'ferinstance"
4. Ady says that he has a lot of 35×5 tube available
5. Murray's rejection of various 'internal stiffeners' was very valid, but my suggestion requires none.

MichaelG.

The man doth protest too much, methinks

Neil

[Michael CheckleyParticipant @michaelcheckley34085]

Got me on this one, but a good exercise . I was looking forward to drawing a Mohr's circle but it didn't get that far!

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 08/06/2015 21:52:13:

The man doth protest too much, methinks

.

Given this Shakespearean accusation from our Moderator, and no comment from anyone else; I wasn't going to bother posting … but it may useful to someone, sometime:

According to the SketchUp plug-in [mentioned above]: the welded cluster of three tubes [*] is approximately 8.4 times stiffer than the single tube.

MichaelG.

[*] 35mm O.D. x 5mm Wall

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Michael, I had also thought of using three tubes as per your drawing, although my first thought was to use seven tubes together which would be more circular but of course would be much more bulky and be very difficult to weld them together with total success.

Regards Nick.

[Michael GilliganParticipant @michaelgilligan61133]

On the subject of welding [of which I know very little] … I think it would be quite adequate to 'stitch' the joints instead of using continuous welds.

Note: The main stiffening purpose of these welds is to prevent the tubes sliding along each other.

MichaelG.

[Neil WyattModerator @neilwyatt]

> According to the SketchUp plug-in [mentioned above]: the welded cluster of three tubes [*] is approximately 8.4 times stiffer than the single tube.

Impressive, clearly a good way to go if you only have small tube, and stronger and more compact than a tube of twice the diameter.

A tube three times the diameter (i.e. with the same amount of tube in it as three smaller ones) would be 27 times as stiff.

I would be keen to see Ady make up a triple tube accurately enough for a yoke to move up and down it smoothly, though

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 14/06/2015 17:02:50:

I would be keen to see Ady make up a triple tube accurately enough for a yoke to move up and down it smoothly, though

.

It obviously depends upon the quality of the original tube [tolerance on OD and straightness] but the beauty of this arrangement is that it is 'self-jigging' [for want of a better term] … Try putting a tie-wrap at each end of a cluster of three similar tubes and you will see what I mean.

Again though; I must mention that Ady's original question was conceptual, not specific to the drill pillar.

MichaelG.

 

Edited By Michael Gilligan on 14/06/2015 17:35:46

[Ady1Participant @ady1]

I would be keen to see Ady make up a triple tube accurately enough for a yoke to move up and down it smoothly, though

Might be one of my have-a-look-at-that-solution-later jobs…

Thanks for all the input gents

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