Suitability of steel

[Bigjoe]

Morning all,
Ive been asked to make up a mild steel frame for a prototype chair design,the drawing specs 20mm box section but not a wall thickness...on further enquiry the chair will need to support upto 100kg possibly.

Im not sure if 3mm wall thickness would be supportive enough,thats why im consulting eminantly more experience folks on here

 

[dutchman]

You would get an awful lot of stress on those corners, especially the rear ones, but my guess is 3mm wall thickness will hold of welded properly.
If you make a small reinforcement there it is even more likely to hold, but you probably had figured that one out yourself.

 

[Bigjoe]

Hi Dutchman,
The stress on the joints is what is concerning me especially considering its being used as a chair,and if this onw works out there will 5 more to build and other future projects to work on.

and i have to price this one up!

 

[dutchman]

I have a similar chair made of 1.6mm bent tubes, not welded or reinforced.
My guess is that yours will hold, and will be BOMB (or fat @ss) proof with a small reinforcement in the rear corner at the bottom.

You may like this as well:

You might want to read Statics and Dynamics by Hibbler on this subject. Was recommended to me by Storm Monkey and it is a good help.

 

[malcolm]

I'd be more inclined to use a bent tube rather than concentrating all that bending fatigue on a weld. Would be kinder to the carpet too. Could be worth pointing that out to the customer.

3mm would make the chair quite heavy. Can't find any chairs with such a big bending force in the design, but I'd guess 1.5 to 2mm. Though of course those ones are bent tube and not welded.

 

[TechnicAl]

It looks a tad uncomfortable. Is it for something unusual. Do your feet or knees rest against that lower support?

 

[Luke]

it looks like it's an attempt at one of these 'ergonomic typing chairs'.

To be honest, it looks damned uncomfortable, being made of just wood. your ****'ll be hanging off the too-small seat whilst your knees take a hammering on the plain wood knee board!

two single pieces of bent tube sound like a much better idea. and maybe some padding :P

 

[Hitch]

it looks like it's an attempt at one of these 'ergonomic typing chairs'.

To be honest, it looks damned uncomfortable, being made of just wood. your ****'ll be hanging off the too-small seat whilst your knees take a hammering on the plain wood knee board!

two single pieces of bent tube sound like a much better idea. and maybe some padding :P

The drawing has mislead you... it says padded and upholstered
Bending box section to a tight radius can be hard without the right gear, but would be a better job.
Tube would be easier, but thats not whats been asked for.

3mm wall will be plenty, youll probably have a slight flex in it, difficult to guess, but thats what prototypes are for.

You could go lighter, but bigger box, perhaps a 40x20 rectangular for the uprights at the back, that would reduce stress on the joint somewhat.

 

[Drains]

It's based on a Balans kneeling chair.

I messed about with some designs for these things about 15 years ago. They are fantastically comfortable and great for your back, if the geometry is right.

I'm with Malc on the bent steel idea - allows flex, rather than tearing the weld/ haz. It's that or some reinforcing on the corners, maybe. Hmmm

 

[Bigjoe]

Many thanks for all the replies, Hitch i was thinking of recommending the back tube be changed to a 40x20mm section and id love to do bent tubes but i simply dont have the facilities to bend box section

Ive actually sat on these type of chairs and they are really comfortable especially for bad backs (me included).

many thanks one and all

Joe