Bar Strength

[rm5d]

Hello I need to make a an excercise bar for a beddridden person.The lenth will be 4 metres supported on both ends with a single load in the middle. Load is 200 -250 kilograms.
I need to know what is the best thing to use - round,square - hollow,silid and diameter.
I will be grareful for any help.

[joe greiner]

Please check your measurements and/or unit conversions. Your brief is more appropriate for an overhead crane.

Cheers,
Joe

[WelderMick]

Hello I need to make a an excercise bar for a beddridden person.The lenth will be 4 metres supported on both ends with a single load in the middle. Load is 200 -250 kilograms.
I need to know what is the best thing to use - round,square - hollow,silid and diameter.
I will be grareful for any help.

I'm sure someone else here could advise better on the metal sizing, but it would need pretty hefty metal to avoid sagging in the middle with a single beam. It seems to me that a well put together truss structure (made of lighter round tube) will some sort of castor and locking system would be the go. Sort of like a light weight gantry crane. Would need to be well braced with large wheel base.

In terms of metal sizing, I can add that I made up a set of monkey bars (4m span) for the kids out of 2 lengths of 50x4mm round mild steel tube. This still flexes quite a bit if I throw my 100kgs on it.

Cheers

- Mick

[bonox]

simple things - let's assume it's not built in at each end (is ends can't support a significant moment) which gives worst centre moment. Let's also assume you're making this from steel @ 320MPa yield (fairly ordinary mild steel) and you wanted a bar (circular) cross section.

maximum bending moment is 2000mm * 250*9.8N ~5,000,000Nmm

From my experience of bhp, common 'thick' wall size for rsj is 2.6mm.

Stress = My/I where M = 5,000,000Nmm as above, y = centre to outer radius and I is second moment of section = PI*(R_outer^4 - R_inner^4)/64.

let's try a 150mm diameter section with 2.6mm wall.
y = 75mm
I = 3.14/64*(75^4-72.4^4) = 1,550,000mm^4

stress = 5,000,000 * 75 / 1550000MPa
= 241 MPa

giving a factor of safety of 320/241 = 1.32.

That will give you a centre deflection of PL^3/48EI
modulus for mild steel ~ 210GPa or 210000 MPa.

deflection at centre for 250kg = (250*9.8)*2000^3/(48*210000*1550000) = 1.25 mm

You'll reduce the section size considerably if you move to a high moment ratio section like a deep rectangle or an I section (think high span floor bearer dimensions) or you can create a significant moment at each end of the beam (ie building it solidly into a wall rather than just resting it on a support.

That section will also weigh ~ 7800kg/m3 * pi(75^2-72.4^2)*4000/1E9 = 37.5kg

Better off with a truss or I beam in steel or timber for that kind of load.

Bear in mind if you go timber, you'll need a reasonable margin for dynamic loading unless you use an engineered product like LVL.

PS: If you use a thin wall section like above, it would be prudent to perform a buckling/crippling evaluation if your margin is low like the one given.

Hollow sections of big overall size will give you the best performance by weight, more area away from the centreline of bending is also better

[Dave J]

I am glad you could work all that out, it would take me weeks.LOL

Dave

[Dropcat]

I am glad you could work all that out, it would take me weeks.LOL

There's a bunch of calculators on 'net, google 'bending beam calculator' will turn them up.

Metal, wood, plastic, solid, tube, round, square, etc, hours of fun for the whole family.

[rm5d]

Well firstly thanks for the responses.A 150mm tube would be far to thick to be able to grab onto.

The idea is to have it fixed to the wall on both sides.

The other things I can think of is:

1: a solid bar- normal steel
2: a solid bar - type that is used on barbells ect.
3: a I section for the main support supporting a tube to hang onto attached undernearh the I beam

If there are better ideas I am open to suggestions.

Any and all help would be great - person had several strokes and gets weaker and heavier from no excercise .

[Oldneweng]

I would go with the I beam. It can be attached to the ceiling or at least at ceiling height and so out of the way. Fix a bar to it with chain if that would work. Could be swung out of the way. If the bar is only to support this person there is no need for solid. Heavy wall tube has almost the same strength as solid of the same diameter and would be lighter to move. It would not need to be very long, just long enough to do the job. Unless there are other requirements you should get away with about a one metre length. A piece of one inch galv pipe that long would be strong enough to support a person as the weight will be near the chains or whatever at the ends anyway.

Dean

[Dropcat]

1: a solid bar- normal steel
2: a solid bar - type that is used on barbells ect.
3: a I section for the main support supporting a tube to hang onto attached undernearh the I beam

As far as bending strength goes, a pipe & a solid bar are about the same. I'm assuming weightlifting bars are solid to prevent damage.

It's not a strength problem, it's more how far the pipe will bend (deflect). Short bars deflect less, as do thicker ones (even for pipes with the same wall thickness).

What about suspending something from the roof (assuming it's up to it)? Or putting two stands next to the bed (like a weight bench has), this will allow a short bar (less bending) and can be moved out of the way more easily.

[Dave J]

If something could be on the underside, you could stitch weld say a 100mmx8mm flat bar to a piece of 50-65mm pipe.

Dave

[Bryan]

Hospital beds have a pole that fixes vertically to the bedhead and bends over the patient. It has a chain and a handle so they can lift themselves. They are tubular steel, at a rough guess 30 - 40 mm (Don't trust that, do the sums!). The 4 metre length in your original idea will make it hard and cost much more. But maybe you have particular exercises in mind that need varying anchor points?

[rm5d]

No special needs - just room is 4 metres wide.will a short pole with a chain mounted to the wall at the head of the bed work it was my first idea.

What diameter pipe would i use - and how big would the wall plugs/rawlbolts have to be it is not a hospital bed.

The thing with a pole across the room could be used for help with wheel chair acses ,but their is nice to have and what is practical.

Again thanks to those that posted and anymore that might,

Mike