Flagpoles and Salesman

[Pud]

Our sales guy has volunteered us (i.e. me!) to redesign a flag pole support.

To keep it simple, imagine a tubular socket on a flat plate of infinite thickness, the socket having a length of 290 mm and an internal diameter of 60mm.

There are various length poles with various size flags on the top. The flag pole manufacturer has given a maximum bending moment for the flagpoles of 2.18KnM

I am looking at using a material with E=17000MPa and Tensile strength at break of 220Mpa.

Assuming a transition fit of the flagpole in the socket, what will be the minimum required socket outer diameter.

Assume ideal everything – I'll add safety factors.

Cheers


Harry

http://www.tynevalleyplastics.co.uk

 

[zdas04]

First, is "KnM" "kilo Newton meter" (usually wouldn't that be "kNm")? When you say "the flagpole maker has given a maximum ..." do you mean that that is the maximum allowable or the maximum that the pole configuration will experience under the most adverse conditions (e.g., in a major hurricane with a monster flag still waving)?

Finally, I'm not understanding the socket outer diameter. You say it is a socket (which I'm interpreting to be a hole) in the flat plate that accepts the pole. This plate is infinitely thick and has some finite boundaries. Where does an outer diameter of the socket come in?

David

 

[Pud]

David,

Sorry for lack of info!

Yes it's kNm!

Socket is a effectively a tube on a base - see attached pic. The product is to be clamped to the ground by a vehicle tyre resting on the two legs. The flag is stuck in the socket.

The bending moment is the flag itself - 60mm dia x 6m high + flag. Glass fibre tube. e.g. These:

http://www.flagpoles.co.uk/flagpole/telescopic-flagpoles-2/5353

cheers

Harry





www.tynevalleyplastics.co.uk

 

[patprimmer]

I thought it should be tensile at yield rather than break as once it yields it is ruined.

Regards
Pat
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[zdas04]

That makes it a lot clearer (I just love it when someone in an SI country says "If the pole if 17m tall, the biggest flag allowed is 5 yd", I'm assuming they mean 5 sq yards, or 4.18 m^2, but that is not germane).

My statics was a while ago, but the force at the top of the pole and the force at the bottom of the sleeve go in one direction and the force at the top of the sleeve goes in the other direction (equal to the sum of the other two). Also the moment (F*L=2.18 kNm) has to be offset by the moment at the bottom of the sleeve. All that together seems to give you a maximum force of 10.6 kN at the top of the sleeve that has to be contained by the sleeve.

My strength of materials classes were too long ago to convert a point force and a Young's Modulus into a wall thickness.

David

 

[IRstuff]

Do you know why it needs to be redesigned?

TTFN

FAQ731-376

 

[Pud]

David: I still convert small mm dimensions to "thous" to visualise it (using the fact that a head hair is around .003" dia. AND I still work out the price of a pint of beer to £.s.d occasionally. (Beer used to be 1s/8d per pint - it's now around £2.50 in new money...£2.10s!!)

Pat: No stress at yield given. Also this stuff (Grivory GV5H) has only 2.5% elongation before break.

IRstuff: The above picture is not a "design". It's a proposal, with wall sections guessed in 30% GF nylon. The (ex) moulder who had the job actually only moulds polythene buckets and paint trays. 30% GF nylon is an "exotic" to them!!

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[Compositepro]

One of my products is carbon fiber tubing used for similar applications. Your fiberglass pole is likely a pultruded fiberglass. These are relatively inexpensive and have some good properties, but the also have some real weaknesses. The high tensile strength means little. Cantelevering a flexible pole in a stiff socket causes large stress concentrations. The pole will fail in axial compression buckling of the wall where it exits the socket due to the contact pressure crushing the tube wall. Another common failure mode is axial splitting. When a round tube is flexed it becomes oval in cross-section. This creates a hoop stress in the tube. Pultruded poles have low hoop strength because there is little reinforcement in the hoop direction.

There is no easy answer to your question. You can probably double the amount the pole will flex before failing by optimizing the design of the socket to have a trumpet shape at the top to act as a stress relief.

I once spoke to a representative of a Chinese pultruder at a trade show about improving the toughness of their product. His response was why would you want to do that, because a broken pole means the consumer must buy more product.

 

[Pud]

compositepro: these are not pultruded. They are either hand layed up over mandrels (tapered) or filament wound - or aluminium extrusions. (We looked at pultruded tubing when we used to make kayak paddles - rubbish for that application)

The company is the UK's No.1 flagpole outfit.

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[GregLocock]

M/I=sigma/y



Cheers

Greg Locock


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