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Aluminium tube


sandall

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No direct connection with rigging, but a cautionary tale....

 

Some of my extended family are into competitive scurry racing. A scurry is a 2-person 2-wheeled cart, pulled by 2 ponies, either side of what is basically an aluminium scaff pole. A few weeks ago at a big competition one of these poles just snapped. Fortunately this was just after a race, & the ponies were walking, so nobody died or was seriously hurt, but it was a big wake-up call. In the aftermath they discovered that aluminium is apparently known to have a (limited) working life & aluminium parts used in aircraft are routinely replaced after 5 years (as an aside - as a child I had a couple of small cast-aluminium B17 bombers. When I found them again many years later, one of them had broken into 2 pieces; the other was, & still is, fine).

 

Does this have an implication for all these lengths of aluminium tube we routinely hang things on (or built sets with)? Even a load-test only indicates that it didn't break during the test :huh:.

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The issue with aluminium is fatigue- multiple applicationsof load, (which can be a deliberately applied, or form vibration) at below thelimits that the ‘elastic limit’, can change the properties of the materialsignificantly, to the point at which cracks grow and the part fails. This is analogousto flexing a paperclip a number of times to break it.

 

 

The example you give is one where the part will be flexingmany tens of thousands of times over it’s life. You are indeed correct thatthis is a major consideration in the design and maintenance of aircraft.

 

 

In this industry, most items are used with static loads, andso it is unlikely that fatigue will be an issue, but there will be cases whereit should be considered.

 

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Superpants is right, bend an aluminium can a few times and you can tear it in half. Most metals work harden as they are put under repetitive forces and so become brittle. Ali is also prone to hairline surface cracks as it flexes and bends.

 

I think that thick walled ali tube is harder to source and what I used in my youth just isn't available nowadays. I may be wrong but I would think that this application makes regular inspection, if not replacement, necessary.

 

Rostra legs are in compression and towers are trussing so the forces are different. We helped design a stage trussing with thin walled tube I could bend over my knee that, once welded up into quad, could take an impressive load.

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Rostra legs are in compression and towers are trussing so the forces are different

True, but I often see ali legs used underneath Steeldeck for dance shows, where (however well the stage is braced) the lateral & percussive forces on the legs are considerable. The individual sections of hire-shop-type scaff towers can also take quite a pounding.

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For aluminium alloys there is a proportionality between the stress applied and the number of cycles before the material fails. If a product is designed correctly (as the rostra legs and scaff tower examples should be), then the number of cycles of stress will be up in the many millions. This is dependent on many factors including the presence of stress raisers such as holes, bolts or rivets, different grades of aluminium and heat treatment. Items such as aircraft wings, suspension struts and lifting equipment may well be expected to see many millions of stress cycles and hence the design of the product in these areas becomes dominated by the fatigue behaviour, rather than the outright strength of the materials.

 

Tom is correct in pointing out that this is why we see steel used in some applications. Steel has the property that below a threshold stress level, fatigue does not occur. It is this reason that railway wheels and bogies for example make use of steel as it won’t be affected by fatigue at all if designed correctly.

 

 

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Not all fractures are caused by fatigue, only a metalurgist with a microscope can tell by looking. What was the history of the tube before it became part of the cart?

I would imagine it was new (thick-walled tube) when the scurry was made. No idea how old that particular one is, but the races are quite short, so I doubt if it sees anything like an hour of intense activity per month.

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I lied about the number of wheels (it's 4 if the link doesn't work), but this * gives an idea of where the pole sits. The ponies pull the cart directly, & the pole is attached to it with a swivel, so its only purpose seems to be to stop the ponies tripping each other up, meaning the forces on the pole itself may be far less than I thought. This cart was built by a pukka engineering firm & uses thick-walled scaff tube; some other competitors use (possibly thin steel) square-section tube. The sport itself is a bit like the Ben Hur chariot race in a show-jumping arena.

 

(* Despite several attempts I'm not at all convinced that the postimage link works :angry:, & I can't remember how to just drop in the image, so you may have to just imagine a small 4-wheeled metal cart, driver in front, passenger / counterweight behind, 2 ponies harnessed to front of cart with a pole to keep them apart attached to front of cart with a swivel)

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Some good pics on the Hartland siteHERE.

 

I think that these shafts are to stop the carriage overrunning the ponies and were mostly timber and ply constructions. In the pictures they don't appear on the scurries so perhaps shafts are a separate item or part of the harness?

Much smaller than the ones on the Hartland site - see https://osbornerefri.../galleries.html , but I think you are right about it also being there to prevent over-running.Having found a close-up I think the swivel may actually be a clamp, so presumably there is a lot of stress at that point, but that's not where it was described as snapping.

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