Exploding oxygen cylinders on Qantas: what’s it got to do with tyres?
Qantas have found out the hard way that when oxygen cylinders operate at very high pressures and ‘fail”, they can do an extreme amount of damage.
From the evidence released so far, it seems that the valve screwed into the top of the bottle ‘let go explosively”, coming through the floor and hitting a door handle, luckily not a passenger. The bottle then took off like a rocket would under the same circumstances, straight through the side of the plane. This failure can be due to the thread being over-tightened and stretched as the valve is inserted. The same thing can happen to wheel nuts over-tightened on their studs, the over-stretched thread fatigues, and the wheel comes off because the wheel nuts were too tight!
So what’s this got to do with tyres? Well, I once heard a ‘supersingle”, a 15R22.5 Tubeless tyre with 105 p.s.i. of air in it, on the FRONT wheel of a truck, blowout (fail catastrophically was the term used) from 5 kilometres away on the Murray Valley Highway. That gives you an idea of the force involved when a tyre blows out.
Any pressure vessel is designed and tested to a much higher presuure than it’s operating pressure – the “factor of safety”. . For example, the air compressor operating in the corner of the workshop has a safe operating pressure stamped on it, and the “Test Pressure” as well, which will be several times higher than operating pressure.
So what do you reckon the “Factor of Safety ‘of a tyre is? It doesn’t just sit there like the oxygen cylinder. It operates in a dynamic environment, having to cope with all kinds of abuses such as heat, potholes, broken road edges, and the like.
The answer is experience. Each tyre is designed to do a particular job. For example, an off-road tyre would have a different factor of safety to a speedway or race car tyre, which hopefully will never hit a pothole at speed. An off road truck tyre should have a stronger casing than a highway design, if service experience dictates it. This is achieved by either using more cords in the casing, or stronger cords. In other words, the design is different.
An interesting twist on all this is that it is the rim that fails before the tyre casing bursts when due entirely to over-inflation. There is so much side force on the rim flanges that they buckle and fail. To check their computer calculations of tyre casing strength, tyre engineers regularly blow up their tyres on very very strong rims in a safety cage. But they do it with water! They might get wet, but they don’t get hurt.