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Circle or Square?

July 29, 2013 at 3:49 PM

Which is the Ultimate Shape of a Cable?


Circle or square; which is the ultimate shape of a cable? The shape of a down hole cable has serious implications on the success or failure of a down hole system. While both have their own advantages and disadvantages there is only one ultimate cable shape. If we apply a compressional load on a piece of wire with a square cross section, the wire will deform in the middle of the face or will deform into a parallelogram. This is because the majority of the loading is directed through the face and not out to the edges. Apply the same load to a wire with a circular cross section and you get a very different result; the wire will deform slightly at the middle but the majority of the force is distributed around the cable. This is because when a load is applied to a circle the majority of the force travels down through the arc of a circle.

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In terms of pure axial loading, a square is 30% stronger in tension. However due to less material being required when producing a circular cable of equal width; the circular wire costs less and is 30% lighter.

If the cable becomes twisted a circle is the best shape to resist this. This is because stress concentrations occur at the corner of the square wire, which results in the wire failing quicker. A circular wire bends more easily than a square wire. One huge problem with square wire is that depending on the orientation of the wire to the bending motion: the wire will become harder to bend. This can cause problems for storage and deployment. For storage being less able to bend means bigger spools, which means less space to store wire. For deployment being less able to bend means a greater difficulty getting the wire through bends and down hole. It should also be noted that a square wire is far more likely to form kinks than a circle because of the sharp corners on a square.

Tags: downhole instrumentation wire cable engineering
Category: Business Philosophy

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