Lightening hole

Lightening holes are holes in structural components of machines and buildings used by a variety of engineering disciplines to make structures lighter. The edges of the hole may be flanged to increase the rigidity and strength of the component. The holes can be circular, triangular, ovals, or rectangles and should have rounded edges, but they should never have sharp corners, to avoid the risk of stress risers, and they must not be too close to the edge of a structural component.

Drilling out components was a popular way to lighten racing bicycle components in the 1970s. Eddy Merckx's hour record-winning bike has heavily-drilled handlebars and chainring. Cyclists jokingly refer to the incorporation of "drillium" in the machine, as if it were an exotic metal which confers lightness.

Lightening holes are often used in the aviation industry. This allows the aircraft to be lightweight as possible retaining the durability and airworthiness of the aircraft structure.

Lightening holes have also been used in marine engineering to increase seaworthiness of the vessel.

Lightening holes became a prominent feature of motor racing in the 1920s and 1930s. Chassis members, suspension components, engine housings and even connecting rods were drilled with a range of holes, of sizes almost as large as the component.

[The] wisdom of the day was to make everything along the lines of a brick shithouse [...] and then drill holes in the bits to lighten them.

This drive towards lightening was based on a misunderstanding of the component's mechanical behaviour. The assumption for a H girder was that all of the resistance to bending stresses was carried in the two top and bottom flanges of the girder, with the central web only carrying out a spacing function. The central web could thus be drilled indiscriminately, supposedly without weakening the overall girder. This was based on two fallacies: firstly that the only forces on the beam were simple bending forces in the plane of the web. In practice, a more complicated force, such as an unexpected torsional twisting from a sudden suspension bump overloaded the now-weakened central web and the lightened beam failed immediately. Secondly, the assumption that the ideal forces were separated into the top and bottom flanges was increasingly unrealistic with the development of stressed skin and monocoque designs, where loads were more evenly shared. In these designs there was no "unloaded web" that could be safely drilled.

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