Lightening holes

Lightening holes in Bulleid Firth Brown (BFB) railway locomotive wheel

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.[1] The holes can be circular, ovals, or rectangles with 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.[2]

Motor racing

1935 car chassis, with lightening holes

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.
— David Kelsey, Lotus Cars[3]

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.

In comparison to aircraft design, it was noticeable that motor racing 'lightening' was carried out after the design of the components, usually without understanding the forces in the structure and without re-performing any stress calculations afterwards. Owing to the increasing number of dangerous failures, the practice of drilling lightening holes was forbidden by the race scrutineers. Lightening was still permitted, but only where it had been specified by the original designers, not by simply drilling holes afterwards.

See also

References

  1. ↑ Fournier, Ron; Fournier, Sue (1989). Sheet Metal Handbook. HPBooks. p. 25. Retrieved 2012-06-08.
  2. ↑ Gerr, Dave (2000). The Elements of Boat Strength: For Builders, Designers, and Owners. International Marine. p. 322. Retrieved 2012-06-08.
  3. ↑ Ludvigsen, Karl (2010). Colin Chapman: Inside the Innovator. Haynes Publishing. p. 150. ISBN 1-84425-413-5.
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