Many of the blue 5-castor office chairs that we received in 2001 have a design flaw. The plastic back is attached via a far too small metal bracket. Modest leaning back causes the plastic back to break horizontally, right above the metal bracket, often within a year of normal usage:
A first symptom of this failure is that the back cover starts to fall off repeatedly.
We tested a simple modification of such a chair. It can prevent this failure on still intact chairs, and it can equally be used to remedy the failure in already broken chairs:
The above image shows our repair technique. It involves the addition of two 30 × 30 × 200 mm angled steel bars, into which we drilled two 7 mm holes 30 mm apart (centered 40 mm and 70 mm from the bottom and 9 mm from the inner edge). Drawing: PDF, qcad (DXF)
Most chairs come with M6×20 screws, which are a bit short after we insert our 3.5 mm thick metal support, so we replaced them with M6×25 screws. The complete repair kit was produced using a pack of screws and a metal bar, both bought at Mackays of Cambridge, which we cut, filed, and drilled on the Lab’s pillar drill in GW09:
There is enough space left between the back and its cover to install two 30 × 30 × 200 mm support bars. (If we search a bit longer for suitable material, we might come up with a slightly flatter and lighter structure.)
We’ve used up the material we bought to repair several chairs in the Security Group. Given sufficient interest, we might be able to buy more and perform this procedure also for other research groups, but we would have to charge £15 per chair (£5 materials, plus £10 research assistant time for preparation and installation). In comparison, a new chair costs £60.
Markus Kuhn and Steven Murdoch
created 2005-10-24 − last modified 2005-12-01 − http://www.cl.cam.ac.uk/~mgk25/chairs/