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10/12/2008
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One man whose vision helped in the building of many M25 bridges has turned his sights upwards again. Tom Shelley reports
According to NHS Direct, DIY ladder and stepladder accidents currently send 41,000 people to hospital each year in the UK, while there are about 1,000 further accidents involving professionals. There are approximately 50 deaths and 4,000 serious injuries. At the same time, the total number of domestic and industrial accidents involving ladders across Europe is believed to be around half a million. Making users safer is therefore a real imperative
Enter Lance Sheffield, a construction manager who has come up with a number of ingenious means and mechanisms to improve the safety and stability of ladders. Among these is a ratchet mechanism that greatly increases the load-carrying capacity of such devices. Ultimately, it may have applications elsewhere.
Sheffield, of Otford in Kent, is no stranger to such challenges. He played a major part in the construction of many of the M25’s bridges, and has had a lifetime of dealing with construction site health and safety issues, including the avoidance of ladder accidents.
First among his ladder safety developments are adjustable feet that can be dropped down in response to simple pressure from a user’s boot. This is applicable to both conventional long ladders and stepladders, although the prototype he showed Eureka had a thumb-operated cable release. This is a must on working sites, where the ground is rarely firm and even. “The construction industry can’t do anything level,” says Sheffield.
The ratchet pawl that locks each foot in place engages multiple teeth in a rack simultaneously – the one we saw engaged nine teeth at the same time - which would allow it to be made out of plastic. But what makes the design really ingenious is the idea of having a load-carrying back stop to take full force.
If the load were entirely borne by the pin through the pivot, as is usual in such devices, this would quickly result in the formation of elongated holes in the sides of the rectangular tubular extrusion used for the ladder legs. Hence a thrust plate, made of aluminium or plastic, against which the load presses the pawl, takes most of the load. .
Adding two diagonal straps between the legs, in addition to the usual parallel stays, has made further enhancements to the stepladder. Indeed, the straps vastly improve the stability of the structure by preventing parallelogram distortion, which stays do little to reduce.
The stability of conventional vertical ladders has been much accelerated by having swing-around legs. For transport, these fold flat against the length of the ladder. When deployed, they can be swung out on hinges, so their feet rest against the ground behind and to each side of the main ladder, retained by straps. These legs perform several functions. First, sideways stability is taken to new heights, literally, while the possibility of the base of the ladder slipping sideways is all but eliminated. Secondly, the ladder can be pulled out to a much shallower angle than the usual 1 in 4, or 76 degrees, recommended for conventional ladders, without the base slipping outwards. As well as making them safer to climb, the shallower angle makes carrying loads up the ladder much easier.
All of Sheffiled’s innovations are protected by patent and available for licence through his company Kick Kwik Enterprises.
Pointers
* The ladder legs are extendible and locked in place by a ratchet pawl and rack in which multiple teeth are engaged at once, and a thrust plate takes most of the load of the pawl pin
* Step ladders are further stabilised by the addition by the addition of cross bracing straps that prevent parallelogram distortion of the frame
* Conventional long ladders are further stabilised by swing round legs, on hinges, that give load spreading and four point stability and allow the ladders to be safely used at shallower inclinations
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Author
Tom Shelley
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