Softening the drop

If somebody is working on a high building these days, they will be equipped with a safety harness and a lanyard attached to a steel cable attached to the roof. However, if the user falls, they are going to apply a massive shock load to this cable. If the cable is very rigidly attached to the roof, the user will suffer a massive shock when the lanyard is fully extended, while the cable mountings will additionally apply a massive shock loading to the roof. This can be mitigated if the mountings are designed to absorb the shock. Shock absorbers in compression are commonplace, but shock absorbers in tension are less so. Mountain climbers use ropes that stretch under shock loads, but safety lanyards do not do this. If the cable attachments are very firmly attached to a roof, the shock is liable to be absorbed either by some of the mountings pulling out, or the roof distorting. This, as well as being unsafe, is likely to be expensive to repair. The Challenge What is needed, then, is a linear shock absorber that is able to absorb the force exerted by, say, up to three people falling, without either hurting the falling workers or the roof. It thus needs to bring them to a stop in a reasonable distance, and exert no more than 10kN on either people or roof. If the maximum load is 300kg, this requires that the falling person or persons be brought to a halt in a few hundred mm. The other requirements are that the device should be compact, inexpensive, and being a piece of safety equipment, totally reliable, even if left without maintenance for years at a time. These requirements rule out pieces of machinery with centrifugal clutches and rubbing surfaces. Such devices are inevitably going to be relatively complex and expensive, but most importantly, are unlikely to work when left unlubricated for a period of years. Something that involves something being crushed would work, but if it is going to function over a few hundred mm, is going to be at least that size if not a little larger. Springs are possible, but they are going to have to be big. Anchoring workers to cables with bungee is also possible, but bungee and climbing ropes require care in storage, and constant inspection to ensure they are still good. There is however, an excellent alternative, based on sound mechanical engineering principles, developed and manufactured by a British company that fulfils all requirements. It has been thoroughly tested, and is in service on a number of prestigious buildings. Once you see it, you may consider it obvious. See if you can come up with anything better. The solution will be fully described in the next edition of 'Eureka'. Solution to last month's Coffee Time Challenge The Portable Train Weigher or PTW is one of a family of products developed by Sheffield company Weighwell, which started in 1988 by servicing and maintaining weighing equipment of all types, ranging form small laboratory scales to large weighbridges. It consists of load cells which are attached to the insides of the rails, in such a way that as long as a train passes reasonably slowly, the weight of each wheel is briefly taken by the flange as the flange passes up a small ramp, onto the section with the load cell and down a second ramp. The load cells are held in place by a pair of expanding rods, placed between the rails. The extension is accomplished by rotating large nuts which ensure the extension of screw sections. When the load cells are in place, the screws are locked into position using lock nuts. Development was aided by DTI 'Smart' innovation awards in 1995 and 1997. The device is approved by Railtrack and widely used in the UK, Continental Europe, the Far East, Australia and the USA.