Secure holds have sure release

Problem: Fire doors need to shut when needed and smoke vents need to open when needed. One of the accepted methods employed in both cases is the use of electromagnets. These may be used to hold doors open and, when switched off, allow them to close; conversely they perform a similar task with smoke vents, normally holding them closed but releasing them for opening. Potential problems include the ways in which door-hold electromagnets can be easily damaged in vandal-prone locations, and with vent-holding devices which often have to be fitted into confined spaces. Solution: Stephenson Gobin, best known to Eureka readers for its electromagnetic brakes and clutches, has developed (and continues to develop) electromagnetic latches for special purposes, including floor-mounted electromagnetic fire door retainers encased in thick aluminium castings with wiring that is only accessible by detaching the cast units from the floor. The company's 'Slamlock' electromagnetic window latch engages a pin on the window or vent in a slot in the side of a rotating disk which links to a flat plate through a crank; the plate being secured by the electromagnet. Through mechanical advantage, this arrangement doubles the effective holding force and allows the whole mechanism to be fitted into a thinner space. Plans are afoot to link both types of device to alarm systems through wireless links to reduce wiring costs. Applications: The cast aluminium floor mount units are finding uses in locations as hostile as schools and prisons and have holding forces of 200 or 500N. The 'Slamlock', on the other hand, has a holding force of 1,000N. Both devices can be de-activated by manual switches. The crank mechanism employed could have further uses in other kinds of latches and fastenings, as well as those based on magnetics. TS