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Background During the development of a new stair-lift, SA was invited to contribute to a comprehensive review of cost and safety in partnership with the Bison in-house team of design engineers. Several cost-saving opportunities were identified. This was one of them.

The Final Limit switch is a key safety feature of any stair-lift. It switches off the drive motor if the lift travels beyond its normal park position. The switch might not be triggered for many years (if ever), but always has to be ready to prevent the damage that would result if the lift hit its dead-stop buffers with its motor running.

Challenge The Final Limit switch assembly shown to SA comprised several metal pressings, two switches, four springs, sliding rods, spacers, pivot pins and a lot of small screws and washers. The intricate assembly had 25 parts in all.

SA was able to show how the complexity of the assembly not only added cost, but also was likely to contribute to its unreliable operation. A safety improvement requirement was therefore added to the brief.

Solution SA created a new design that took maximum advantage of the properties of plastic mouldings. The springs and all the fastenings were eliminated and replaced by flexible beams and snap-fits.

All the flexible features were stress-analysed to ensure they never ran the risk of failing to work after several years in service. Another innovative move saw one of the switches, and all its associated connectors and wiring, entirely eliminated by designing a two-way cam-actuator and then negotiating a re-think of the control circuits with the electronics engineering team.

Deliverables Our analysis and re-design replaced a 25-part assembly with a 4-part assembly. These are impossible to miss-assemble and nothing can work loose in service. So effective is the design and so easy is it to assemble that the five minutes needed for the original has been cut down to a mere 20 seconds!

In addition, the wiring loom was simplified, giving further cost and time savings and more reliability.

Results This safety critical assembly has given 6 years of fault-free service in many hundreds of Bison Stair-lifts.