Non-contact safety switches survive in aggressive environment

Manufacturing businesses clearly have to give machinery safety a high priority, but without losing sight of the fact that the company exists to make profits. Machinery safety components - from relays and emergency stop switches to light curtains and gate switches - must therefore be specified with care, as products with a lower initial purchase cost may not prove to be the most cost-effective in the long run.

For example, if a magnetic gate switch fails, it should 'fail safe' so that its final act is to fulfil its safety function and prevent the machine from operating. Until the switch can be replaced, however, the machine will be idle, which could be very expensive in terms of lost production. For some applications, there could be other implications in the event of a safety switch failing to a safe condition - such as damage to electrical equipment as a result of a circuit breaker tripping.

Because of this relationship between safety, reliability and availability - and, ultimately, profitability - many specifiers are moving away from tongue-operated mechanical gate switches to non-contact magnetic switches for guards that have to be opened frequently, as non-contact switches have no mechanical parts that can wear. And for machinery with greater hazards, coded non-contact switches (typically using the same technology as RFID devices) are virtually impossible to defeat.

Look beneath the surface

Nonetheless, simply specifying a non-contact switch does not guarantee long-term reliability, especially when the switch is located in a harsh environment. When Pilz launched its PSENmag non-contact safety switches a few years ago, the intention was that they would, indeed, be robust, long-lasting and suitable for use in aggressive industrial environments. One application in the UK that demonstrates the switches' reliability is at Kaby Engineers Ltd in Leicester, a sub-contract precision engineering company with the capability to produce large volumes of hardened and ground shafts in a wide range of diameters.

Kaby specified Pilz PSEN 2.1p11 switches to replace tongue-operated switches and non-magnetic switches from alternative suppliers, none of which had survived the frequent operation and harsh environment on an induction hardening machine. After three months the engineers at Kaby were satisfied that the Pilz switches were more reliable than anything they had tried before. Now, some three years later, they still have no doubts.

To give an indication of what the switches are achieving, the induction hardening machine on which they are installed runs 24 hours per day, seven days per week, and the guards are typically opened ten times per day. The switches are mounted approximately 250mm from the induction coil and are subjected to magnetic flux and heat radiated form the workpiece, as well as steam from the quench (the quench medium contains a highly aggressive polymer additive that attacks many plastics).

No in-service failures

The switches were recommended by a distributor, Controls and Drives Ltd (CDL), whose Lee Clarke comments: "In all this time, the switches have only been changed once - and that was just as a precaution when the machine was down for routine maintenance. As the switches use plugged connections rather than flying leads, it really was very quick and easy to swap the switches without disturbing the wiring."

CDL has supplied switches from the Pilz PSENmag range to a number of other customers and has had no negative feedback whatsoever. Lee Clarke says: "These switches are incredibly tough and reliable. They are also compact, so they can be easily installed on bespoke machines or used to replace other switches that are suffering reliability problems. I have not heard of any Pilz magnetic switches failing and, given the extreme conditions on the Kaby machine, I would have thought that if the switches can survive on there, they will survive anywhere."