Product category:
Safety relays, interlocks, control systems, electrical components
News Release from: Pilz Automation Technology | Subject: PSEN switches
Edited by the Manufacturingtalk Editorial
Team on 16 January 2004
All-electronic safety relay proved more
reliable
Historically Kaby Engineers had experienced problems with switch failures on induction heater guard gating until changing to a system based on an all-electronic safety relay.
Historically Kaby Engineers had experienced problems with switch failures on induction heater guard gating until changing to a system based on an all-electronic safety relay that has proven to be much more reliable expensive
This article was originally published on Manufacturingtalk on 28 Jul 2008 at 8.00am (UK)
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Moreover, the PSEN switch came with a plug-in lead Kaby Engineers is one of only a small number of sub-contract precision engineering companies with the capability to produce large volumes of hardened and ground shafts in a wide range of diameters.
The company's success means that the factory in Leicester operates 24 hours per day, seven days per week.
One of the items of equipment that is fully utilised is an induction hardening machine that automatically feeds, hardens and quenches cylindrical components that are up to approximately 160mm in diameter.
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Hazards presented by this machine include heat generated on the surface of the components (in the region of 800 degrees C), the movement of high-powered rams that manipulate the components, pinch points where the components rotate on rollers and, perhaps the greatest hazard, live electrical components.
Fixed guards surround most of the machinery, but there are two hinged guards; one allows batches of components to be loaded and the other gives access for changing the induction coil (to suit workpieces of different diameters) and for removing parts for inspection during set-up.
This guard will typically be opened ten times or more in any 24-hour period.
The need for high-integrity guard switches is clear, but historically Kaby Engineers has experienced problems with switch failures, although the switches have always failed to a safe state.
Given the exceptionally harsh operating conditions, the high failure rate is easily understood.
For various reasons it is impractical to locate the switch on the second hinged guard anywhere other than towards the top of the guard, which means that it is approximately 250mm from the induction coil and almost directly above it.
As such, it is subjected to heat radiated from the workpiece and steam from the quench medium.
If the quench were pure water, the clean steam would be problematic enough, but the quench medium contains a polymer additive that is highly aggressive; many plastics - and sealing materials - are attacked by this steam, leading to degradation and cracking.
Furthermore, the extremely high magnetic flux emanating from the induction coil has been known to cause failures in electrical and electronic components located up to 10m away.
Lastly, the frequency with which the guards are opened has seen traditional tongue-type mechanically actuated gate switches fail regularly, even when steel-headed and floating-head varieties have been fitted.
If a gate switch fails, either totally or by momentarily producing an electrical glitch that is detected by the associated safety relay, the induction hardening process and all movements are immediately halted.
It is usually not worth annealing and re-hardening the part-hardened component, so this tends to be scrapped.
However, the biggest potential problem relates to damage to the 3kHz generator that drives the induction hardening coil.
If the generator is shut down suddenly by the 1000 A breaker tripping, rather than in a controlled manner, it can lead to a failure of the power supply that costs around GBP 5000 to replace or repair.
And the downtime would be at least three days, which is unacceptable for a machine that is operating non-stop.
In order to avoid such direct and indirect costs, Kaby Engineers sought to replace the gate switches on the two hinged guards.
Tongue-type switches were failing too frequently due to physical wear and tear, so non-contact magnetic switches were tried.
These failed due to the plastic housings being attacked by steam from the quench medium, so Controls and Drives (CDL), the Leicester-based distributor, was invited to source an alternative.
CDL offered Kaby Engineers two types of gate switch that it believed would do the job.
Both were similarly priced when taking into consideration the cost of the associated safety relay, but the compact Pilz PSEN 2.1p11 switch with the PNOZelog 3.1p all-electronic safety relay was marginally less expensive.
Moreover, the PSEN switch came with a plug-in lead that meant changing the switch would be significantly quicker, should it ever fail.
Bearing in mind the true cost of downtime and Pilz's reputation for high reliability, Kaby Engineers chose the Pilz products.
After three months of trouble-free operation, Kaby Engineers is convinced that the Pilz switches are in a league of their own.
Lee Parsons, Development Engineer and the person responsible for making continuous improvements to the manufacturing process efficiency, says that installing the Pilz devices has proved highly beneficial: "It would have been obvious straight away if there were going to be problems, but the switches have operated perfectly; nothing else has ever coped so well with the heat, steam and hard daily use.
Being realistic, however, we have to be prepared to change them, but they will be easier to replace than other switches.
First, they can only be installed one way round and, second, the plug-in lead means the switch can be changed in minutes, without having to rewire anything.
It could even be possible for a trained, competent fitter to change them, rather than a maintenance engineer - which is important because we cannot guarantee to have on-site maintenance cover throughout the night shifts and over weekends." Although the Pilz data sheet recommends that the switches should be mounted on a non-ferrous surface, the devices installed at Kaby Engineers are installed on steel and are protected by steel covers.
While this reduces the operating range for the switches, it in no way adversely affects the safety-related performance characteristics.
While many manufacturers of switchgear use lower-cost plastics for switch housings, Pilz manufactures the IP67-sealed PSEN switches using PBT (polybutylene terephthalate), a polymer that has low water absorption, is resistant to temperature extremes, and has extremely stable electrical properties.
As a result of these factors, the components inside the housings are well protected from the harsh environment of the induction hardening machine.
As far as immunity to interference from the induction coil is concerned, the Pilz switches are manufactured to conform with the requirements of the Electromagnetic Compatibility Directive (89/336/EEC, as amended by 92/31/EEC and 93/68/EEC), the Low Voltage Directive (73/23/EEC, as amended by 93/68/EEC) and the Machinery Directive (98/37/EEC).
In addition, Pilz has the in-house capability to conduct tests to check performance and compatibility for unusual applications.
Lee Parsons concludes: "Following the successful installation on the first induction hardening machine, there are other hardening and welding machines where we are considering upgrading the gate switches to the Pilz devices - and these switches are so compact that they are easy to retrofit to existing machinery.
So long as the price is reasonable, which it is in this case, we are prepared to invest in equipment that will give us improved machine availability in the long run." For more information about the PSEN switches and electronic safety relays, look at the 'Products' section of the Pilz Support web site.
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