Feature Machine safety Harmonising productivity with machine safety
Safety gates, protective covers and machine guards prevent hazards associated with moving parts. However, the correct functioning of a safeguard can only be guaranteed by using appropriate sensor technology. Alex Bryce, Pilz Automation Technology sales manager, considers the sensor systems available and the criteria designers should consider
ensor systems operate using different principles so it is impor- tant designers fully understand the implications of using each type. The challenge is to try to har- monise productivity with machine safety and user-friendliness. If a safe- guard is not user-friendly, there may be a temptation to manipulate a safety guard, putting the machine operator at risk. Designers need to identify cases in which safety devices might be manipulated and eliminate these at the machine development stage. Certain considerations will help designers decide which components should be used for the safe monitoring of movable guards in accordance with EN 60947-5-3. Typical questions to ask include: is the cover frequently opened and closed? Does the process need pro- tecting as well as the operator? Are components exposed to high vibration or extreme temperatures, or are they used in potentially explosive areas?
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reduced considerably. Mechanical safety bolts are complete solutions comprising a safety switch, handle and bolt. These provide protection where safety gates are difficult to adjust, as well as on safety gates that are frequently opened and closed. A mechanical guide on the actuator ensures the actuator engages with the safety switch correctly when the safe- guard is closed.
Mechanical safety switches For simple safety gate applications where there is no increased risk of manipulation, mechanical safety gate switches are widely used. These are used for simple safety gate monitoring or with a mechanical guard locking device, which uses increased extrac- tion force on the actuator to prevent the gate or cover from being opened unintentionally. Many applications require two mechanical switches to guarantee the required level of safety, which increases engineering costs. Another consideration is that swarf can be deposited on the sensor, restricting its function. Also, if safety gates drop over the course of time, the lifetime of a mechanical switch is
Mechanical hinge switches provide an elegant solution for rotating or hinged gates and flaps. These provide better protection against manipulation compared to other mechanical solu- tions because they are concealed within the hinge, which also saves on installation space.
Non-contact safety switches Gates, covers and flaps often need to be opened repeatedly for operational or assembly purposes, to enable operators to access or reach into the protected area. Here, the risk of a gate being mechanically misaligned is high. The disadvantage of mechanical guard locking devices is they only have limited tolerance for misaligned gates. However non-contact safety
It is important to choose the right sensor technology when installing machine safeguards
switches have a higher response toler- ance, allowing greater flexibility in the way they are attached to the machine. Rugged non-contact switches are also suitable for applications that require protection types higher than IP67. These switches are able to meet safety requirements up to PLe of EN ISO 13849-1, and SIL 3 of EN/IEC 62061, or Category 4 of EN 954-1. When it comes to deciding on the acting principle of the sensor, impor- tant distinctions need to be made. Whilst magnetic safety switches pro- vide only limited protection against manipulation due to the technology used, coded safety switches achieve up to 100 per cent protection against manipulation. As a result, magnetic safety switches should be concealed. If a safety switch cannot be con- cealed or if 100 per cent protection is required, RFID coded safety switches can be used. These operate in accor- dance with the ‘key lock principle’ whereby only one key, a unique ver- sion, fits a lock. Given that the actuator is installed using one-way screws, which can only be loosened via a spe- cial tool, a very high level of protec- tion against manipulation is achieved. Coded switches rather than mag- netic switches are suitable if metal swarf is present as RFID is unaffected by metal swarf. Various coding types mean that dif- ferent levels of security against manip- ulation can be achieved. The greatest protection is provided by systems in which an actuator is uniquely assigned to a switch. Sensors with ‘normal’ coding are also available if the demands on manipulation protec- tion are less stringent.
Pilz Automation Technology T: 01536 462202
www.pilz.co.uk Enter 326
he latest edition of ‘A practical guide to machinery safety’, sponsored by Laidler Associates, part of the machinery division of TÜV SÜD Product Service, is now available for free download from the Laidler website. The easy to read guide has recently been updated to cover the latest information on legislation including the EMC and
Machinery safety guide available to download T
Low Voltage Directives and the most recent changes to the Machinery Directive. It also covers risk assessment and hazard analysis, and step-by-step advice on CE marking and the Provision and Use of Work Equipment Regulations (PUWER). Free to download and available to request as a hard copy at
www.laidler.co.uk ‘A practical guide to machinery safety’
provides companies with suggestions and guidance for the best route to compliance. Laidler Associates
T: 0333 123 7777 30
www.laidler.co.uk Enter 327
Gate closed in case of danger Risks can remain even after a machine has been shut down, due to moving parts. Here the use of a safe guard lock- ing device is mandatory. This means the gate cannot be re-opened until standstill has been safely detected or a time delay has elapsed.
Integrated systems
More can be achieved by using inte- grated safety gate systems that combine safe monitoring, safeguard locking and control elements, including additional functions such as E-STOP and escape release. All components, whether mechanical or electronic, are inte- grated. Complete systems such as these help to reduce the time and costs asso- ciated with project configuration, design, documentation, purchasing and installation.
NOVEMBER/DECEMBER 2011 Factory Equipment
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