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Safety in pneumatics

John Hill, Marketing Services Manager for Parker Hannifin's Pneumatics Division, explores safety issues within the industry.

While engineers might debate the relative merits of each new development in pneumatic technology, few would contest the fact that any innovation designed to enhance safety is to be welcomed. There exist relatively few pneumatic component safety critical standards and correspondingly there are only a small number of safety products or components available. However, with the advent of the new Machinery Directive 2006/42/EC, now is an appropriate time to re-evaluate pneumatic safety, reacquaint ourselves with key safety features and look at what the future holds.

Some pneumatic safety products have become familiar in the workplace, such as the two-handed start, which prevents an operator from activating a pneumatic device unless both hands depress interconnected switches within a fraction of a second, thus ensuring that one of those hands is not within the machinery concerned. Air fuses have also provided a valuable level of defence in preventing the danger of whiplash that can result from accidentally severed or ruptured air lines, by instantly shutting off the main air flow if the volume of air exceeds a set limit. Such devices as these can help prevent common causes of injury on a daily basis, protecting employees from harm and safeguarding businesses from the serious consequences of negligence.  

Nevertheless, to achieve and maintain the best standard of safety, every item requires correct installation and maintenance, a factor that is crucial if companies are to comply with the current Machinery Directive and successfully sell their equipment around the world. Even the familiar two-handed control unit must be correctly positioned and installed to ensure that its operator cannot be in proximity to the danger zone during start-up.

When designing and building pneumatic machinery, it is helpful to break down the three key areas of function - air supply and preparation, control mechanisms, and moving elements - and pose some simple safety questions. For example, can the system be safely set up?  Is there potential for uncontrolled movement? Can correct operating pressures be guaranteed? Consideration of these and other factors has been made simpler, and more urgent, by the arrival of the Machinery Directive 2006/42/EC (formerly 98/37/EC). This will impact on all those involved in the design, manufacture and operation of machinery with moving parts because the responsibility for compliance now rests with the builders of systems, rather than the suppliers of components.

The Machinery Directive 2006/42/EC references two standards, EN 13849-1 and EN 62061, each covering a specific group of systems. EN 13849-1 covers the general design principles for machine safety and safety related control systems, including pneumatics, while EN 62061 defines the functional control of electrical, electronic and programmable electronic control systems only. The standards extend procedures for risk assessment, with these now having to be specified as Performance Level (PL) in the case of EN 13849-1, or Safety Integrity Level (SIL) for EN 62061. These ratings will be associated with a given safety function, with definitions for diagnostics capabilities and common cause failures.

Issues such as the guarding and safety of operators and the overall interaction between man and machine are made clearer for pneumatic designers and engineers by the definitions and processes of the Machinery Directive, which aims to prescribe functional, systematic and safe designs. Though new legislation can often be seen as a threat and a hindrance, the new Machinery Directive will ensure that machines operate in a safe manner, gaining machine builders a degree of protection from litigation in the event of accidents arising through machine failure or misuse.

The Machinery Directive now clearly specifies EHSR (Essential Health and Safety Requirements) for assemblies of machines; this means that such assemblies must be considered as complete new machines and CE marked, and it is the legal responsibility of the system builder to ensure compliance. It is therefore important that machine builders involve their suppliers from the earliest design stages to ensure that the reliability and safety of the integrated machine structures are considered from the outset. This can be achieved by the selection of proven, reliable components and safety principles that minimise the manifestation of critical faults.

For the evaluation of components within a safety circuit, the Machinery Directive specifies the calculation of MTTFd (Mean Time to Dangerous Failure). This involves entering such values as the number of cycles and the mean number of operations into an equation. This equation can yield impressive results for robust, high quality components.

An interesting benefit of the Machinery Directive is that by placing responsibility on machine builders it will tend to encourage the use of higher quality parts. Cheap parts may be classified as safe at the time of installation but can soon degrade and become inadequate or dangerous. The Machinery Directive, with its criteria for estimating component failure, will make better components the only option, resulting in better, safer systems.
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