Machine Safety: Designing safety with accessibility

Tom Shelley reports on the problem of ensuring safety in machines that require frequent access, such as those used in the food industries

Machines that require frequent access for cleaning or clearing blockages pose a particular design problem. Since operators cannot be protected by locking the machines up in cages, guards have to be placed over potentially dangerous parts, and either firmly welded in place, or, if used in the food industries, equipped with interlocks capable of surviving cleaning chemicals. Regulations that must be conformed to include the Supply of Machinery (Safety) Regulations 1992 (SMR), the Provision and Use of Work Equipment Regulations 1998 (PUWER 98) and the Machinery Directive. The problem arises because, although the Machinery Directive covers safety requirements in machines supplied to the whole of Europe, there is no definitive interpretation of what is required, and individual health and safety officers in both official bodies and companies often have their own interpretations. Or, as Mike Wilkinson, managing director of The Food Machinery Company, based just outside Rochester in Kent puts it: "Each country in Europe has its own bodies that interpret and enforce the Machinery Directive in their own way." Wilkinson is particularly well-placed to assess the problem, as his company imports machines from all over the world and then remodels them to ensure they meet UK requirements. He does much of the design work himself using AutoCAD 2010 and either adds guards and interlocks (using safety switches, interlocks and safety relays from Schmersal) in his own workshop for small numbers of machines, or contracts this out for large orders. The requirements for designing guards for food machinery are more demanding than for other manufacturing machines, since food processing machines have to be stripped down and cleaned every day, whereas metal manufacturing machines might only need to be accessed once in several months for maintenance, or on the occasion of their developing a fault. Hence, total enclosing cages are not an option. An interesting example at the company was a Taiwanese machine for manufacturing samosas and wontons, to which the makers had affixed a prominent CE mark in the belief that the enclosures protected users from all working parts. However, Wilkinson pointed out that the chain guard could be lifted off and accessed by fingers from underneath. For use in the UK, says Wilkinson: "We will weld on a guard that totally encloses the chain and prevents finger access." Another machine highlighted another problem. Says Wilkinson: "It had a specific pinch area, and the guard allows fingers to get in, but the mechanism still has to be accessed for cleaning so we will add a cage and a safety switch." It is worth noting, in this regard, that The Machinery Directive requires the use of safety switches that cannot weld themselves into the closed position. Even in the food industry, different machines fall into different safety categories and, while a Category 1 machine only requires the use of simple safety switches (Switching A relays), Category 3 requires two safety circuits going through stop switches and the use of a suitable safety relay. Says Wilkinson: "You need to think of all the possible ways in which the machine can be party to an accident", underlining the point with reference to a machine that had a large section that had to be swung out for cleaning, which in some circumstances could still be connected to a live feed. An example of a Category 3 machine Wilkinson showed was a Chinese-made packaging machine with a number of opening and closing mechanisms, but only a lift-off guard with no interlock. This was to receive an interlocked cage and safety relay. However, it is not just Chinese or other far eastern machines that require adaptation, but a significant number made in other European Union countries as well. Wilkinson says: "This is a major issue that nobody seems to have thought about, since it makes something of nonsense of the single market." How acute this problem is was demonstrated by a number of machines present at the site. Some, like a bag-sealing machine with lots of pinch points that could easily be made safe with a simple guard and a Danish-made cooking kettle that already had a mechanical lock to stop the stirrer when the lid is lifted, did not require much work. However, as Wilkinson pointed out, while they may not have required much work, they still need some. He says: "Just a bit of forward thinking makes something perfectly safe" A German-made vegetable preparation machine with fast moving chopping blades brought to light another issue. Although it already came with interlocks to meet all German-interpreted Machinery Directive requirements as well as full European CE Approval, was still potentially problematic in the UK. Says Wilkinson: "You could still access the blades if you put your hand in from underneath. [In Germany] this is not considered to be a problem. But in large companies in the UK where they employ semi-skilled labour, they have to prevent people from wanting to hurt themselves. Normally, for such customers, we design our modifications in conjunction with the firm's health and safety officer." Fearsome as the working parts of some of Mike Wilkinson's machines look, analysis of injuries investigated by the HSE in the food and drink industries over a four-year period showed that the worst culprits (30% of all injuries) were conveyors, followed by fork lift trucks (12%) and bandsaws (5%), with other categories down in small or less than single percentage points. 90% of conveyor injuries occurred on belt conveyors, of which 90% involve well known hazards such as in-running nips, transmission parts and trapping points between moving and fixed parts. In addition, 90% of accidents occur during normal operations. Good, clear advice on how to design out such hazards can be found in HSE Food Information Sheet No. 25, 'Safeguarding flat belt conveyors in the food and drink industries', which may be downloaded, free of charge from the HSE web site. Another good source of information on all matters concerning automated food production is the Centre for Food Robotics and Automation in Doncaster. One of its particular capabilities is their use of computer simulation technologies with which it can create interactive 3D layouts of whole factories.