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Brake presses are widely used in industry, especially in small and medium sized establishments because of their relative affordability and flexibility. They enable the production of a wide variety of sheet metal parts of various sizes and shapes.
The operation of brake presses, involves various important risks for the operators, as studies by Ngô et al. (1994), Venditti (2005) and Tran (2009) have investigated. Brake press operations most often contain an important human-machine interaction. In most press work stations, the operator manually feeds, holds and retrieves the part from the machine. The operator holds the piece-part and actuates the closing motion with a foot pedal, in most applications. An hazardous situation is thus created from the proximity of the workers hands to the press closing motion. A possible accident in such a situation is then that the worker gets his hands caught between the closing dies.
Safety regulations and standards therefore require that machines such as brake presses be equipped with protective devices which either prevent entry of the operator in the hazardous zone or stop the hazardous motion when parts of the workers body are in the hazardous zone. However, before implementing an effective safeguarding device, an effective risk assessment methodology is in order. The motivation for such an approach can easily be found in legislations, regulations and safety standards around the world. For example, in Québec, the Act respecting occupational health and safety at work requires that employers must take the necessary measures to protect the health, safety and physical integrity of the workers. Commonly used approaches to risk assessment rely on subjective perceptions. A quantitative approach provides more objective results. In this thesis, fault tree methodology will be used.
Fuzzy numbers in Risk assessment and fault tree analysis
In traditional fault tree analysis precisely. probability values are not always known. There is always uncertainty surrounding them. It has been pointed out, in many papers , see for instance Kabir (2018), that the probabilities associated with the events making up the tree are seldom known with precision and gathering the required information has proved to be very difficult. To remedy this difficulty, fuzzy logic concepts have proved useful. Fuzzy set theory first proposed by Zadeh (1965), has proven to be a useful methodology to cope with these cases where uncertainty and scarcity of data are important features.
So, fuzzy numbers can help in handling uncertainty about data. But the question remains, how to obtain the necessary data in the first place? The fuzzy logic approach can use the opinion of field experts to extract the probabilities or failure rates required in a fault tree analysis. This could be done informally, but in this thesis, this process will be done following guidelines enunciated in the literature under the denomination Expert Elicitation. These expert opinions take the form of qualitative linguistic estimates of the elicited probabilities. For example, the answers given can be expressed as ‘’the probability is low’’ or ‘’ It is very low’’. Fuzzy methodology is then utilized to convert these linguistic estimates into a quantitative expression.
Machine safety conceptual framework
Over the recent years (starting in the early 1990s), standards have been developed on machine safety, especially on the aspect regarding risk analysis. Two of the most regarded such standards are CSA Z432 and ISO12100. Furthermore, in Quebec many safety guides have been developed to help industry understand and implement the underlying ideas on machine safety and risk:
Risk is viewed in this segment of the literature in a framework composed of different elements which were illustrated in the above example and which we will know introduce.
The basis of any situation involving a risk is a hazard: a potential source of harm. In the above example, two main hazards were mentioned:
• Engine fuel: highly volatile, flammable, explosive, toxic, carcinogenic, damaging to the environment
• Vehicles with speeds of 120 mph
In machine safety, hazards are, in particular, created by the motion of machine components and parts .
These hazards can create many hazardous situations which are defined as situation in which people are exposed to these hazards:
• People can inhale fuel vapours
• People can inhale nitrogen oxide produce by the burning of the fuel
• People can expose their skin by handling liquid fuel without proper clothing
• Thousands of vehicles will circulate in close vicinity to each others, creating possibilities of collisions
• Vehicles will circulate in all weather conditions .
In machine safety, hazardous situations are created when operators find themselves, by necessity, in close proximity to machines, notably when they must perform common tasks such as:
• Placing tools, parts in the machine
• Installing tools, fixtures in the machine
• Removing parts from a machine after a normal production cycle
• Cleaning in or around machine
• Un-jamming a part in a machine
• Inspecting a part.
Krϋger and al. (2009) give a comprehensive survey of human-machine interaction in a complex robot-assisted assembly line.
From these hazards and situations, hazardous events, which are events which can lead to harm, can be produced, such as:
• Health problems
• Fires and explosions
• Collisions between vehicles
In the field of machine safety, hazardous events commonly stem from two main sources:
• Technical (machine-related) failures and
• Human failures resulting from actions taken by operators, like for instance reaching into a press while it is in its downward motion to retrieve a misplaced part.
These hazardous events can therefore produce harm or more generally damages to property and the health and safety of humans. Before we continue, it is worthwhile to review the literature on the definition of «risk», a concept which turns out to be surprisingly slippery and elusive.
Definitions of risk
Indeed, as Bahr (1997) puts it, «Risk is probably one subject we all feel we understand yet admit that we know nothing about.» Kumamoto and Henley (1996) mention in their seminal treatise on risk, that risk is defined differently by various people. «This disagreement causes serious confusion in the field of risk assessment and management.» The Webster’s Collegiate Dictionary, for instance, defines risk as the chance of loss, the amount of possible loss, the type of loss that an insurance policy covers, and so forth. Le Petit Robert gives, in French, a similar definition. Such dictionary definitions are not sufficiently precise for our purposes. To further illustrate the complex and multifaceted nature of risk, let us mention an example given by Horlick-Jones (1998): «In 1992, Britain’s Royal Society, one of the world’s most prestigious scientific institutions, published a report entitled Risk: Analysis, Perception and Management, In his carefully worded preface to the preface, Sir Francis Graham-Smith, Vice-President of the Society, reflected upon its lineage and composition, and went on to say that: «Chapters 5 and 6 differ somewhat, in style and in content, from the earlier chapters. In particular, chapter 6 sets up, as an expository device, as series of referenced points of view as opposed positions in the debate. Some of the contending positions will undoubtedly strike many practitioners as extreme…» .
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