Avoiding danger

Marginal Column

March 2012

Functional safety of machines begins at conceptualization. The first step is a systematic risk assessment. But how is this performed?

 
 

Manufactures are obliged to ensure state-of-the-art safety of their machines. However, the compulsory requirements of the European Machinery Directive 2006/42/EC has brought this is­sue to the forefront. In accordance with Rexroth’s systematic approach, the first of the ten steps toward being able to meet these requirements is the thorough risk assessment required by the directive. Be­cause we can only minimize those dangers that we recognize. But this risk assessment also offers companies legal security for products not covered by the directive. So a documented risk as­sessment should always be carried out during planning and de­sign. This process combines risk analysis, risk evaluation, and risk reduction and can be divided into five working points:

1. Determining the limits of the machine

2. Identifying the hazard

3. Risk analysis

4. Risk evaluation

5. Risk reduction

Ideally, an interdisciplinary team from Design, Quality, Sales, and Service works through these points. The timing is key: if the assessment is performed too early, it may not identify all the required measures. If it is performed too late, it may no longer be possible to implement the best solution approaches, or only at great expense.

1. Determining the limits of the machine

Here, the project team defines the spatial, chronological and external limits of the machine as well as its intended use. Spatial limits take into account aspects like the “elbow room” required by all the axes in a machine tool. The chronological as­pect relates to the service life – from transport and operation to disposal – or to periods of use stipulated in standards. For example, according to ISO 13849 a safety-relevant control unit must be replaced after 20 years. Ambient conditions and the properties of the materials to be worked determine the exter­nal limits.

 

2. Identifying the hazard

Examples of hazards (excerpt from ISO 12100).

 

This is the most important part of any risk assessment: the systematic identification of foreseeable hazards in all phases of the machine’s service life. Once the life phases have been analyzed, the next step is to identify the tasks that the users perform in the various working steps. For example, during commissioning the operator must adjust, inspect and set up the machine. In normal operation, the tasks are then filling, removing, shutting down, restarting. The hazard for each of these tasks must be identified. The project team needs to take the following aspects into account:

  • What human action is performed in which phase?
  • What are the possible operating states? (This also includes malfunctions in which the machine does not work as in­tended.)
  • What might unintentional operator behavior or intentionally improper use of the machine look like? A list of hazard types, situations and outcomes forms the basis for identifying risks.

3. Risk analysis

The team can employ a variety of methods to determine the likely severity of an event and the probability of its occurrence. It is normal to quantify the risk by distinguishing between the extent of the event, the frequency and duration of exposure to the hazard, the probability of occurrence, and the possibility for restricting or preventing its occurrence.

 

4. Risk evaluation

As soon as the project team has quantified the risk elements, it must evaluate whether risk reduction in the form of appropriate protective measures is necessary. These measures should result in an “adequate minimization of risk”. To achieve this, the fol­lowing conditions must be met:

  • All operating conditions and user interactions are taken into account.
  • The risks have been minimized as much as possible, or eliminated.
  • New hazards resulting from the implemented actions are considered.
  • The users are adequately informed of residual risks.
  • The protective measures are compatible with one another.
  • Foreseeable improper use has been taken into account.
  • The user-friendliness of the machine has not been impaired.

5. Risk reduction actions

Multiple alternatives may be possible for individual hazards. For example, ISO 12100 recognizes three types of protective measures; their order must be observed to comply with the standard:

1. Inherent (design or operating properties relating to the machine) safety through improvement of the machine’s design.

2. Technical protective measures and possibly supplementary protective measures, such as user information and data sheets.

3. User information on the residual risk.


 

More information on risk assessment (sample forms, help in quantifying risk elements and in risks evaluation, etc.) as well as the other nine steps for machine safety are described in the guide “10 steps to performance level”, including numerous practical examples and information for all kinds of control technologies. Rexroth also offers individual consulting, service and training on the topic of functional machine security.

 

10 steps to performance level Handbook for the implementation of functional safety according to ISO 13849.

Bosch Rexroth AG

July 2012 – 264 pages – € 49.90 ISBN: 978-3-9814879-2-3