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2017
Conference Paper
Titel
Probabilities in safety of machinery - risk reduction through fixed and moveable guards by standardized impact tests, part 1: Applications and consideration of random effects
Abstract
The uncontrolled ejection of a workpiece, a tool or parts of it can cause a perforation of the safety guards of machine tools with severe consequences for the operator. Therefore, experimental tests have been conducted in Germany and Italy during the last two decades in order to design specific safety guards with sufficient penetration resistance. As a result, a worldwide database has been established for metal- and woodworking machinery, and machine-specific design provisions have been collected in CEN and ISO product safety standards. Similar to all other experiments in the field of ballistic impacts, there are many parameters influencing the penetration behaviour: material of the target and its delivery condition, size, weight and shape, trajectory and impact angle of the projectile. In order to support the designer with practice-oriented provisions, different guard materials have been collected in design tables, in which the impact energy is allocated to the necessary thickness of the selected material. Formulas to estimate the worst-case projectile energy through technical machine specifications are also given. For the purpose of comparability, the details of the impact test procedures are also defined in the safety standards. This part 1 of the article describes how impact tests are being performed for different machine standards at IWF and INAIL laboratories. The assumptions and simplifications of the results are discussed on the background of the existing database. The deterministic interpretation narrows several significant influences on the results of the impact tests to a single test arrangement. It is not considered that the empirically determined penetration resistance, even in an accurate test mounting, has a statistical variation. Based on results of the IWF, it is shown that there is a considerable difference between a deterministic and a probabilistic interpretation. Consequently, a probabilistic review of existing design guidelines is given by focusing on a critical example with a reasonable statistical fundament: the perforation of a 12 mm thickness polycarbonate guard. As the backwards compatibility of existing design procedures is all important to manufacturers, at least the following two questions emerge: 1) How can the existing database be interpreted in a probabilistic way? 2) What is required to improve the current impact test and evaluation procedure to provide a more realistic picture of the risk of penetration? The commonly known "safety factor" seems to be the answer to ensure backwards compatibility with existing standards.