Guards on machine tools must be designed such that they are capable of resisting the impact of objects ejected at high velocity from the working zone. The windows fitted within the guards to allow the manufacturing process to be observed frequently represent a weak spot. The tests performed to date, which have been limited to the materials themselves, show polycarbonate to be particularly suitable as a material for vision panels in guards. The results available do not permit adequate assessment of the impact resistance of the window design, however, as other factors, such as the mounting situation and the design of the windows themselves, must also be taken into account. The objective of this project was to define principles for the design of guards with integral vision panels which comply with the safety requirements.
A pneumatic impact test bench was employed to study the impact strength of various designs of vision panel manufactured from polycarbonate up to 12 mm in thickness. The impact tests were performed in accordance with the provisions of the European safety standards for turning machines, milling machines and machining centres, and involved projectiles with a mass of between 0.1 and 2.5 kg. In order to obtain data for the dimensioning of vision panels, the forces generated by the impact test were measured.
Polycarbonate vision panels should be fitted to guards on machine tools by clamped or adhesive joints, in order for their retention capacity to be fully exploited. Clamped or adhesive joints enable the vision panels to be fitted under low stress, with transmission of the force over a large surface area, and with adequate allowance for expansion. Arrangements involving bolted joints are less suitable, as the bolted joint prevents deformation of the panel. Constuctions with clamping profiles, which are frequently encountered on older machine tools, proved in fact to be unsuitable for the absorption of higher impact energy levels. An adequate overlap of the vision panel and the surrounding panel must be ensured, whether the vision panel is clamped or bonded. Should the overlap be insufficient, the vision panel may be forced partly or completely out of its mounting and projected away, without itself being destroyed. The size of overlap to be selected depends partly upon the level of energy to be absorbed and the thickness of the vision panels. Based upon the test results available, a minimum overlap of between 12.5 and 37.5 mm should be ensured, according to the values of these parameters. If the retention capacity of polycarbonate vision panels is exploited to the full, impact forces of up to 90 kN may occur briefly, for which allowance must be made in the design analysis of the structure as a whole.
mechanical engineeringType of hazard:
Mechanische Gefährdung, SicherheitstechnikDescription, key words:
guards, machine tools, impact strength, vision panel designs, mounting arrangements, impact tests