Safety for Gravity Loaded Axis from Standardized Requirements to Real Design of Safety

One of the design measures adopted for the risk reduction of machinery is the de-energization of all the components of the machine to ensure a "safe stop state". Therefore, for gravity loaded axes (GLA), the risk of unintended gravity descent in the de-energized state shall be considered in the risk assessment.
In case of power failure, the gravity loaded axes are held solely by the braking or counterweight systems, which are installed in the machine. For example, due to undetected failures or poor maintenance, an inadequate response of the system after power failure is possible. Moreover, in multimodal machine tools (e.g. automatic, setup, maintenance modes) the exposition of the persons to the GLA can vary depending on the machine and the specific use, thus requiring different system design for each case. In this paper the design and optimization of a real mechanical lock system for automatic stopping of GLAs is presented in full details. First, the evaluation of the power failure detection time is discussed, with measures able to assure a proper detection functionality: the mechanical lock shall be active before the GLA begins to move. Then, considering the damping devices of the full GLA mechanism, the real magnification factor due to the dynamical response of the system under gravitational loads is presented. This factor, when determined, is useful for designing the GLA system with a proper safety factor using a simple equivalent static design. Finally, some critical aspects related to standardized requirements for GLAs are discussed based on real systems design.

Keywords: Safety of Machinery, Energy failure of machinery, Machine Design.