Failure analysis of a ball transfer unit

The component called Ball Transfer Unit is often used in the conveyor belt system for the automatic and rapid transport of luggage within the airport handling systems. The analysed component is a bearing in which a largest ball rotates over a belt of other small balls. Some of these components grip, but only the ones present in one airport near the seaside. The aim of the present examination is focused on the reliable identification of the causes which have produced the loss of the rotational attitude which represents the main functional property of this mechanical bearing component. Visual inspection, microscopic analysis (optical and SEM) and computational simulation have been carried out to understand the failure mechanism. In particular the computational analysis permits to calculate the stress state and the deformation induced on the bearing. In a first time the damage has been attributed on the different and often not regular depth of the carburised and hardened layer supporting the belt of the little ball. This irregularity seems to produce an excessive deformation of the supporting body and the consequent detachment of the zinc coating. The presence of debris in the bearing component seems to be the element which deteriorates the rotation of the balls involved in the system. On the other hand, the performed study has revealed that the cause of the disease is not the irregularity of the depth of the hardened layer, but the presence of an abrasive body composed by SiO2, CaO and Al2O3 of exogenous origin which can start up a wear mechanism.