Resumen en inglés

The widespread use of pneumatics in automatic processes requires high reliable components and as a consequence the specific necessity to define and assess the performance of pneumatic cylinders as far as regards the service life determined by the behavior of constitutive parts such as seals and guide bearings which have a critical influence in the efficiency of these devices. The guide bearing, generally located in the front end cover of cylinders was found to be a particular cause of failure since they lead the seals to collapse and the subsequent occurrence of leakages. The scope of this thesis is then, the design and development of a test bench capable to perform reliability assessment of pneumatic cylinders by testing which allows evaluating the behavior of pneumatic cylinders under the action of radial load with accelerated working conditions. The test bench has been developed in such a way that it can implement two types of tests: the wear test and the leak test, which during its execution, the bench must be in the capability of providing information about the pressure drop in the chambers, the sealing capacity of the gaskets and finally the lifetime of the cylinder. To this end, a test bench that allows achieving the objectives set out above by operating in a fully automatic mode has been developed, implementing a PLC as the main controller of the process and as a signal acquisition system, besides of a SCADA software that serves as the Human–Machine Interface (HMI). In the other hand, three analytical models were defined in order to estimate the wear of the guide bearing of the cylinders. To judge mathematically the wear of the guide bearing and thus determine the lifetime of the cylinders, it was necessary to find the wear coefficient of the materials and the way that the acting pressure distributes along the bearing, the former being found from the synthesis and processing of results of reliability tests carried out previously at the Mechanical Department of Politecnico di Torino. The definition of the models goes from the most simple and distant from the reality of the phenomena occurring between the coupling, to the more complicated and that best fits to what actually happens between the piston and the guide bearing. Therefore, the first model takes into consideration the pressure distribution along the guide bearing as uniform, the second model suggests that the pressure distribution on the bearing is trapezoidal since the piston rod is moving misaligned in reference to the axis of the cylinder due to the load acting radially and finally, the third model, plays with the clearance between the tribological coupling, assumes that the pressure distribution on the bearing is not linear and calls for the contact mechanics occurring between the bodies. The final objective of the experimental tests, together with the analytical formulations, is the assessment of the wear coefficient and an outlook for the service life of pneumatic cylinders.