Meccanica (2012) 47:51–61 DOI 10.1007/s11012-010-9415-8 Experimental contact pattern analysis for a gear-rack system Massimiliano Pau · Bruno Leban · Antonio Baldi · Francesco Ginesu Received: 29 July 2010 / Accepted: 27 December 2010 / Published online: 12 January 2011 © Springer Science+Business Media B.V. 2011 Abstract Gears perform their main task, namely load transmission, by means of very small contact areas originated by tooth interaction and thus, the analysis of phenomena occurring at the interface between mating teeth represents a critical issue in ensuring the optimal functioning of such devices. Nevertheless, while liter- ature proposes a huge amount of numerical tooth con- tact analyses (TCA), a lack of experimental validation of such approaches is to be noted, since it is extremely difficult to inspect a contact interface which is, by its own nature, closed towards the outside world. One of the most promising techniques employed in investigat- ing contact in metallic interfaces is based on the use of high frequency ultrasonic waves; their reflection from the interface (which is known to be related to contact conditions) can be graphically processed to build maps from which it is possible to assess geometrical features of the nominal contact area and, after a suitable cali- bration procedure, contact pressure distribution. This study proposes the application of the ultra- sonic method to investigate the contact of a tooth be- longing to a spur gear with a plane, thus reproduc- ing the case of a gear-rack system. Experimental tests were carried out under different conditions, including regular contacts, misalignments and excessive wavi- ness of the teeth; in all cases analyzed, this tech- M. Pau ( ) · B. Leban · A. Baldi · F. Ginesu Department of Mechanical Engineering, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy e-mail: pau@dimeca.unica.it nique exhibited interesting capabilities in faithfully re- producing the contact features, thus allowing us to state that it may represent a useful tool in arriving at a deeper insight into contact problems in gears and, above all, in effectively validating numerical TCA and wear models. Keywords Spur gears · Experimental · Ultrasonic analysis · Tooth contact analysis · Contact area · Contact pressure Notation TCA Tooth Contact Analysis NDT Non-Destructive Testing RCA Real Contact Area Z Acoustic impedance of a medium R Reflection coefficient of the ultrasound wave FE Finite Element 1 Introduction and purposes of the study Although gears represent one of the most widespread machine components, even after more than a cen- tury of investigations their design and performance analyses are still of primary importance in addressing the ever-increasing demand for load-carrying capacity, speed and life-cycle duration. One of the most critical issues in their opera- tion involves the way in which stresses are transmit- ted through the contact region between mating teeth.