The student Julen Elizegi Aiertza obtained an OUTSTANDING qualification with 'CUM LAUDE' mention

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The student Julen Elizegi Aiertza obtained an OUTSTANDING qualification with 'CUM LAUDE' mention

THESIS

The student Julen Elizegi Aiertza obtained an OUTSTANDING qualification with 'CUM LAUDE' mention

2022·07·18

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Thesis title: "Gainazal nekearen ondorioak karbono altzairuzko engranaje zilindrikoetan"

Court:

  • Chairmanship: Edurne Iriondo Plaza (UPV/EHU)
  • Vocal: Amaya Igartua Aranzabal (Tekniker)
  • Vocal: Arkaitz López Jauregi (Laulagun)
  • Vocal: Haritz Sarriegi Echeberria (Escuela de Ingeniería IMH en Alternancia)
  • Secretary: Jon Ander Esnaola Ramos (Mondragon Unibertsitatea)

Language: Basque

Abstract:

One of the most common failure modes in gears is surface fatigue, called pitting or spalling. It results from cracking in subsurface that appear when the tooth contact pressure exceeds the permissible contact stress. Due to repetitive loading, these cracks propagate to the surface, releasing fragments of material and causing holes known as spalls. These, modify the mechanical behaviour of the gears and reduce their life. It is also believed to cause the gear system to switch off for various reasons, such as excessive levels of vibration and/or noise or tooth root breakage. To avoid these unacceptable consequences, once the working conditions are defined, they are dimensioned based on the ISO 6336-2 standard, which determines the minimum dimensions of the gears to avoid the appearance of cracks during their service life. However, regardless of the application of the gears, the same criteria related to the damaged area are always used to define spalling failure, with the most limiting being considered when 4% of the contact area of a tooth is damaged. Furthermore, it is not explained the reason for it to be critical. Despite this, the dimensioning standard also mentions that if the gear manufacturer and the user agree so, gears with teeth up to 100% of the contact area damaged by spalling may be permissible in certain applications where the working conditions are not demanding, conditions for which, among others, gears made of carbon steels are used. Therefore, experience has shown that these criteria are too severe for certain applications, leading to the manufacture of oversized gears and unnecessary increase of costs.

Currently, the state of the art regarding the effect of the spalling exceeding the specified failure criteria is largely unknown. On the one hand, it is not known whether exceeding this level of damage increases the risk of tooth root breakage. On the other hand, there is insufficient knowledge to ensure that the effects of increasing the level of surface fatigue on the behaviour of the gears are negligible, nor to know whether possible reductions in life have to be considered. To sum up, there is a lack of knowledge about the reasons why spalling is critical, which implies that the relationship between these and the level of spalling is unknown.

In response to this problem, firstly, experimental tests have been carried out to analyse the consequences of exceeding the failure criteria used to define critical spalling in cylindrical gears of carbon steels under different working conditions. The evolution of the surface affected by surface fatigue has been relatively stable in all experimental tests, increases in vibrations during the tests have been slight and, despite the high levels of spalling, no tooth root breakage has occurred.

It has therefore been demonstrated that levels of spalling far above the critical levels of the dimensioning standards are admissible, and that the risk of gear tooth root breakage due to surface fatigue is low. Therefore, a hybrid methodology is then proposed to define the critical level of spalling for a particular application, regardless of the geometry and working conditions of the gears. This hybrid methodology is based on the increment of vibration and noise produced by spalling, which are calculated with a dynamic model of one degree of freedom in which the numerically computed time varying mesh stiffness of a pair of gears damaged by spalling is imported. In the analysed case study, it has been observed that the increases in vibration and noise produced by the spalling are more evident in helical gears than in spur gears. Moreover, in cases where it has been ensured that the gear tooth root will not break, it has been concluded that in order to determine the criticality of the spalling it is more appropriate to base on the sum of the total area affected in all teeth rather than just on the most damaged one.