Search

Spur gears are one of the most commonly employed machine elements for power transmission. During their operating life, they are subjected to variable and cyclic loading. Consequently, material fatigue may occur in the tooth root region, more commonly referred to as bending fatigue. Steel gears are typically subjected to heat treatment processes such as carburizing to reduce wear. Additionally, the gears may be shot peened to induce a beneficial, fatigue-resistant surface layer of compressive residual stresses. However, this may result in bending fatigue crack initiation below the surface, i.e., subsurface bending failure. This type of crack is hardly detectable during regular service intervals. Hence, it may often go unnoticed, grow, and propagate rapidly through the brittle carburized layer, resulting in tooth breakage and gear failure. This doctoral thesis establishes a computational model for predicting the bending fatigue crack location (surface vs. subsurface) and the required number of cycles for surface-hardened spur gears. Due to the relative complexity of the gear’s geometry and loading conditions, the finite element method is employed to obtain load-induced stresses and strains. The multilayer method is employed to account for inhomogeneous material, while the strain life approach (ε – N) is used to predict the bending fatigue life. The research is divided into multiple stages. During the initial stages, bending fatigue with surface/subsurface crack initiation is investigated on relatively simple geometry, such as surface-hardened gear steel specimens. Then, the model is gradually upgraded to account for more complex loading conditions, residual stress distributions, and inhomogeneous material until the final model applicable to the running gear pair is acquired. According to the obtained numerical results and confirmed by the experimental investigations from the available literature, subsurface bending fatigue crack initiation tends to occur only in carburized and additionally shot-peened gears. The critical region for subsurface failure is located in the proximity of sharp loss of compressive residual stresses and still relatively high load-induced stresses. The probability of subsurface crack initiation can be reduced by modifying the gear’s geometry (choosing a smaller normal module) or increasing the beneficial compressive residual stresses by prolonging the carburization time. Zupčanici su jedni od najčešće korištenih strojnih elemenata u prijenosnicima snage i gibanja. Vođeni strogim proračunskim kriterijima prema relevantnim standardima kao što je ISO 6336, čelični zupčanici s ravnim zubima u pravilu zadovoljavaju minimalni očekivani radni vijek. Međutim, postojeći standardi ne uzimaju u obzir zajednički utjecaj dodatnih postupaka površinskog otvrdnjavanja, kao što su cementiranje i sačmarenje, na pojavu zamora materijala u korijenu zuba. U takvim slučajevima, javlja se potreba za računskim modelom koji može procijeniti ne samo broj ciklusa do pojave zamorne pukotine, već i mjesto nastanka spomenute pukotine (površinski naspram potpovršinski nastanak). Mjesto nastanka zamorne pukotine od posebne je važnosti zbog njene otežane detekcije tijekom redovnih servisnih intervala. U okviru rada razvijen je računski model temeljen na metodi slojeva (eng: multilayer method) te metodi konačnih elemenata (eng: finite element method – FEM) koji predviđa mjesto te broj ciklusa do nastanka zamorne pukotine u korijenu zuba zupčanika. Zbog relativne kompleksnosti modela, istraživanje je započeto na uzorcima izrađenim od materijala tipično korištenim za izradu čeličnih zupčanika. Predloženi model postepeno je unaprijeđen kako bi se simulirala stvarna geometrija te opterećenje zupčanog para.

In this paper, a numerical model is proposed to predict the location and the number of cycles for bending fatigue failure of surface-hardened gears. The predictions are validated against experimental results. Linear-elastic stresses and strains obtained by a finite element simulation are imported into the fatigue model. Bending fatigue lives at the observed tooth root region are estimated based on the multilayer method, hardness method, and strain-life approach. The critical plane approach is employed due to non-proportional loading. Good agreement between actual and predicted bending fatigue lives was observed, as well as between the estimated and actual subsurface failure location.

Surface hardened spur gears are one of the most commonly used machine elements regarding the power transmission. During gear service life, variable and cyclic nature of the stresses at the tooth root may lead to material fatigue, commonly known as bending fatigue. The bending fatigue may cause crack initiation and consequently propagation at the tooth root fillet, eventually resulting in a tooth breakage. In this paper, the authors presented a computational model for predicting the number of cycles required for crack initiation. Two cases of driven gears are considered. Elastic finite element analysis is used to generate the numerical models which accounts for the nominal tooth root stress change during the gear meshing. Additional effects due to friction and centrifugal force are also considered. Fatigue models are generated using the strain-life approach and finite element analysis results. Mean and residual stresses, along with the local plastic deformation effects, are also considered. The bending fatigue life results are in good agreement with the experimental data from the available literature for the first case and conservative for the second.