beam loaded on the flank with a tangential force \(F_{t}\) rotated by an angle \(\alpha_{Fen}\) with respect to the tooth axis (Figure 1). The determination of the normal chordal dimension \(s_{F\!t}\) of the tooth root critical section and the bending moment arm \(h_{F\!e}\) relevant to load application at the outer point of single pair gear tooth contact according to ISO 6336--Method B is shown in Figure 1.  
The occurring stress \(\sigma_{F}\) (Equation (1)) results  
\[\sigma_{F}=K_{A}\cdot K_{V}\cdot K_{F\beta}\cdot K_{Fa}\cdot\frac{F_{t}}{b\cdot m _{H}}\cdot Y_{F}\cdot Y_{S} \tag{1}\]  
where \(Y_{F}\) is defined as in Equation (2)  
\[Y_{F}=\frac{\frac{\delta h_{F\!e}}{m_{a}}\cos\alpha_{Fen}}{\left(\frac{s_{F\!e} }{m_{a}}\right)^{2}\cos\alpha_{n}} \tag{2}\]  
is the form factor. It takes into account the influence of the tooth form considering that the load is applied at the outer point of single contact. \(Y_{S}\) is the stress correction factor and considers the notch effects due to the variation of the section at the tooth root and is used to compensate the errors introduced by the method used to determine the most critical section (oblique lines as prescribed by the ISO 6336--Figure 1) and it is defined as in Equation (3)  
\[Y_{S}=(1.2+0.13L)\cdot q_{s}^{\frac{1}{12+2\cdot 3/L}} \tag{3}\]  
where (Equation (4))  
\[q_{s}=\frac{s_{F\!t}}{2\cdot\rho_{F}} \tag{4}\]  
is the notch sensitivity and \(L\) is defined as in Equation (5)  
\[L=\frac{s_{F\!t}}{h_{F\!e}} \tag{5}\]  
The factor \(K_{A}\), called application factor, is used to take into account the effect of external overloading. It depends on the application. The dynamic factor \(K_{V}\) considers the internal dynamic loads. The factors \(F_{F\beta}\) and \(K_{Fa}\) are used to model the uneven load distribution in the contacts along the face width and in the transverse direction. These can be caused, for example, by manufacturing errors or deflection of the system under load. Figure 1: Determination of normal chordal dimensions of tooth root critical section according to ISO 6336-3:2019—Method B. The occurring stress \(\sigma_{F}\) is compared with the permissible bending stress \(\sigma_{FP}\) (Equation (6)). The limit value for tooth root stress should preferably be obtained from experimental tests using gears as test pieces. In this way, most of the effects such, for example, the one of fillet at the tooth roots, are automatically included in the results. According to the ISO 6336--Method B, the permissible bending stress \(\sigma_{FP}\) can be calculated starting from the nominal stress \(\sigma_{Film}\) which represent the bending stress limit relevant to the influences of the material, the heat treatment, and the surface roughness of the test gear root fillets. \[\sigma_{FP}=\sigma_{Film}.Y_{ST}.Y_{NT}.Y_{BetLT}.Y_{RrelT}.Y_{X} \tag{6}\]  
\(Y_{ST}\) and \(Y_{NT}\) are the stress correction factor and the life factor for tooth root stress, respectively, both relevant to the dimensions of the reference test gear. \(Y_{BetLT}\) is the relative notch sensitivity factor, and \(Y_{RrelT}\) is the relative surface factor. Finally, \(Y_{X}\) is the size factor relevant to tooth root, which is used to consider the influence of tooth dimensions on tooth root bending strength. It considers the influence of size on the probable distribution of weak points in the structure of the material, the stress gradients, which, in accordance with strength of materials theory [20; 21], decrease with increasing dimensions, the quality of the material as determined by the extent and effectiveness of forging, the presence of defects, etc. [11]. According the standard ISO 6336--Method B, the size factor can be estimated as a function of the material and the normal module according to Table 1.  
It can be appreciated that, independently from the material considered, the size factor is always equal to 1.0 for gears having a module below 5 mm. [PERSON] et al. [19] performed systematic tests on small size gears made of 16 MnCr5 having a normal module \(m_{n}=0.45\div 0.6\) mm.