ISO/TS 6336-4-2019 pdf free.Calculation of load capacity of spur and helical gears – Part 4: Calculation of tooth flank fracture load capacity.
Tooth flank fracture is characterized by a primary fatigue crack in the region of the active contact area. initiated below the surface due to shear stresses caused by the flank contact. Failures due to tooth flank fracture are reported from different industrial gear applications and have also been observed on specially designed test gears for gear running tests (images of tooth flank fractures can be found in Reference I9I) Tooth flank fracture is most often observed on case carburized gears but failures are also known for nitrided and induction hardened gears. Tooth flank fracture is sometimes also referred as subsurface-initiated bending fatigue crack, sub-surface fatigue or tooth flank breakage. The main failure characteristics are:
— tooth fracture is due to a crack located in the active flank area, often at approximately half the height of the tooth;
— primary crack initiation is at a considerable depth below the surface of the loaded gear flank, typically at or below the case-core interface;
— the primary crack starter is often but not always associated with a small non-metallic inclusion;
— the primary crack propagates from the initial crack starter in both directions — towards the surface of the loaded flank and into the core towards the opposite tooth root section;
— due to the high hardness in the case, the crack propagation towards the surface is smaller than through the core;
— the angle between primary crack and flank surface Is approximate 400 to 50°;
— due to the inner primary crack, secondary and subsequent cracks may occur which originate from the surface;
— the crack propagation rate rapidly increases as soon as the primary crack has reached the surface of the loaded gear flank;
— the final breakage of the tooth is due to forced rupture; typically developing according to local bending stress;
the fractured surfaces show typical fatigue characteristics with a crack lens around the initiation point and a residual zone of forced rupture;
— in many cases (but not all), no indications of surface related failures such as pitting or micropitting are observed on the gear flanks.
Due to these characteristics the failure type of tooth flank fracture can be clearly differentiated from the classical tooth root fatigue failure that is caused by tooth bending stresses in the tooth root area and also from classical pitting damage that is initiated at or close to the flank surface and characterized by shell-shaped material breakouts from the loaded flank surface. Furthermore, tooth flank fracture may occur at loads below the rated allowable loads for pitting and bending strength as well as on gears, which have completely fulfilled all the requirements regarding gear material, heat treatment and gear quality according to existing standards. Failures due to tooth flank fracture occur typically in excess of 1O load cycles pointing out the fatigue character of this failure type.
5 Basic formulae
5.1 General
The calculation method for tooth flank fracture load capacity is based on a local comparison of the total occurring stresses (load induced stresses and residual stresses) and the material strength for each considered point of contact and over the material depth. For the herein presented procedure, the occurring stresses are expressed by the local equivalent stress, rff.cp(y), and the material strength is described by the local material shear strength, Tr.Cp(J). The calculation of rcfr.cp(y) and rr,Cp(y) is performed with help of an approximate calculation approach in closed form. This approach was numerically matched with sophisticated calculation methods based on the SIH (see References [1QJ,[121,1131 and IJJ) and was verified by experimental investigations and experiences from industrial application.
The quotient of the local equivalent stress, rcIf,Cp(y), and the local material shear strength, rper.cp(y), is expressed as a local material exposure, AFF.CP(Y). The local material exposure, AFFCP(Y), should be calculated for discrete contact points, Cl’, in the contact area along the tooth width and tooth height and in each considered material depth.y (Method A), If there is no detailed information about the local I-Iertzian contact stress calculated with a 3D load distribution program, the Hertzian contact stress and the resulting material exposure can also be determined with the formulae according to Method B for some specified points of contact which shall be chosen based upon a reasonable distribution of the contact area. Influences of tooth flank modifications on the pressure distribution shall be appropriately considered.ISO/TS 6336-4 pdf download.