One of the main sources of noise in a gearbox is the variation of the tooth meshing stiffness. This variation causes a deviation from the nominal transmission ratio at the output of the gearbox termed transmission error.

A first approach to reduce the transmission error is to find a gear geometry< with integer contact ratio µ± and/or overlap ratio µ≤. However, due to the varying deformation of the teeth under load additional measures should be taken. The classical profile modification is a tip relief which is produced by a section on the tool with a larger pressure angle. A new development is a wave-form-like profile modification which is generated by form grinding. Specialised software like KISSsoft should be used to find the right form and amount of modification.

Fig. 1 shows a typical pair of gears. The transmission error for this example is presented in Fig. 2 as it is calculated in KISSsoft. The upper part of Fig. 2 shows the results for the unmodified tooth form, the lower part shows the altered transmission error due to a tip relief of 26º. In Fig. 3 the effect of the new form of modification is shown: the extra ‘bump’ on the flank causes a larger transmission error for lower loads, but compared to Fig. 2 the error for full load is significantly reduced.

KISSsoft AG is based in Hombrechtikon, Switzerland. www.kisssoft.ch