Tooth Root Testing of Steels with High Cleanliness

ABSTRACT

The power density of gearboxes is continuously increased through different research activities. Besides new material developments, steel cleanliness comes to the forefront in order to meet future requirements regarding load carrying capacity of gears. The experimental quantification of load carrying potentials for high quality steels is the basis for introducing cleanliness as a design parameter. 

In this paper, investigations on the tooth root load carrying capacity of steels with different cleanliness levels are presented. The investigations are carried out on a pulsator test rig with a standardized FZG-C gear geometry. To determine and compare the different behaviors of the tested steels, correct force application in the test rig needs to be ensured. By this, it is possible to clearly separate the endurance strength for different cleanliness levels within the same steel grade composition.

For the pulsator testing, an approach for checking and ensuring correct clamping of the gears is presented. Using this procedure, endurance tests on conventionally manufactured gears with different cleanliness levels are carried out. Resulting mean values of the tooth root strength as well as scattering of test results is evaluated, and the influence of higher cleanliness on an increasing mean value and decreasing scattering is proved. The confidence level of the mean value is discussed regarding the overall number of tests.

As a conclusion, the impact of steel cleanliness on increasing endurance strength and decreasing scattering is separated from manufacturing and testing influences. A higher level of cleanliness takes into account the influence of the occurring failure mechanisms. Especially for applications with a high manufacturing and surface quality, high quality steels show a high potential for increasing the load carrying capacity and thereby the power density of the gearbox.