Development and validation of analogy specimens for the determination of tooth root load capacity on high-strength case-hardening steels of large size

Experimental investigations on the tooth root load capacity are usually carried out on spur gears in pulsator tests [4, 5, 16]. This allows statistically verified strength parameters to be determined at low cost and in a short time. Typical test geometries are in the size range module m= 3 ... 5 mm. Literature shows that the load carrying capacity decreases with increasing size (module) depending on the material [16, 25, 26]. For gears of large sizes (for case-hardened gears from a module of approx. 15mm), however, widely used pulsator test rigs, in the force range up to approx. 250kN, reach their load limits. Therefore, new test geometries are required in order to determine experimentally verified strength values for gears with a module of 20mm and above. As part of this work, analogy specimens of different sizes were developed using the finite element method (FEM), which exhibit comparable stresses in the critical cross-section both in magnitude and in orientation to test gears in the critical cross-section (30 degrees tangent). Furthermore, test fixtures were also developed with the aid of FEM, which can safely transfer the loads that occur. The calculated analogy specimens were then manufactured and endurance tests were performed by using an electromagnetic pulsating test rig. The samples were taken in such a way that the highly stressed area of the analogy sample was taken from the area of the raw material in which the critical tooth root cross-section would also be present for a gear of the same size. The experimental test results were compared with test gears of standard geometry [6, 21, 25, 26] of module 5 mm and module 10 mm. The test results on the analogy specimens and the corresponding test gears were found to be in very good agreement. The test methodology developed in this work thus represents a suitable procedure for reliably carrying out experimental investigations on the tooth root load capacity of materials with high fatigue strength and specimens of large size. The hereby gained strenght values can be used in designing new gear stages.