Single tooth bending fatigue testing at any r ratio

Single tooth bending fatigue (STBF) testing has long been used as a simple and cost effective alternative to running gear bending fatigue tests. In typical STBF testing, the test tooth root fillet is subjected to a cyclic tensile load until bending failure occurs, while the test load is reacted with an opposing tooth along the line of action. This results in a rather simple fixture design, however the typical STBF fixture design does not allow for any reversal of the loading in order to stress the tooth roots in compression. The stresses in the root fillet for this type of test vary from a maximum tensile value to a percentage of the maximum, resulting in a positive R ratio (ratio of minimum to maximum stress). It is well known that the root area of a gear tooth is subject to both tensile and compressive stresses as the tooth approaches and subsequently meshes with mating teeth. For a single pair of meshing gears, the loading experienced by the root fillet results in a slightly negative R ratio in practice. For idler or epicyclic planet gears, stresses are fully reversed and the extreme case of an R ratio of-1.0 occurs. The Single Tooth Reversible Bending Fatigue (STRBF) test outlined herein overcomes previous limitations by allowing compressive stresses to be applied to the test tooth root in any magnitude in conjunction with the typical tensile stresses. This test setup involves three teeth of the test gear, with the upper and lower teeth providing the reactions in the up and down load directions and the test tooth being subject to test loads in both directions. Any R ratio applicable to gear bending fatigue testing up to and including fully reversed loading (1 R-1) is possible. Non-dimensional examples of fatigue data from a recently completed fully reversed testing program are shown.