Effect of sursulf treatment on the performance of 0.14% C steel gears

Experimental and analytical study of gear micropitting initiation and propagation under varying loading conditions

Abstract: Micropitting damage is one of the failure modes commonly observed in gears leading to destructive failures, which in turn results in unplanned shutdown and expensive replacement, such as those observed in wind turbine gearboxes. This study investigates gear micropitting initiation and propagation when subjected to varying torque loads under a constant rotational speed. The study employs both experimental gear testing and analytical evaluation based on the ISO Technical Report of Gear Micropitting, ISO/TR 15144-1:2010 and the recently revised ISO/TR 15144-1:2014. Initiation and propagation of micropitting are assessed in testing by quantifying the development of micropits and their progressive rate after specific numbers of running cycles at step-up torque levels. The analytical study is conducted to validate the prediction of micropitting using the ISO/TR recommended procedures by comparing the results with the occurrence of micropits in the tested gears. The gear test results show that micropitting initiates at the pinion dedendum but escalates at the addendum, because of the greater severity of progressive micropitting at the dedendum of the mating wheel where the tip relief area first comes into mesh. The analytical results, based on varying surface roughness measurements obtained from the tested gears, confirm that the maximum contact stresses and minimum specific lubricant film thicknesses occur in these regions. The specific lubricant film thickness varies considerably because of changes of surface roughness after gears are subjected to various running cycles under varying torque levels. It has found that the excessive loading, gear tooth micro-geometry, surface roughness and lubricant film thickness are the main factors affecting micropitting.

The influence of contact stress distribution and specific film thickness on the wear of spur gears during pitting tests

Abstract: One of the main gear damage mechanisms is the formation of pitting and spalling on the tooth flank. Several factors have significant influence on the damage formation, such as: contact stress level; tooth profile type; relative contact speed; surface finish and lubrication conditions. This work comprehends the global observation of all such parameters and was carried out to explain the phenomena related to this wear mechanism. The wear test equipment uses the power recirculation principle and is commonly known as FZG test rig. The gears were made from AISI 8620 steel and had two types of surface finishing (by shaving or by milling). The wear experiments were performed with two torque stages: 135 N.m (running-in) and 302 N.m (steady-state), and two test temperatures: 60oC (running-in) and 90oC (steady-state). The wear level was determined by using image analysis. In order to calculate the specific film thickness and friction coefficient, the roughness of tooth flank was measured at each test stop. After the experiments were completed, it was possible to confirm that, for both manufacturing processes, the boundary lubrication regime was adopted at the tooth flank and the specific film thickness presents a different behavior when compared to addendum, pitch diameter and deddendum regions. The wear on the gear flanks depended on the lubricant film thickness and it was higher for the milled gears.

Advanced Mechanics of Materials

Abstract: Introduction. Theories of Stress and Strain. Linear Stress-Strain-Temperature Relations. Inelastic Material Behavior. Applications of Energy Methods. Torsion. Bending of Straight Beams. Shear Center for Thin-Wall Beam Cross Sections. Curved Beams. Beams of Elastic Foundations. The Thick-Wall Cylinder. Elastic and Inelastic Stability of Columns. Flat Plates. Stress Concentrations. Fracture Mechanics. Fatigue: Progressive Fracture. Contact Stresses. Creep: Time-Dependent Deformation. Appendix A: Average Mechanical Properties of Selected Materials. Appendix B: Second Moment (Moment of Inertia) of a Plane Area. Appendix C: Properties of Steel Cross Sections. Author Index. Subject Index.

Pitting of Gears and Discs

Abstract: Pitting tests using 127 mm (5 in) centres distance gear rig under controlled conditions are described. These are compared with similar disc tests using the same materials and lubricants. Tests of both types confirm Dawson's conclusion that an important factor influencing pitting life is the ratio of surface roughness to the calculated lubricant film thickness. It has been shown that using a hunting tooth ratio, particularly when associated with a rough harder surface and a surface finish oriented normal to the motion, increases the likelihood of wear. Most importantly, the results show that using disc tests can greatly overestimate the pitting life of gears. These experiments and other evidence from the literature suggest that the most probable reason for these differences between gears and discs lies in dynamic gear loads.