Current thought is that the main sources of dynamic excitation in spur and helical gearing occur along the line-of-action and are due to time varying tooth stiffness and static transmission error. This paper examines friction forces as a potential dynamic excitation source in the gear mesh of involute parallel axis spur and helical gearing. The friction forces act in a direction perpendicular to the line-of-action, defined as the off line-of-action direction. To support the claim that friction force is a potential dynamic excitation source, experimental evidence is presented in the form of shaft motions measured near the support bearings in the line-of-action and off line-of-action directions. These experimental results show that the off line-of-action motion is of the same order of magnitude as the line-of-action motion and at times the off line-of-action motion at gear mesh frequency is several times larger than the line-of-action motion. The motions are related to the forces through the bearing stiffness matrix. Other potential explanations of the large off line-of-action shaft motion such as bearing cross coupling phenomena, reduced bearing stiffness, bearing clearances, and system dynamics are examined. The results suggest that through a combination of a small friction force excitation at the gear mesh, and the force transmissibility properties of the gear-bearing system, the friction force can produce forces at the bearing comparable to those generated by transmission error excitations, particularly at low to medium speeds.
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