Impact of Root Geometry Manufacturing Deviations from a Theoretical Hob Rack on Gear Bending Stress

摘要

High reliability, superior efficiency, and light weight are key requirements of mechanical power transmission systems, such as automotive transmissions. The competing design requirements pose a challenge to gear designers. Rigorous engineering analysis and sophisticated computational tools might be needed to help in finding the best compromise of design parameters and product performance requirements. One important aspect to be considered during the gear design phase is the manufacturing process. Different manufacturing processes can be used to produce gears, such as hobbing, shaping, and milling (Ref. 1). Each one of them has its advantages and limitations. Gear hobbing is a cost-effective and widely used method of cutting gear teeth (Ref. l). This generating process makes both the tooth involute flanks and root fillet. The involute flanks can be finished by a post-process such as shaving, grinding, or honing (Ref. 1). In the hobbing operation, root fillets are generated by the hob rack tip corner. The generated root fillet is not a true radius, but a trochoid form (Ref. 2). An undercut root fillet is formed when the "trochoid lies inside a line drawn tangent to the involute profile at the point of intersection of the involute and the trochoid" (Ref. 3) (Fig. 1). Under certain conditions the trochoid form may intersect the tooth involute flanks above the start of active profile, resulting in undercut (Ref. 4). Undercut is generally considered an undesired result of the generating process because it may affect load distribution and reduce gear load capacity (Refs. 3, 5). Undercut was comprehensively investigated by Su and Houser (Ref. 4), and Pedrero et al. (Ref. 5).