A LENGTHWISE MODIFICATION FOR FACE-HOBBED STRAIGHT BEVEL GEARS

Face hobbing has been successfully applying in manufacturing straight bevel gears using a virtual hypocycloidal straight-line mechanism. This method is a continuous indexing and double-flank cutting process, and is recognized its high productivity and precision. In order to improve gear contact condition, three types of flank modifications are frequently used in gear industry: profile crowning, lengthwise crowning, and longitudinal twist. In the design of the spiral bevel and hypoid gear, under satisfying a specified accuracy requirement, three types of modifications are blended properly during gear design to absorb assembly and manufacture errors. Circular cutter blades are normally adopted to accomplish the first type modification. The second can be achieved by a cutter radius change or a cutter tilt with adjusted pressure angles. The last can be achieved by a cutter tilt or modified tool path (for example, helical motion and modified roll). This paper proposes a lengthwise crowning method for face-bobbed straight bevel gear (SBG) using a hypocycloidal mechanism. This modification is applied to the pinion only. A numerical example drive with point-contact tooth surfaces is adopted to validate the proposed mathematical model. Finally, two evaluations, ease-off topography and tooth contact analysis (TCA), are made to investigate the contact condition of this numerical case.