Improvement of Tooth Root Strength of Aircraft Steering Drive Worm Gear through Tooth Modification

A single-enveloping worm gear was considered as the final reduction stage of an electro- mechanical actuator for an aircraft steering. When an aircraft is towed by a vehicle on the ground, free castering by towing must be possible. To incorporate a worm gear for castering, a high lead angle is required so that free castering is not hindered. A high lead angle tends to render the contact stress distribution between the worm and worm wheel biased on the worm wheel tooth surface, which in turn, causes the root stress to be concentrated at one end. Therefore, tooth modification was applied to the worm wheel to relieve the stress concentration. Using finite element analysis (FEA), the stress distributions in the worm wheel before and after the modification were analyzed and compared. The 3-D worm gear model used in the FEA was accurately modeled using differential geometry from the mathematical definitions of the worm and hob cutter.