Tooth Tips: William Crosher

July 09, 2013

When laying out the sequence of operations, the teeth must run true with the diameters and faces that will be used for their actual mounting.


When the face width exceeds 24 inches, a minimum of three supporting ribs or webs is required. The rim thickness must provide full support for the tooth root. The gear arms are subject to complex stresses, and must transmit the torque between the hub and the rim. In addition, the arms have to resist any side bending from thrust loads and axial tension from the centrifugal forces.

The gear hub must be of adequate section for the shaft diameter and keyway, also providing a rigid support for the supporting arms of the wheel. The hub length is usually from 1.25 to 2 times the shaft diameter in order that the teeth run true without any wobble. The wheel must have the strength and rigidity to withstand the forces that will be applied to the teeth. When gears are to be carburized, the usual practice is to add pining along the joint. Table 1

Based on the quality specifications, tolerances are set for the critical gear dimensions that will include the outside diameter, bore, face, total runout, and length through the blank. Maintaining these tolerances throughout the manufacturing process is of fundamental importance, as is the necessity for a complete inspection of the blank before any gear cutting. It is considered more economical to produce blanks with tighter (minimum) tolerances than the gear that is to be produced. Table 2

When laying out the sequence of operations, the teeth must run true with the diameters and faces that will be used for their actual mounting. It is, therefore, practical to mount the blank using the same locations that will be used to mount the gear in its finished state. In the image embedded in this column, AGMA provides tolerances for a variety of quality specifications {Ref: AGMA 390.03} {Din: 3961}. Not all gear operations are capable of achieving the higher quality levels. Each method achieves above average quality levels only with special attention to the manufacturing process. 

About The Author

William P. Crosher

is former director of the National Conference on Power Transmission, as well as former chairman of the AGMA's Marketing Council and Enclosed Drive Committee. He was resident engineer-North America for Thyssen Gear Works, and later at Flender Graffenstaden. He is author of the book Design and Application of the Worm Gear.