Tooth Tips: William Crosher

June 11, 2013

When a gear is produced by machining, it must begin with a blank that is properly designed and produced in full accordance with the specifications.

When a gear is produced by machining it must begin with a blank that is properly designed and produced in full accordance with the specifications.  The blank is subject to tangential, separating, and axial forces during the manufacturing process, and must therefore be resistant to distortion from these stresses.  To avoid distortion the blank design must avoid thin cross sectional areas.  

Gear calculations currently do not determine the critical fatigue stress for gears that are thin rimmed or have special blank geometry.  Rim thickness is defined as the shortest distance from the root of the tooth to the bore surface.  Keyways are taken into consideration as they reduce the section.  If the rim thickness is undersized distortion will occur with or without the keyway.  The section of material under the root should be no less than the tooth height and, preferably, one and a half times this height. 

The recommended minimum rim thickness for gears with a larger outside diameter than fifty inches = 0.03in × OD.  As further insurance against the development of a crack through the rim a minimum thickness based on tooth size is established as:

                    Rim Thickness = 3 inches            
Inferior gear quality frequently results from poorly designed, and/or inaccurately produced blanks.  When the blanks are stacked together and the bores are not perpendicular to the sides, problems increase.  A Chicago manufacturer purchased their blanks with one face ground allowing them to be stacked together during the hobbing process.  The outsourced blanks were inconsistent in bore quality and face parallelism.  When the teeth were cut all the gears were scrapped at a financial loss to the manufacturer.  Many gears are produced on blanks that are located by their bore on the cutting arbor.  When the bores are not a tight fit, eccentricities are inevitable. Should the blank run eccentric to the operating centerline the teeth will not be of the required geometry.  Throughout the gear’s production the blank provides the centerline and locating surfaces.  The blank must permit the work piece to be supported just inside the gear's root diameter during the tooth cutting.  Normal practice is to finish the blank in a lathe prior to the cutting of the teeth.  The locating face or faces must be finished with the final bore perpendicular to the face and concentric to the outside diameter. 

The gear blank design is frequently influenced by the cost.  The ideal blank is solid, machined and capable of being through hardened or surface hardened.  This design reduces the centrifugal stress limitations present with fabricated blanks.  The method of preparing the blank is influenced by the quantities.  Low volume gears can be produced from bar stock, medium size runs by forgings or welded blanks, high volume runs by forging, casting, extrusion or powder metallurgy.  Cold extrusion is the most popular method used for high volume automotive gears.     

Many gear blanks with an integral hub are produced from a cost effective open-die forging.  It is a fairly simple matter to provide changes in the dimensions of the forged blanks to reduce the amount of surplus material to be removed.  Frequently die-forging is the best way to produce quantities of gear blanks with an integral hub.  A wide variety of other methods are also available for the production of blanks such as machining from bar stock, castings, extrusions and cold forming.               

North American practice is for blanks larger than fifty inches to be of welded construction.  Improved welding methods, such as buttering, make the welding of gear steels such as 4140 and 4340 practical.  Rim hardness’s of 300 to 340 BHN are usual.  When the tooth hardness exceeds the range 300-340HB, additional material and strengthening is required.  The rim may be an alloy steel rolled ring, heat treated to the required tooth hardness, while a lower cost steel provides the strength and rigidity in the ribs.  The two main full penetration welds joining the ring to the center.  To ensure the soundness of the weld after the first and final weld pass magna-fluxing is necessary.  Welded blanks are generally limited to pitch line velocities of 25,000/fpm.  Every welded gear blank should be stress relieved. 

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.