The universal demand for gears necessitates that they be produced in many geometric forms, numerous materials, and in a wide range of quality levels that will require different manufacturing methods. Cast gears are formed from molten metal; wrought gears are shaped by the deformation of hot or cold metal; metal powder gears are molded from compressed powders and/or extruded; and gears are manufactured by various machine processes. Two manufacturing categories can be considered: large volume gears produced to a finished or near-finished shape in virtually one operation, and any volume of gears produced by a number of machining operations.
Each category breaks down into a number of groups. Whatever the method, the gears are expected to transmit uniform angular velocity. Inaccuracies are unacceptable because they subject the teeth to excessive dynamic overloads. Inconsistencies cause variations that affect the free play of the gears as they rotate. Gear manufacturing must result in gears that have identical interacting features. As the teeth enter and exit the mesh, a high frequency ripple is created whose magnitude is influenced by the tooth accuracy and flank surface finish. Current manufacturing methods allow the manufacture of gears to any quality within the full range of established levels, from AGMA A14 (Q3) through A2 (Q15). In the year 2003 AGMA adopted a similar numerical system to ISO, introducing an A quality grade to replace the former Q.
Commercial gearing is usually produced in the quality range of AGMA A11 (Q6) to AGMA A7 (Q10), carburized, hardened, and finish ground gears to AGMA A6 – A5 (Q11 to Q12), with high powers combined with high speed requiring the highest quality AGMA A3 – A2 (Q14-Q15). As higher accuracy is required, the number of requirements also increases. Medium accuracy requires the additional parameters of total helix and total profile. High accuracy includes parameters for slope and form for helix and profile.
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 50 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/DP
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 workpiece 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. We will continue our discussion in the April issue of Gear Solutions magazine.