The gear blank design is frequently influenced by the cost. The ideal blank is finished with the final bore perpendicular to the face and concentric to the outside diameter. The gear blank can be 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, and 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 50 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 hardnesses 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. Figure 1
Weldments are generally limited to pitch line velocities of 25,000/fpm. Every welded gear blank should be stress relieved. 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 two 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 pinning along the joint. The accompanying figure shows welded and pinned blank designs.
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 (see figure), as is also 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.
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.