Gears must have the rigidity and strength to resist a complex network of stresses. Steel weldments can meet these requirements and offer the designer economies in weight and cost when properly designed. North American practice is for blanks larger than 50 inches to be of welded construction. The pressure between the contacting teeth generates several forces that act on all the gear’s components. 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 torque produces a bending moment on the arms, and the assumption is that it is uniformly divided among all the arms. Each arm is considered to be a cantilever beam fixed and supported at the hub and loaded at the pitch circle. Torque is not used directly, instead the maximum bending moment is used that is directly proportional to the tooth’s tangential force. A problem with the analysis is to determine the arc length of the rim segment that is to withstand the radial force. A conservative approach is to consider the arc length as equal to the diametral pitch by assuming one tooth will resist the full bending moment that results from the radial force. As this estimate affects the cross section’s moment of inertia, an engineer’s judgment is required to determine the amount of any modification to this approximation.
Improved welding methods like buttering make the welding of gear steels such as 4140 and 4340 practical. Rim hardnesses of 300-340 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 and 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 join the ring to the center. To ensure the soundness of the weld after the first and final weld pass, magna-fluxing is necessary. Weldments are generally limited to pitch line velocities of 25,000/fpm. Every welded gear blank should be stress relieved. Different steels are easily welded. 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. Minimum rim thickness below the root circle usually equals the tooth thickness at the pitch circle.
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 so 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. Web plates can be placed between the two discs to provide an I-beam section. This provides the gear with sufficient rigidity in line with the shaft to absorb the end thrust force.
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, and reference tolerance charts are available. A complete inspection of the blank before any gear cutting is also necessary. It is considered more economical to produce blanks with tighter (minimum) tolerances than the gear that is to be produced. A difficulty in specifying welded joints is the determination of its quality. The welder’s experience and skill is critical. There have also been considerable advances in welding equipment, and with remote current capability the settings are within the machine. An inspector should look for porosity, incomplete fusion or joint penetration, an unacceptable weld profile, and existence of cracking. Five key factors include environment, skills, methods, equipment, and materials.
