![]() The next step is to locate the “pitch line,” drawn as an extension of the common tangent cone element, for the gear set. The intersection point of the axes is the “pitch apex” (Figure 1). The vast majority of bevel gears are designed for a shaft angle of 90 degrees, which we will use for our examples. Step one in drawing any bevel gear set is to lay out the pinion and gear axes at the desired shaft angle. The curved teeth are set at an angle of zero to 10 degrees. ZerolÆ bevel gears are a special case of spiral bevel gears. ![]() In a similar way, spiral bevel gears are represented by two tangent cones rolling on each other without slipping. Helical gears can be theoretically represented by two tangent cylinders rolling together without slipping. Whereas helical gears are characterized by straight teeth set at an angle on an imaginary pitch cylinder, spiral bevel gears are characterized by curved teeth set at an angle on an imaginary pitch cone. A comparison can be made to helical gears, which also have their teeth set at an angle to their axes. Spiral bevel gears feature curved teeth that are set at an angle, generally 35 degrees, to the axis of the pinion and of the gear. It is the connecting step between the gear tooth design and the creation of the rest of the structure - the gear blank, its bearing arrangement, input or output features and, ultimately, the housing itself. Taking the data listed on a gear dimension sheet and converting it into a drawing is the key next step. This can be an iterative procedure involving several sizing trials before a dimension sheet is finalized. The resulting estimated life is compared to the requirement. An initial size with pitch, face width, tooth pressure angle, and spiral angle is chosen, and the bending stresses and contact stresses for the resulting design are calculated. This information includes the applied loads, speed, shaft angle, offset, gear materials, heat treatment, lubrication method, operating temperature, required life, and operating conditions affecting the driving and driven loads. To create a dimension sheet, certain information is usually required. Publications on the subject are also readily available from the AGMA (American Gear Manufacturers Association) or The Gleason Works. As a gear design and manufacturing company, Nissei provides gear dimension sheet information to our customers as part of our service. This basic information is usually available in the form of a gear “dimension sheet,” which may have been calculated by your own gear engineer. See the appendix at the end, and in Part 1, for the definitions of these items. Each guide assumes that the basic gear tooth design information is already at hand (shaft angle, pinion and gear pitch diameters, outer cone distance, face width, pinion and gear pitch angles, face angles, root angles and the outer pinion, and gear addendums and dedendums). ![]() This is the second of a three-part series of instructions to assist engineering designers and detailers with the process of correctly laying out bevel and hypoid gear teeth.
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