In the movie “Field of Dreams,” the character Ray Kinsella is told, “if you built it, he will come.” In engineering, there is a belief that if you can design something, it can be built. Most engineers use tools such as computer aided design (CAD) in order to create a visual representation of the mechanical system and the corresponding components. Unfortunately, the ability to create a visual representation in CAD does not guarantee manufacturability.
When producing a spur gear or a helical gear, the gear teeth are produced such that they are parallel to the axis of the bore. The gear needs sufficient material between the bore and the hub in order to maintain manufacturability. For example, if we choose a module 2, 24-tooth spur gear, the maximum recommended hub diameter is 38mm. If the design calls for the bore to be opened to 36mm, this will leave a hub thickness of only 1mm. A 1mm thickness will be too thin to machine without distortion. If the design requires a tapped hole in the hub in order to fix the gear to the shaft, this thickness of 1mm will not permit enough threads to exist in order for the set screw to hold. If the same gear has a bore of 30mm and a standard 6mm keyway, there will only be 1.2mm of material from the center of the keyway but there will be no material in the corner of the keyway.
Another design flaw is to detail a spline or keyway size that does not belong to a recognized standard. For spline bores, the applicable metric standards are ANSI B92.2M, ISO 4156, DIN 5480, DIN 5472, and JIS D 2001:1959. Although you can draw a spline of any size, deviating from these standards will result in the need for custom tooling. For keyways, the applicable standards are ANSI/AGMA 9002-B04 and ISO/R773. For a standard 25mm bore, the ideal keyway is 8mmx3.3mm. There is nothing to stop a designer from specifying a 7.5mm or a 9mm key width except for the lack of a key in that size.
A common design issue is the introduction of a D shaft on the motor as it requires a corresponding D-shaped bore on the drive pinion. When a motor shaft has a key slot, it is easy to add a keyway to the gear bore and assemble the pinion to the shaft using a standard key. When a motor shaft has a spline, it is necessary to machine the mating spline form into the bore of the drive pinion. Provided that the spline standard is known and the drive pinion has the proper machining clearances, the pinion can be properly assembled.
However, there is currently no standard for the D shape on a motor shaft. A motor with a 10mm shaft might have the D shape height that varies anywhere between 6.5mm and 8mm. Since there are no established standards for D shapes, every corresponding drive pinion needs to be custom made. Additionally, the process of producing a D shape bore on a gear is not practical for the machining of the teeth. In order to machine the gear teeth, the gear must rest on a shaft that matches the bore. This means that for every unique D bore shape, a unique arbor needs to be developed. Alternatively, a gear can have the teeth hobbed first and then the bore produced. However, producing a gear this way can result in a degradation in gear quality as the teeth and the bore are produced independently.
The concept of using CAD in gear design is a great tool to visualize the design concept. Unfortunately, a paper design is just that and does not insure manufacturability.
