June 14, 2022
The initial motivation for changing outside diameters was the avoidance of undercutting. Designers, engineers, and theoreticians quickly discovered that the practice had other advantages — plus a few problems. On the plus side, the tooth strength of the “enlarged” part increased. This is a result of increased tooth thickness and a higher transverse pressure angle. A slight reduction in tooth strength occurs on the “contracted” member it mates with. We’ll discuss this in greater detail in another posting, but for now please remember that you have to calculate tooth bending strength on both parts before declaring victory. The tooth shape changes with diameter modification, and one upper limit on outside diameter is the thickness of the “top land;” at some amount of enlargement the teeth become pointed and nothing the designer does can reverse that. Most modern computer programs check for this problem and provide error messages when the “safe limit” on top land is breached. That “safe limit” is subject to some debate, but all agree that pointed teeth are unacceptable. Over time, our understanding of the effects of addendum modification has grown and methods have been developed to calculate the “best” amounts of modification to achieve balanced bending strength, best durability rating, the minimum to avoid undercut, and the most scoring resistance. Not every method or goal is universally agreed to within the community of gear “experts.” This is why designers need to understand the basic principles and not blindly obey the directions of the software. A “fatal error” on top land thickness to one expert may not be a serious concern to another. Scoring may not be an issue for your application. You may have assembly reasons for having a specific outside diameter. Ultimately, each designer has to understand the consequences of their choices. Do not risk the success of your device by relying on inherited “rules of thumb” or messages from a commercial computer code.