In the last blog posting, I covered the importance of developing a Product Specification and getting all the “stakeholders” to agree on what a successful outcome would look like. In sports, we might compare that step to going over the ground rules. No one wants to lose momentum by having to stop and argue over how many points you get for a field goal, where the sidelines are, or where the finish line is. The good news is, you do not have to invent a new “game” every week; you can reuse and modify the product specifications that have worked well in the past.
Similarly, your design work — or as I like to call it, the fun part — will build on the successful efforts of the past. For example, you will know how many stages of gearing your device will need. The physical arrangement of the required input and output interfaces will inform your choices for the types of gears needed. That “family recipe” we have talked about previously will guide you in creating geometry for each stage.
Those “preliminary” gear designs now have to be tested against that product specification. Is the overall reduction ratio within tolerance? Do the “numbers,” safety factors or projected service life, meet requirements? Are the de-rate factors, quality levels, and material assumptions correct in the computer input? Be especially careful with duty cycles and variable load/speed applications to test all the possible “worst conditions.” Double check the availability of cutting tools. Also verify that your gear sets will fit within the interface.
Why so many cautions? Because the next step in the process will not be much fun if part way through it — or after you have completed it — the study layout gets blown up by a very preventable error. There are sufficient pitfalls ahead that you will be kicking yourself over if you start off with a built-in error. I often describe the design process as a three-dimensional Sudoku puzzle; you do not want to start off with a basic error in arithmetic.
Designing the gear sets is often the easiest part of a project. And this makes perfect sense when you consider the investment made over the years in standards, tooling, and software. We, as a community, really know about gears. The challenge is in integrating those beautiful gears into a system that can deliver reliable service over the entire design life. That means we have to deal with shafts, bearings, seals, housings, retainers, shims, couplings, and lubrication. We have to think about how the device will be machined, how it will be assembled, and how it will be maintained for many years under field conditions.
Gear design is the “fun” part of the job. Our whole magazine is devoted to helping you create the best gears in the world. Unfortunately, we have much less content on the rest of the gearbox; more thoughts on that next time.