June 27, 2022
Regular followers of this blog know whenever I say “it all depends,” that I am compelled to credit industry legend Ray Drago for stealing his catchphrase. Upon further review, I have concluded that Ray is just paraphrasing St. Thomas Aquinas’ book on situation ethics. Things are right or wrong depending upon their circumstances. Enough philosophy; back to how to decide on what helix angle to use. Our last posting ended with an account of the complications roller bearings brought to gear and gearbox design. Many opinions on helix angle selection are founded in the designer’s (or organization’s) history with bearings, hobbing machines, or legacy tooling. From a technical standpoint, only the bearing life effect is “science” — the rest is “policy.” Policy (or design practice) is wonderful for simplifying decision making, but can result in time passing your organization by. Reasons behind the “policy” can be invalidated by something as simple as a new piece of capital equipment. You may be operating under rules that are laughable to your competitors and not even know it because they are so deeply imbedded in your old family recipe. A late friend of mine spent his entire career with one company; in retirement, he became a consultant. I helped him learn a third-party computer program by loading up one of my recent designs and nearly gave him a heart attack. “You actually have gear sets out in the field with a 28.765 degree helix angle? Weren’t you ever taught that 20 degrees is the safe limit?” I challenged him to find a single peer-reviewed paper to back up that claim. We both had extensive reference libraries, but it bothered him enough to scan dozens of volumes before admitting there was nothing in the canon to support the “rule” he had first learned 45 years earlier as an engineering intern. Asking around for back-up, he discovered other firms had “baked” different values into their custom software. Some were as low as 15 degrees, which was traced back to their “standard” tooling being a 15 degree transverse DP system with a 20 degree transverse pressure angle (a topic for a future blog). Others insisted 23 or 30 degrees were the absolute upper limit that they would use. I have also talked to people who insisted helix angles had to be integer values such as 8, 10, 12, 15, or 20 degrees; none of that decimal or minutes/seconds nonsense for them. Like my wonderful friend, they were surprised the rest of the gear tribe did not know and respect these fundamental truths. Ultimately, the roller bearing tribe is right: if the numbers fall within the comfort zone of our experience, you are good to go. Higher helix angles do some “good” things for gear capacity while creating larger bearing loads. If the bearings can support the load, the gearbox will be fine.