Servo manufacturer here. Old-ish thread bumped for further discussion.
Actually, Ti (titanium) isn’t as strong as people think ‘and’ it scratches easily (important because gears function by surfaces sliding against each other). Anyway, yes it’s true Ti has a higher tensile strength than steel - per mass - not by volume. This is important because as a practical matter - ounce for ounce - while a piece of Ti is stronger than a piece of steel, to get the ‘same’ strength as steel, that piece of Ti (e.g. a gear) will be larger than than the one made of steel. Digest that because if the gears are the same size, then the Ti-gear is actually weaker than the steel-gear! And with respect to the few grams difference in weight between Ti and steel gear sets, it’s only important in theory, not in the real world.
As background, Ti gained ‘street cred’ due to the SR-71 Blackbird because it captured the popular imagination - especially with aviation enthusiasts. However, there are other real world examples where cost-is-no-object - to include F1 racing transmissions. Guess what? Those gears are made of steel instead of Ti. Why? It’s because steel is more compact for the same strength and it’s not as brittle, which in case it’s not obvious, is exactly why we prefer steel for our servo gears!
Finally, the secret sauce for ensuring the tiny gear profiles within servos - regardless of whether the underlying material is Ti or steel - last a long time is a surface treatment (kind of like case hardening). So not only are steel gears better than Ti because it isn’t as brittle, but once you harden the wear surface of both alloys (ion or plasma nitriding) it no longer matter in the least what’s at the core because both are plenty hard enough to wear a long, long time. In fact, surface treatments are so effective we use them with aluminum alloys for gears as well and guess what? They give good service and last a long time also!
In closing, please allow me an analogy; in football, the halfback is generally more agile than the fullback because he’s lighter. Like a wear coating on Ti and steel, once both are armored with helmet and pads, both can survive the rigors of a hard fought football game. The difference come when you need 1-yard for a critical first down. The coach usually send the fullback to get it - steel is your fullback.
Last thing; if you want to get down in the weeds regarding the pros and cons of Ti vs. steel, you’re going to need a degree in Mechanical Engineering, plus a masters in Materials Engineering, plus you really have to digest something like the Handbook Of Mechanical Alloy Design by Dekker - this just to follow the conversation! My point isn’t to insult robotic hobbyist (many smarter than me) but to observe the ‘real’ reason for Ti gears within servos is because it’s great for getting rubes to reach for their wallets more quickly. Or put another way, marketing!
