proper selection of grease for gears?

[coppercone2]

With two gears that interface each other, what criterion are there for selection of optimal grease?

Like, surface finish, operating temperature, operating pressure (based on precision of gear geometry and contact area during rotation), rotational speed (relates to kinematic viscosity of grease?), gear material?

Is there really a optimal grease or do you need to make a decision based on some other design choice?

[mgraves235]

In my experience, there is no single "optimal grease" for every application. everything thing you mentioned is a factor that needs to be considered.   Gear material being primary, IMO
in my career i've worked with and had lots of training and experience with drive mechanisms with various type of plastic gears.  nylon gears, in most uses, need to extra lubrication, where as Pet or PETE gears will eat themselves in short order if run dry. plastics on the more brittle end will shed particles as it wears and will mix with lube to make an  abrasive paste that will cause premature gear failure.
sintered bronze gears only need slight lubrication, if any, but there generally used in service replaceable environments, and have a rated cycle life.
hardened steel gears are usually ok with a conventional hypoid lube, but the method in how they mesh becomes a factor, i.e.  a ring and pinion setup requires a different lubrication method than vertical drive gears.

long story short, you are correct in that there are many variable in determine what lube to use in which scenario, and i'm certainly not the authorty to list all the conditions and uses. i remember in machinists school, in one of our textbooks it had a chapter devoted to that subject, i would suggest heading to your library, or looking around online for a similar updated text, and it will educate you far more than someone here can, in a single post.