spur gear ratios
spur gear ratios
(OP)
I usually see spur gear ratios stated as best for a range between 1:1 and 6:1 (with efficiencies of 98-99%). What happens as you go to higher numerical ratiosn, e.g. 9:1, do efficiencies go down (a little, a lot, enough to make the increased complexity of a 2-stage reduction a better choice)? Where's the sweet spot assuming 20 degree pressure angles?





RE: spur gear ratios
With higher ratios you get pinions that are quite small and gears that are relatively large. This could mean that your pinion ends up with significantly less strength rating than the gear. A gear and pinion with balanced rating means you aren't "overdesigning" one of the elements of the gear mesh.
Also, Big Gear = More Expensive Gear. Mesh efficiency does go down with an increase in ratio. This I believe has to do with an increase in the Specific Sliding Ratio of high ratio gear meshes relative to lower ratio gear meshes. There is always sliding action in a gear mesh but you want to keep it to a minimum since the sliding action creates wear and I believe also contributes to noise.
RE: spur gear ratios
RE: spur gear ratios
My recomendation is to keep your pinion tooth counts high enough to avoid these types of problems. The actual tooth count that you can get depends on the diametral pitch, pressure angle, mating gear number of teeth, loading, and quite frankly how good a designer you are.
Generally with 20 degree pressure angle systems, I recomend novice designers to start with a minimum of 16 teeth on the pinion and work your way down as your can develop your design. In most cases I can accomodate 12 teeth in my designs. You may get to 10 depending on the diametral pitch of the system. If you need to go lower - consider helical gears to increase contact ratio.