I spent today revisiting my choice of 48DP gears for the timing gear train. 48 DP means that a 48 tooth gear will be 1 inch in diameter (really the theoretical pitch circle is 1 inch, the tangent circles where the gears mesh). The alternative is to use Module .5 gears. .5 module means that a 48 tooth gear will have a pitch circle of 24mm(or .9449 inches). That is, 48 teeth * .5 Module = 24 mm pitch circle.
Ron brought up a good point that made me question my original decision. In my earlier post about the gear train, I laid out nine criteria for the design. Ron is suggesting another one: Use gear cutters that are readily available and inexpensive. I did an internet search and I also found that Module .5 gear cutter sets are more available and less expensive (if you are willing to buy from China or Новосибирск in the, Russian Federation) than 48DP gear cutters. I make my own gear cutters, but if another builder wants to buy gear cutters or gears, metric gears may be a better choice, even in the US.
It was actually quite straight forward to pull up my old 48DP gear study and overlay a couple alternative Module .5 analysis.
My previous solution was to use 5 gear types, 16, 30, 32, 40, and 56 with a diametrical pitch of 48. By using Module .5 gears I can still get away with 5 gear types, 18, 28, 36, 40, and 64. teeth. I like this solution for the following reasons:
- .5 Module Involute gear cutters are less expensive than 48 DP gear cutters.
- Still have the same number of gear types.
- The crankshaft Pinion will have more gear teeth, 18 instead of 16 and will be ever so slightly larger.
- The basic geometry is not impacted, for example there are still 4 gears between the crankshaft pinion and the camshaft spur gear.
The down side is the magneto drive train is a little close to the edge of the gear tower and the engine is 1/16″ shorter.
The other change this will drive is the gear shaft diameter and metric bearings. I had settled on 3/16″, but metric gears in this size typically use 5mm shafts, so instead of a 3/16″ X 3/8″ X 1/8″ bearing. Bearings with a 5mm ID commonly have a 2.5, 4 or 5mm thickness. The timing gear tower was designed around the 1/8″ bearing thickness, so this also needs to be adjusted. Running some numbers, a gear with a width of 5mm and two bearings with a width of 4mm each will fit nicely in the timing gear tower. So MR115-2RS Bearings with 5mm ID, 11mm OD and 4mm thickness should work.
Original 48 DP option above
.5 Module solution with 6 gear types above, clean magneto gear mesh
.5 Module solution with 5 gear types, not optimal magneto drive gear mesh.
I have three solution:
the 48DP option that has:
- 2 ea 16 tooth gear
- 1 ea 30 tooth
- 4 ea 32 tooth
- 2 ea 40 tooth
- 2 ea 60 tooth
A .5 Module options has:
- 2 ea 18 tooth gear
- 1 ea 32 tooth gear
- 5 ea 36 tooth gear
- 1 ea 48 tooth gear
- 1 ea 56 tooth gear
- 1 ea 64 tooth gear
A second .5 module solution with fewer gear types:
- 2 ea 18 tooth gear
- 1 ea 30 tooth gear
- 5 ea 36 tooth gear
- 1 ea 40 tooth gear
- 2 ea 64 tooth gear
I spent way too much time on this today, I think I will noodle on it a bit more before committing one way or the other.