I need to define the gear train that drives the camshafts and magneto (distributor) from the crankshaft. My goal is to define the pitch and tooth count of all the gears.
Below is a picture of the Midget Offy’s gear train.
Source:https://www.youtube.com/embed/A0KM6XxpNIg?wmode=opaque&start=0
The constraints for the gear train include:
- I would like to use a “standard” gear pitch. The smallest that can be considered standard is either a 48DP or a .5 Module (which is about 50DP).
- The camshaft has to turn 1/2 the speed of the crankshaft.
- Would like to have a single plane gear train for simplicity. The Mighty Midget has a dual plane gear train as can be seen in the photo above. The dual gear in the gentleman’s hand is the idler between the crankshaft pinion and the gear train in the gear tower.
- The camshaft gear, as well as all of the others, has to fit in the gear tower housing
- The crankshaft should have as many teeth as possible.
- There should be as few bearing sets as possible. This is an advantage of a two plane gear arrangement, multiple gears can use a single bearing set. In a single plane gear train this means the number of gears between the crankshaft gear and the camshaft gear should be minimized.
- A plus would be to have as few gear types as possible, also “standard” tooth counts would be preferred.
- There are other mechanical constraints such as the 88 degree inclusive angle between the two valve banks and the distances between the camshaft and crankshaft.
- Also the magneto needs to turn at the same speed as the crankshaft.
If a single plane gear train is used, the camshaft gear has to have twice the number of teeth as the crankshaft gear. It does not matter how many gears are in between or how many teeth they have. I made an excel spreadsheet to prove this to myself. I think I will settle on 48DP, .5 Module does not give me a measurable advantage in tooth count. The largest camshaft gear that fits has 32 teeth; this gives me a 16 tooth crankshaft pinion. I would prefer to have more teeth, but this is acceptable. I might design a dual plane gear train just to see if it would be worth it.
I will be using the gear pitch circle in my diagrams, this diameter is calculated as the number of teeth divided by the diametrical pitch. If I am using 48 DP then it is easy to translate back and forth between pitch diameter and number of teeth. My go to book for info on gears is Ivan Law’s “Gears and Gear Cutting”.
The above example uses a 16 tooth crank pinion, 32 tooth camshaft gear used in three places and two 50 teeth gears. Below is another option, but it does not fit in the gear tower as the idler gear interferes with the highlighted bolts.
Would like to see if I can fit an 18 tooth pinion and 36 tooth camshaft gear. Below I try this with the gear tower housing.
Opps, the gear driving the magneto needs to have the same number of teeth as the crankshaft; the magneto needs to turn at the same speed. This is because the magneto will be used as our distributor and this is a four stroke. I add this to the list of constraints.
Also I have been using the gear’s pitch circle, when we look at the gear outside diameter, as below, we find that the 36 tooth gear does not really fit. The distance between the gear and the outside wall is only 1/16″ of an inch, too small. Back to the 32 tooth cam gear and 16 tooth crank pinion.
With this configuration I have 4 gears between the crank pinion and the camshaft spur gear, I think this is the minimum. I have four different tooth counts, 16, 32, 40, and 56 with a diametrical pitch of 48. I will move forward with this configuration for now, I believe it meets all of the criteria I laid out. I may find more issues as I make more progress elsewhere.
This is where I ended up on this Christmas Eve.