There are a number of factors that go into gear design. In simplest terms for regular spur gears, you need to size the tooth to handle the loads it will see. Root stress, bending stress, contact ratio(how many teeth are in contact at once), etc. Using the input torque (from the handles) and the resistive torque from the wood, you translate that into the force that occurs at the point of contact on the tooth. You can then calculate the stresses to determine if the stress is within the yeild stess of the material with sufficient margin of safety(for the given ratio etc). Once you've found the suffecient tooth design, the ratio used will determine the diametral pitch and the overall diamter of the gear. So overall diamter, or the more commonly used pitch diameter, is more a function of the tooth design and ratio needed. I actually started out designing gears in my first job. It's been awhile, but I could pull out my books again if I needed to.
I've been wondering if any of the boring machine gears are worth quoting a limited run? If a gear company could hob or shape some gears at a cost people would be willing to pay. Has anyone tried this? What gears would be first on the list to have done? I was just thinking...
The diametral Pitch radius would factor into the calculation of torques to forces and visa versa, and the caculation of stress on the tooth.
There are a few different terms when dealing with bevel straight gears, like on the boring machines, but the concepts are basically the same. Bevel gears allow you to change the direction of motion by 90 degrees per pair, but the drawbacks are that there are higher toques on the shafts for bevel gears, the mounting distances must be accurate so that the teeth mesh properly, and one wheel of such gear is designed to work with its complementary wheel and no other, without having to change the center distances/mounting distances.
