“Design: Nearly all modern industrial gears built in the world use the involute tooth form. The involute shape can be most easily described as the shape of the line that is produced by unwinding a string from a cylinder. See the figure to better understand the principle of the involute.

The involute shape can be applied to two different diameters of gears and yet the two gears roll together while transmitting smooth motion if both sets of gear teeth have the same base pitch. The two gears can then be put together to make a gearset. Base pitch is defined as the distance between the same flanks of two adjacent teeth measured perpendicular to the involute surface. It is pictorially shown in the figure. While both the large and small parts are generically called “gears” when they are separate, the small member is generally called a “pinion”, while the larger member remains known as the “gear” when they are put together in a gearset. By definition, the pinion will have a fewer number of teeth. Because a tooth of the pinion will predictably mesh with a tooth of the gear as the parts rotate, it can be understood how the gear will turn slower than the pinion as determined by the gearset ratio. The gearset ratio can be obtained by simply dividing the number of teeth in the gear by the number of teeth in the pinion.

As the driving gear teeth push on the driven gear, two stress areas are of interest: the Hertzian contact stress in the contact area of the teeth and the resulting bending stress in the root of the tooth. If the contact stress is too high, the result is a gearset with distress on the working flanks. If the stress in the root of the tooth is too high, tooth breakage will occur.

As the gear teeth roll through the contact zone, the contact moves from the tip to the root of the tooth on the gear that is being driven and from the root to the tip on the gear that is driving. Visualizing the figure, it can be seen that the ratio of sliding to rolling is high at the extremes of the line of contact, and that when the contact point coincides with a line between the centers of the gears, the contact is pure rolling motion.

Two of the most important factors in the successful operation of a gear set are the choice of material and heat treatment. The trend is to use the hardest and most durable heat treating process available.”