To maximize the functionality of a gearset while managing cost and maintenance, it is important to consider many different factors while selecting the gears. This includes the operating loads, operating environment, available lubrication, system cost, durability requirements, and noise requirements. The following list has been summarized from a KHK Gears webpage: HOW TO SELECT GEARS.
Select a gear type and tooth profile
If the input and output shafts are parallel and non-intersecting, spur gears are an excellent choice if noise is not a concern. They are cheaper and lower cost than other options.
If the input and output shafts are parallel and non-intersecting, but noise and vibration is a concern, helical gears should be used.
Select a gear material
If loads are light and NVH is a concern, as is the case with small consumer appliances, automotive seat and latch actuators, or toys, plastic gears are a good choice.
If loads are high and costs are not a concern, ground, hardened steel gears are an excellent choice. Steel gears can also be shaped, hobbed, or milled.
For applications where the gears cannot be submerged in oil or lubricated regularly, bronze or brass gears can be paired with steel gears to mitigate wear.
Select a manufacturing process or gear vendor
For mass production, injection molded or sintered / pressed metal powder gears are a good choice.
For gears requiring moderate precision, metal gears can be hobbed or shaped. The resulting tooth to tooth error is larger than grinding, but acceptable for most applications.
For gears requiring high precision, metal gears should be ground. This process produces the most precise gears, especially if top of the line machines are used.
Ontario Drive and Gear, located in New Hamburg, Ontario, produces metal gears using hobbing, shaping, and grinding processes. They can produce and measure gears and shafts to tolerances less than 10 microns.
RushGears.com produces a variety of gears on short turnaround times using a variety of processes, including grinding.