Gears have played a role since the invention of rotating machinery. Because of their force-multiplying characteristics, they were used by early engineers to lift building materials. Gears were also used to hoist ship anchors and for catapult pretensioning. Another application was wind and water-wheel machinery, as the rotational speed could be increased or decreased as required for the specific machine to be powered.
The earliest gears were made from wood with cylindrical pegs for cogs and were often lubricated with animal-fat grease. Metal gears saw rapid development in 18th-century Britain with the onset of the industrial revolution. The science of gear design and manufacture took a giant leap forward during the 19th century. The metallurgy of material development and thermal processing offers the greatest promise for gear advancement today.
Gears serve multiple purposes – with the most important being gear reduction and direction of rotation change. An example of gear reduction would be an electric screwdriver, which has very large gear reduction because it needs a lot of torque to turn screws. Since the motor produces a small amount of torque at a high speed, gears are used to reduce the output speed while increasing the torque. An example of the directional change is the differential in a rear-wheel-drive car or truck. The driveshaft transmits the power down the center of the truck, and the differential turns the power 90 degrees to apply it to the wheels.
Different gears do a variety of jobs. Spur gears have straight teeth and are mounted on parallel shafts. The teeth on helical gears – used in most car transmissions – are cut at an angle to the face of the gear. As a result of gradual tooth engagement, helical gears operate more smoothly and quietly. Bevel gears can have different types of tooth configuration but are useful when the direction of the shaft’s rotation needs to change. Worm gears are used to make large gear reductions. They are also useful in applications – such as conveyor systems – where a braking action is needed because while the worm can easily turn the gear, the gear cannot turn the worm due to the shallow angle on the worm. Rack-and-pinion gears are used to convert rotation into linear motion.
Design, in addition to optimum thermal processing, keeps gears operating reliably. Early development showed that rolling contact was required during tooth engagement to reduce surface wear. Proper lubrication also prevents direct contact of the entire tooth surface.
Material and thermal processing are the key variables in gear-manufacturing technology in the 21st century. Powder metallurgy has proven a successful gear technology due its reduced component cost. Several automotive applications use this technology; one of which is a gear set used in a motorized lift gate for an SUV.
The objective of effective heat treating of gears is – through a conventional harden, quench and temper – to produce an optimal case depth with high residual compressive stresses on the surface. Thermal processing of gear teeth is done in a variety of ways. Flame hardening is one of the most common methods of gear hardening. Although precision flame hardening exists (Dec. ’06, p. 65), basic flame hardening can be performed with the least investment in equipment and training. In a similar fashion, lasers are also used to surface harden gears.
Another method of heat treating gears is induction. Induction hardening is done in a variety of ways, using either a single- or dual-frequency approach. The geometry of a gear tooth makes heat treating difficult because it hardens most where it is not needed and least where it is needed most. Process developments using a dual-pulse, single-frequency method or a simultaneous dual-frequency method have solved these problems.
So the next time you grind the gears in that stick-shift car or truck, you can hope that the gear-tooth profile has plenty of rolling contact and the gear-tooth case depth and surface residual stresses have been optimized through effective heat treatment.IH