Gear

A gear, also known as a gearwheel, is a rotating machine part used to transmit motion.

These components use a series of teeth to engage with compatible teeth on another part. This engagement allows the transfer of rotational motion or torque. Torque is the twisting force that causes an object to rotate. By meshing gears together, engineers can control how much force is applied and how fast a part spins. When two or more gears are connected in a sequence, the system is called a gear train.

Gears function as a type of lever machine to trade speed for power. When a small gear drives a larger one, the rotational speed decreases. However, the torque increases due to mechanical advantage. The smaller gear in a pair is often called a pinion.

Conversely, a large gear driving a small gear increases rotational speed but reduces torque. These changes are determined by the gear ratio, which is the ratio of the tooth counts between the two gears. Gears can also change the axis of rotation or invert the sense of rotation. For example, one gear turning clockwise can cause the next gear to turn anti-clockwise.

There are many specific types of gears designed for different mechanical tasks. Most gears are round and have equal teeth to ensure smooth operation. Some are non-circular, meaning they have irregular shapes. The Geneva drive is a notable example that produces extremely uneven motion by design.

Another variation is the rack and pinion, where a circular gear engages with a straight bar called a rack. This setup converts rotational motion into linear motion.

Other mechanisms, like sprockets, engage with link chains, while timing pulleys engage with belts.

Human history with gears is both ancient and incredibly diverse. Interestingly, the oldest functioning gears are not man-made; they are found in the hind legs of the insect *Issus coleoptratus*. The earliest preserved man-made gears date to 4th-century BC China.

In Europe, the Antikythera mechanism is a famous example of early, intricate gearing.

Constructed between 150 and 100 BC, this device calculated astronomical positions. Later, in the 14th century, Giovanni Dondi dell'Orologio built the Astrarium. This complex astronomical clock featured 107 moving parts to show planetary positions. The Salisbury Cathedral clock, built in 1386, remains the world's oldest working geared mechanical clock.

Materials used for gears vary based on the required strength and cost. Historically, large gears for flour mills were often made of wood. These were called cogwheels and used wooden pegs, or cogs, inserted into a rim.

As technology progressed, metal became more common. Steel is a primary choice today because it has a high strength-to-weight ratio and is cost-effective. Aluminum is also used because it is lighter and easier to machine, though it is less strong than steel. Recently, engineering plastics and composites have become popular for moderate speeds. Plastic gears are often used as "sacrificial" parts. This means they are designed to fail first during a jam to protect more expensive components.

Manufacturing processes for gears are highly specialized to ensure precision. Many gears are produced through net shape molding, which includes plastic injection or metal die casting. About 80% of all gearing produced worldwide uses this method as of 2014. Other techniques include powder metallurgy, where metal powder is shaped and then sintered. For high-precision needs, gear cutting methods like hobbing, milling, or broaching are used. Metal gears used in heavy-duty vehicle transmissions often undergo heat treatment. This process makes the teeth hard and wear-resistant while keeping the core tough.

Gears are essential components in many modern systems. Most vehicles use a gearbox, or transmission, to manage torque. A gearbox allows an operator to vary the torque applied to wheels without changing the engine speed. These systems use terms like "first gear" or "reverse gear" to describe torque ratios. Gears are also vital in precision instruments like watches, where they ensure exact tracking of time. Because gears provide rigid, interlocking movement, they are preferred over belts or chains in applications requiring high accuracy. From micromachines only a few micrometers wide to mining equipment over 10 metres in diameter, gears drive the mechanical world.