Gears are rotating mechanical devices employing 'teeth' in order to transmit torque between separate axes. Two or more cooperating gears are called a transmission and can produce a mechanical advantage by changing speed, torque or rotation direction.

Gears use teeth, also called cogs, to engage and transmit rotational motion each other. One gear has the movement supplied to it by the turn of its shaft and is known as the drive. Interlocked teeth pressure the corresponding gear to turn in a ratio. If the drive turns a larger gear then torque is improved; if the drive turns a small gear, the rotational speed is increased. A gear does not necessarily always turn another gear, like in the instance of a rack.

Gears - How it's Made Video credit: Science Channel via Youtube

Types of Gears

Depending on the mechanical requirements, multiple gear types may be suitable. Each type has advantages and disadvantages; follow the links to the individual gear selection guides to learn more.

Bevel and miter gears transmit motion between intersecting shaft axes via the use of teeth on a tapered plane. They can be employed at nearly any angle. Miter gears are bevel gears with a 1:1 ratio.
Helical gears have curved teeth that appear as part of a helix. They are suitable transferring motion between parallel or perpendicular axes.
Herringbone gears possess V-shaped teeth made of conjoined, opposite-hand helices.
Hypoid gears are a style of bevel gear which does not offer intersecting axes. The hypoid gear is offset, and can best be considered a hybrid between a bevel gear and worm drive.
Rack and pinion gears' change rotational movement into translation by the use of an infinite circumference, toothed mechanism.
Spur gears transmit power amongst parallel axes, using similarly parallel teeth.
Worm gear sets consist of a drive and matching gear, that offers high speed reduction and torque multiplication in a small footprint.

Specifications

Axes relationship: Some gearing configurations may require the transfer of motion between oddly angled axes. Hypoid gears allow for mesh between non-intersecting axes and bevel gears can be fabricated to work at nearly any axes angle.
Handedness: Gears with teeth not parallel to the gear axis are subject to left or right handedness. This dictates the direction the teeth curve in and is important for worm drives, hypoid gears, helical gears and some bevel gears.	Left hand | Right hand
Internal gears: Some gears may have an inverted tooth structure, so that gear teeth point inwards towards the gear center, rather than outward like a normal (external) gear. Internal gears have the same rotation direction. Rack and pinions, worm gear sets and hypoids cannot be interpreted at internal gears.
Lubrication: Most gearing arrangements require lubrication to reduce friction and extend the service of the gear. Specialty lubricants are made for high-pressure designs or gears with brass.

Selection tip: Gears must have the same pitch and pressure angle in order to mesh.

Mounting

Consider the gear center, bore diameter and shaft diameter. The gear center can be a bored hole or an integral shaft. The bore diameter is the diameter of the center hole. The shaft diameter is the diameter of the shaft for gears with an integral shaft. Helical gears can be mounted on a hub or shaft. A hub is a cylindrical projection on one or both sides of a helical gear, often for the provision of a screw or other shaft attachment mechanism. Hubless gears are typically attached via press fit, adhesive or internal keyway.

Shaft mounting choices include the following:

Keyway: One or more square cutouts exist in the gear bore for exact mounting on the shaft.
Set Screw: The gear is attached to the shaft by screws through the hub.
Hub Clamping Screws: The gear is attached with a screw that squeezes the inner diameter of the hub to a tight fit around the shaft.
Split: The hub is split into several pieces that are tightened down by a separate clamp to grip the shaft.
Simple Bore: A straight bore designed for adhesive attachment.

Materials

 Gear composition varies by application and affects the gear's service, rotation speed, accuracy and more.

• Cast iron provides durability and ease of manufacture.
• Alloy steel provides superior durability and corrosion resistance. Minerals may be added to the alloy to further harden the gear.
• Cast steel provides easier fabrication, strong working loads and vibration resistance.
• Carbon steels are inexpensive and strong, but are susceptible to corrosion.
• Aluminum is used when low gear inertia with some resiliency is required.
• Brass is inexpensive, easy to mold and corrosion resistant.
• Copper is easily shaped, conductive and corrosion resistant. The gear's strength would increase if bronzed.
• Plastic is inexpensive, corrosion resistant, operationally quiet and can overcome missing teeth or misalignment. Plastic is less robust than metal and is vulnerable to temperature changes and chemical corrosion. Acetal, delrin, nylon, and polycarbonate plastics are common.
• Other material types like wood may be suitable for individual applications.

Applications

When selecting a gear the workload and environment of the gear set should be considered.

• Power, velocity and torque consistency and output peaks of the gear drive so the gear meets mechanical requirements.
• Inertia of the gear through acceleration and deceleration. Heavier gears can be harder to stop or reverse.
• Precision requirement of gear, including gear pitch, shaft diameter, pressure angle and tooth layout.
• Handedness (left or right teeth angles) depending the drive angle.
• Gear lubrication requirements. Some gears require lubrication for smooth, temperate operation.
• Mounting requirements. Application may limit the gear's shaft positioning.
• Noise limitation. Commercial applications may value a smooth, quietly meshing gear.
• Corrosive environments. Gears exposed to weather or chemicals should be especially hardened or protected.
• Temperature exposure. Some gears may warp or become brittle in the face of extreme temperatures.
• Vibration and shock resistance. Heavy machine loads or backlash, the deliberate surplus space in the circular pitch, may jostle gearing.
• Operation disruption resistance. It may be necessary for some gear sets to function despite missing teeth or misalignment.

Resources

Gear Technology - Gears

How Stuff Works - How Gears Work

Popular Science - Wheels That Won't Slip

Image credits:

QTC Metric Gears | WM Berg | Wikimedia | Hymark | Atlanta Drive Systems | Agro Engineers | Chemodex | Royal Purple | TT Net | MISUMI | SDP/SI | Ondrives.US Corp.