What is Gear?

By BYJU'S Exam Prep

Updated on: September 25th, 2023

Gear is a toothed cylindrical or roller-shaped element of a machine that meshes with another toothed cylindrical element to transmit power from one shaft to another. It is primarily used to obtain different torque and speed ratios or to change the direction of the driving and driven shaft.

There are several other power transmission tools like belt drives, chain drives, rope drives, etc., but the main advantage of the gear system is that there is almost negligible or no slippage between the driver and the driven member. Gears are primarily used wherever there’s a short distance between the axis driving and the driven shaft, like a bicycle, motorcycle, car, etc.

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What is Gear?

Gears are ordinary, rotating machine parts that usually have teeth. When they connect to other gears, they transmit torque. That is sort of a twisting force.

Principle of Gear

Gear works on the fundamental principle of thermodynamics, the law of conservation, or the first law of thermodynamics, which states that energy can neither be created nor destroyed. We can say it is conservative. It can be transformed from one form to another. We know that power is the function of the shaft’s torque (force in rotary motion) and speed (P = TV). Therefore, when we connect a small gear on the driving shaft and a larger gear on the driven shaft, the driven shaft speed decreases per unit rotation of the driving shaft.

As we know, power is conservative, so according to this, the torque of the driven shaft increases according to the ratio of driving gear to driven gear, or we can say, according to the ratio of driving shaft velocity to driven shaft velocity. Therefore, by using various gear designs, we can obtain various torque and speed combinations of the driven member.

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What are the Types of Gears?

There are plenty of ways to classify the gears, such as according to the construction of teeth(terminology of gear), use, the direction of motion transfer, etc. However, primary gears are classified according to the design of teeth. The most common types of gears, along with their applications, are enlisted in the below table.

Type of Gear


Spur Gear


Electric Screwdrivers


Watering Systems

Power Plant Machinery

Helical Gear





Bevel Gear


Marine Applications


Railway Track Inspection Machines

Rack and Pinion

Steering Mechanism

Lifting Mechanism (Vertical Movement)

Positioning Mechanisms


Worm Gear

Packaging Equipment

Worm Drive Speed Reducers

Spur Gear

These gears transfer power in the same plane as when the driving and driven shafts are parallel. In this type of gear, the teeth are parallel to the axis of the shafts, so when it meshes with another spur gear, it transmits the power in a parallel shaft, and when it connects with the helical gear, it will transmit power at an angle from the driving axis.

Helical Gear

Teeth on helical gears are cut at a 45-degree angle to the axis. It has helicoid teeth on a cylindrical roller. Helical gears produce less noise and vibration than spur gears since the load is dispersed across the entire helix. They are also commonly used in industries since they are less prone to wear and tear. It’s also used to transmit power in parallel shafts, but it’s also used in non-parallel shafts on occasion. In helical gears, if the pinion (driving gear) is cut with right-handed teeth, the gear (driven gear) is cut in the opposite direction with left-handed teeth.

Double Helical or Herringbone Gear

This gear has both right and left-handed teeth on one gear. Power is transmitted between parallel shafts using the double-helical gear, often known as the herringbone gear. It was created to offset the drawback of single-helical gears’ high-end thrust.

Bevel Gear

A bevel gear is a toothed rotating mechanical device that transfers mechanical energy or shaft power between shafts that cross at an angle or perpendicularly. As a result, the shaft power’s axis of rotation changes. Apart from this, bevel gears can also increase or decrease torque, causing angular speed to change in the opposite direction.

Rack and Pinion Gear

In automotive steering systems, this gear is used. In this gear, teeth are cut on a rack with a straight rectilinear shape, and a pinion is one spur gear. This is the process of converting rotary motion into linear motion. It is referred to as the infinite radius driven gear.

Worm Gear

This gear transmits power to a shaft that does not intersect and is at an angle. In this design, the driving gear is a screw gear, while the driven gear is a spiral-toothed helical gear.

Backlash in Gear

Backlash is a type of motion mistake that happens when gears change direction. There is a gap between the leading face of the tooth behind the driving tooth on the driven gear and the trailing face of the tooth in front of it, and this gap must be filled before force can be transferred in the new direction. In addition to the phenomenon it creates, the term acklash can also be used to describe the amount of the gap; for instance, a pair of gears could have .1 mm of backlash.

It is possible to create gears with 0% backlash, but it would require flawless manufacturing, constant system-wide thermal expansion, and a lack of lubricant. Gear pairs are therefore made to have some backlash. Usually, it is achieved by cutting each gear’s tooth thickness by half to get the necessary gap size. Find out how the size of the gear is usually specified. However, when a large gear and a tiny pinion are used, the backlash is often completely removed from the gear, and the pinion is given full-sized teeth. Additionally, backlash can be produced by spacing the gears apart more. Backlash can be an issue in longer gear trains since it equals the sum of each gear pair’s backlash.

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Shifting of Gears

Gear shifting, often known as changing gear, alters the gear ratio in various devices (like cars) to suit the task being performed. Gear shifting in automobiles has various consequences. Higher sound levels are emitted from vehicles while they are in lower gears in terms of noise emissions. Because the design life of lower ratio gears is shorter, it is possible to utilize less expensive gears that, because of their lower mesh stiffness and smaller overlap ratio than helical gears used for higher ratios, tend to produce more noise. There are various methods for changing gears, such as:

  • Manual transmission
  • Automatic transmission
  • Derailleur gears
  • Hub gears

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