Gears and Gear Trains

A gear is essentially a toothed wheel or cylinder that works in tandem with another gear (or gears) to transmit motion, or to change speed or direction. A gear train is a mechanical system formed by mounting gears on a frame so that the teeth of the gears engage.

At its simplest, a gear train has a drive gear that will typically be connected to a motor and a driven gear that is connected to a shaft in order to perform work of some kind.

Gears can also be used to change the direction of rotation. For example, when two gears mesh together, the second one always turns in the opposite direction. So if the first one turns clockwise, the second one must turn anticlockwise.

You can also use specially shaped gears to turn the direction of rotation through an angle. For example, in the differential between the rear wheels of a car, the power is transmitted by a shaft that runs down the center of the car, and the differential has to turn that power 90 degrees to apply it to the rear wheels.

The use of gears to change speed is important because many devices include a small motor, which whilst spinning fast, does not have much torque. Using gears to reduce the speed of rotation has the effect of increasing the available torque. For example, an electric screwdriver requires significant torque to drive screws into wood. With gear reduction, the speed can be reduced while the torque is increased.

In a gear train the ratio depends only on the number of teeth on the first and last gear. The intermediate gears, regardless of their size, do not alter the overall gear ratio of the chain. However, the addition of each intermediate gear reverses the direction of rotation of the final gear.

An intermediate gear which does not drive a shaft to perform any work is called an idler gear. Sometimes, a single idler gear is used to reverse the direction, in which case it may be referred to as a reverse idler. For instance, the typical automobile manual transmission engages reverse gear by means of inserting a reverse idler between two gears.

Spur Gears

Mechanical Aptitude Tests Spur Gears

The spur gear is the most basic type of gear. The teeth, which are on the outer surface are straight and parallel to the axis so that when two gears come together, they do so in the same plane. The advantages of spur gears are due to their simplicity, which makes them relatively cheap to manufacture and maintain.

As a result of their relative size, spur gears can be used to increase or decrease speed and torque. In any pair of gears, the larger gear will move more slowly than the smaller gear, but it will move with more torque. The bigger the difference in size between two spur gears, the greater the difference in speed and torque.

In a gear train with an even number of gears, the first and last gears will rotate in opposite directions. In a gear train with an odd number of gears, the first and last gears will rotate in the same direction.

Gears with an equal number of teeth will turn at the same speed. If they have an unequal number of teeth then the gear with the fewest teeth will turn faster. To work out how fast one is turning with respect to the other you need to calculate the gear ratio. This can be determined by dividing the number of teeth on the larger gear with the number of teeth on the smaller.

For example: the ratio of a 60-tooth gear running with a 20-tooth gear is (60÷20) = 3. This means that the smaller gear will rotate three times faster than the larger gear.

Worm Gear

Mechanical Aptitude Tests Worm Gears

Worm gears are used to transmit power at 90° and where high reductions are required. In order to transmit motion and power at various speeds worm gears work in sets, rotating on shafts at right angles to one another. The worm usually drives the worm gear.

Worm gears are used in a variety of applications, particularly when a high reduction ratio is required and where the power needs to be delivered at right angles to the drive motor. In addition worm gears are quiet, vibration free and give a smooth output.

Rack and Pinion

Mechanical Aptitude Tests Rack and Pinion Gears

A rack and pinion is used to convert rotational motion into linear motion. A circular gear called the pinion engages teeth on a linear bar called the rack.

Rack and pinion combinations are often used as part of a simple linear actuator, where the rotation of a shaft powered by hand or by a motor is converted to linear motion.

Belt Drives and Chain Drives

Mechanical Aptitude Tests Belt Drives and Chain Drives

Idler gears are often used to transmit rotation between distant shafts in situations where it would be impractical to make the distant gears larger to bring them together but he same thing can be achieved using a belt or chain. Belt drives require minimal maintenance and isolate noise and vibration, as well as protecting the machinery from overload.

They are highly efficiency and represent a cheap solution if the shafts are far apart. In a two gear system, the belt can either transmit drive in the same direction, or the opposite direction if the belt is crossed. Drive belts can slip unless they have teeth, which means that the output side may not rotate at a precise speed, and some work gets lost to the friction of the belt as it bends around the pulleys.

Slippage can be overcome by using belts with teeth in them coupled to gear-like sprockets, or by using sprockets and a steel chain instead of a belt. Chains are often narrower than belts, and this can make it easier to shift them to larger or smaller gears in order to vary the gear ratio. Multi-speed bicycles with derailleurs make use of this.

Key Points

  • A gear is a toothed wheel or cylinder that meshes with another toothed component to transmit motion or to change speed or direction.
  • Gears are attached to a rotating shaft turned by an external force, which is not usually illustrated in mechanical aptitude test questions.
  • Two gears may be connected by touching each other directly or by means of a chain or belt.
  • If gears are connected by a chain or belt then they move in the same direction.
  • If the gears are touching (meshed) then adjacent gears move in opposite directions.
  • A single idler gear can be used to reverse the direction of a driven gear.
  • Meshed gears with an equal number of teeth will turn at the same speed.
  • If they have an unequal number of teeth then the gear with the fewest teeth will turn faster.
  • Worm gears are used to transmit power at 90° and where high reductions are required.
  • A rack and pinion is used to convert rotational motion into linear motion.
  • A belt or chain can be used to transmit rotation between distant shafts.
  • The speed ratios of chain and belt drives are calculated in the same way as gear ratios.