Blog 21: Gear Basics: Parts of Gear


A gear is a rotating circular machine part having evenly spaced teeth cut around cylindrical or conical surfaces. or, in the case of a cogwheel or gearwheel, inserted teeth (called cogs), which mesh with another toothed part to transmit torque. Gears are used to transmit motion or change speed or direction in a machine. Different speeds can be obtained by using a different combination of gears. It can be used for raising the torque, adjusting the speed and changing the direction of movement, etc.
Two or more gears that are doing one work behind another are called transmissions. An advantage of gears is that the teeth of a gear prevent slippage. By interlocking a pair of these elements, they are used to transfer rotation and forces from the driveshaft to the driven shaft. There are many other power transmission devices like belt drive, chain drive, rope drive, etc. but the main advantage of the gear system has almost negligible or no slippage between the driving and driven member. There are different parts of gear.

Gears are operated in mated pairs. The gear or toothed component that is attached to a machine shaft or base component is called as driving gear (i.e., the gear that provides the initial rotational input) and the gear that rotates along with its shaft component is called the driven gear (i.e., the gear or toothed component which is impacted by the driving gear and exhibits the final output). Depending on the design and construction of the gear pair, the transference of motion between the driving shaft and the driven shaft can result in a change in the direction of rotation or movement. Additionally, if the gears are not of equal sizes, the machine or system experiences a mechanical advantage which allows for a change in the output speed and torque

Parts Of A Gear

Some of the important parts of the gear are listed below. This will help to understand the basic geometry of the gear.

Axis: The axis of revolution of the gear, where the shaft passes through

Teeth: The jagged faces projecting outward from the circumference of the gear, used to transmit rotation to other gears. The number of teeth on a gear must be an integer. Gears only transmit rotation when their teeth mesh and have the same profile.

Pitch Circle: The circle that defines the “size” of the gear. The pitch circles of two intermeshing gears must be tangential so that they can intermesh. If the two gears were instead two disks driven by friction, the circumference of those disks would be the pitch circle.

Pitch Diameter: The pitch diameter refers to the working diameter of the gear, a.k.a., the diameter of the pitch circle. You can use the pitch diameter to calculate how far away two gears should be: The sum of the two pitch diameters divided by 2 corresponds to the distance between the two axes.

Diametral Pitch: The ratio of the number of teeth to the pitch diameter. Two gears must have the same diametrical pitch to mesh.

Circular Pitch: The distance from a point on one tooth to the same point on the adjacent tooth, measured along the pitch circle. (so that the length is the length of the arc rather than a line).

Module: The module of gear is simply the circular pitch divided by pi. This value is much easier to handle than the circular pitch because it is a rational number.

Pressure Angle: The pressure angle of a gear is the angle between the line that defines the radius of the pitch circle and the point where the pitch circle intersects a tooth, and the line tangent to that tooth at that point. Standard print angles are 14.5, 20, and 25 degrees. The pressure angle affects how the gears touch and how the force is distributed along with the tooth. Two gears must have the same contact angle for meshing.

Gear ratios:

Gear ratio is the ratio between the number of teeth of the driven gear and the drive gear. It is used to calculate the speed and torque of the output shaft when input and output shafts are connected through the gear train.

The input gear where the torque is applied is called as the driver gear while the output gear is known as the driven gear. A gear used between the driver and the driven is known as idler gear. To calculate the gear ratios we can use the formula:

Gear ratio= ω12 = n1/n2 = d2/d1 = T2/T1

Where  ω1 and ω2 are the angular velocities of the driver and driven gear in radian/sec.

n1 and n2 are the gear speed of the driver and driven gear in RPM.

d2 and d1 are the gear diameter of the driver and driven gear.

T2 and T1 are the numbers of teeth of the driver and driven gear

Advantages Of Gear

  • They are used for transmitting motion over a small center distance of shafts
  • They are used for a large reduction in speed and for the transmission of torque.
  • Gears require only lubrication; hence less maintenance is required.
  • Using gear systems, we can transmit motion between non-parallel intersecting shafts.
  • They are used for a positive drive, so their velocity ratio remains constant.
  • They have a long life, so the gear system is very compact

Disadvantages of gears

  • They are not suitable for large Velocities.
  • They are not suitable for transmitting motion over a large distance.
  • Due to the engagement of the toothed wheel of gears, some parts of the machine may get permanently damaged in case of excessive loading.
  • They have no flexibility.
  • Gear operation is noisy.

Require constant lubrication to move.

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