What is Gear?- Definition, Parts, Types, and Benefits

What is gear?

A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called cogs), which mesh with another toothed part to transmit torque. Gear may also be known informally as a cog. An advantage of gears is that the teeth of a gear prevent slippage.

A gear is a type of machine element in which evenly spaced teeth are cut around cylindrical or conical surfaces. By interlocking a pair of these elements, they are used to transfer rotation and forces from the driveshaft to the driven shaft.

Gears can be classified by shape as involute, cycloid, and trochoidal gears. They can also be classified according to shaft positions as parallel shaft gears, intersecting shaft gears, and non-parallel and non-intersecting shaft gears. The history of gears is old and the use of gears appears as early as ancient Greece in B.C. in the writing of Archimedes.

Why Use Gears?

Gears are a very useful transmission mechanism that is used to transmit rotation from one axis to another. As mentioned earlier, you can change the output speed of a shaft with gears. Let’s say you have a motor that spins at 100 revolutions per minute and you just want it to spin at 50 revolutions per minute.

You can use a gear system to decrease the speed (and also increase the torque) so that the output shaft rotates at half the engine speed. Gears are commonly used in high load situations because the teeth of the gear allow finer, more discreet control of the movement of a shaft. This is an advantage that gears have over most pulley systems.

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A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called cogs), which mesh with another toothed part to transmit torque.

Parts of a Gear

There are a few different terms that you need to know when you are just starting out with gears, as listed below. So that the gears can mesh, the diametrical pitch and the pressure angle must be the same.

  • 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.

Types of Gears

These can be broadly classified by looking at the positions of axes such as parallel shafts, intersecting shafts, and non-intersecting shafts.

There are many types of gears such as:

  1. Spur Gear.
  2. Helical Gear.
  3. Gear Rack.
  4. Bevel Gear.
  5. Spiral Bevel Gear.
  6. Screw Gear.
  7. Miter Gear.
  8. Worm Gear.
  9. Internal gear

It is necessary to accurately understand the differences among gear types to accomplish necessary force transmission in mechanical designs.

Even after choosing the general type, it is important to consider factors such as dimensions (module, number of teeth, helix angle, face width, etc.), the standard of precision grade, need for teeth grinding, and/or heat treating, allowable torque, and efficiency, etc.

1. Spur Gear

Gears having cylindrical pitch surfaces are called cylindrical gears. Spur gears belong to the parallel shaft gear group and are cylindrical gears with a tooth line that is straight and parallel to the shaft.

Spur gears are the most widely used gears that can achieve high accuracy with relatively easy production processes. They have the characteristic of having no load in the axial direction (thrust load). The larger of the meshing pair is called the gear and the smaller is called the pinion.

Check out our Detail Article: What is Spur Gear?

2. Helical Gear

Helical gears are used with parallel shafts similar to spur gears and are cylindrical gears with winding tooth lines. They have better teeth meshing than spur gears and have superior quietness and can transmit higher loads, making them suitable for high-speed applications.

When using helical gears, they create thrust force in the axial direction, necessitating the use of thrust bearings. Helical gears come with right-hand and left-hand twists requiring opposite hand gears for a meshing pair.

3. Gear Rack

Same sized and shaped teeth cut at equal distances along a flat surface or a straight rod is called a gear rack. A gear rack is a cylindrical gear with the radius of the pitch cylinder being infinite. By meshing with a cylindrical gear pinion, it converts rotational motion into linear motion.

Gear racks can be broadly divided into straight tooth racks and helical tooth racks, but both have straight tooth lines. By machining the ends of gear racks, it is possible to connect gear racks end to end.

4. Bevel Gear

Bevel gears have a cone-shaped appearance and are used to transmit force between two shafts that intersect at one point (intersecting shafts). A bevel gear has a cone as its pitch surface and its teeth are cut along the cone.

Kinds of bevel gears include straight bevel gears, helical bevel gears, spiral bevel gears, miter gears, angular bevel gears, crown gears, zerol bevel gears, and hypoid gears.

5. Spiral Bevel Gear

Spiral bevel gears are bevel gears with curved tooth lines. Due to the higher tooth contact ratio, they are superior to straight bevel gears in efficiency, strength, vibration, and noise. On the other hand, they are more difficult to produce.

Also, because the teeth are curved, they cause thrust forces in the axial direction. Within the spiral bevel gears, the one with zero twisting angles is called zerol bevel gear.

6. Screw Gear

Screw gears are a pair of same hand helical gears with the twist angle of 45° on non-parallel, non-intersecting shafts. Because the tooth contact is a point, their load carrying capacity is low and they are not suitable for large power transmission.

Since power is transmitted by the sliding of the tooth surfaces, it is necessary to pay attention to lubrication when using screw gears. There are no restrictions as far as the combinations of a number of teeth.

7. Miter Gear

Miter gears are bevel gears with a speed ratio of 1. They are used to change the direction of power transmission without changing speed. There are straight miter and spiral miter gears. When using the spiral miter gears it becomes necessary to consider using thrust bearings since they produce thrust force in the axial direction.

Besides the usual miter gears with 90° shaft angles, miter gears with any other shaft angles are called angular miter gears.

8. Worm Gear

A screw shape cut on a shaft is the worm, the mating gear is the worm wheel, and together on non-intersecting shafts is called a worm gear. Worms and worm wheels are not limited to cylindrical shapes. There is the hour-glass type which can increase the contact ratio, but production becomes more difficult.

Due to the sliding contact of the gear surfaces, it is necessary to reduce friction. For this reason, generally, hard material is used for the worm, and soft material is used for the worm wheel. Even though the efficiency is low due to the sliding contact, the rotation is smooth and quiet. When the lead angle of the worm is small, it creates a self-locking feature.

9. Internal gear

Internal gears have teeth cut on the inside of cylinders or cones and are paired with external gears. The main use of internal gears is for planetary gear drives and gear-type shaft couplings. There are limitations in the number of teeth differences between internal and external gears due to involute interference, trochoid interference, and trimming problems.

The rotational directions of the internal and external gears in the mesh are the same while they are opposite when two external gears are in the mesh.

Advantages of Gear

  • By using gear trains, large velocity ratio can be obtained with minimum space.
  • Gears are mechanically strong, so higher loads can be lifted.
  • Gears are used for transmission of large H.F.
  • They are used for transmitting motion over small centre distance of shafts
  • They are used for large reduction in speed and for 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 positive drive, so its velocity ratio remains constant.
  • They have 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 toothed wheel of gears, some part of machine may get permanently damaged in case of excessive loading.
  • They have no flexibility.
  • Gear operation is noisy.

FAQs.

What is Gear?

A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called cogs), which mesh with another toothed part to transmit torque. Gear may also be known informally as a cog. An advantage of gears is that the teeth of a gear prevent slippage.

What are the Different Types of Gear?

There are many types of gears such as:
1. Spur Gear.
2. Helical Gear.
3. Gear Rack.
4. Bevel Gear.
5. Spiral Bevel Gear.
6. Screw Gear.
7. Miter Gear.
8. Worm Gear.
9. Internal gear

What are the applications of gears?

Gears are used to transfer motion and torque between machine components in mechanical devices. Depending on the design and construction of the gear pair employed, gears can change the direction of movement and/or increase the output speed or torque.

What are the advantages of gear?

They are used for a large reduction in speed and for 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 its velocity ratio remains constant.