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 drive shaft 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.
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
There are many types of gears such as:
- Spur Gear
- Helical Gear.
- Double Helical Gear.
- Herringbone Gear.
- Bevel Gear.
- Worm Gear.
- Rack and Pinion Gears
1. Spur Gear
Gears with cylindrical pitch surfaces are called cylindrical gears. Spur gears belong to the gear group of the parallel shaft 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 simple production processes. They have the property of having no load in the axial direction (axial 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 meshing than spur gears, are superior quietness, and can transmit higher loads, making them suitable for high-speed applications.
When using helical gears, they generate a thrust force in the axial direction, which makes the use of axial bearings necessary. Helical gears are supplied with clockwise and counter-clockwise rotation, requiring opposing manual gears for a meshing pair.
3. Double helical gears
Double helical gears overcome the problem of axial thrust in single helical gears by using a double set of teeth inclined in opposite directions. A double helical gear can be thought of as two mirrored helical gears mounted close together on a common axis.
This arrangement cancels out the net axial thrust as each half of the gear thrust pushes in the opposite direction, resulting in zero net axial force. This arrangement can also eliminate the need for thrust bearings. However, double helical gears are more difficult to manufacture because of their more complicated shape.
4. Rack and Pinion Gears
Rack and pinion gears are a pair of gears made up of a rack and a cylindrical gear called a pinion. The rack can be thought of as an infinite radius gear (i.e., a flat bar) and is made up of straight teeth that are cut or inserted on the surface of the bar.
Depending on the type of pinion it is connected to, the rack’s teeth are either parallel (when connected to spur gears) or angled (when connected to helical gears). For each of these rack and pinion designs, the rotational movement can be converted into a linear movement or a linear movement can be converted into a rotational movement.
5. Bevel Gear
Bevel gears have a conical appearance and are used to transmit power between two shafts that intersect at a point (intersecting shafts). A bevel gear has a cone as a parting surface and its teeth are cut along the cone. Types of bevel gears include straight bevel gears, helical bevel gears, spiral bevel gears, miter gears, bevel bevel gears, face gears, zerol bevel gears, and hypoid gears.
6. Screw Gear
Screw gears are a pair of helical gears from the same hand with a twist angle of 45 ° on non-parallel, non-intersecting shafts. Since 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 through the sliding of the tooth surfaces, attention must be paid to lubrication when using helical gears. There are no restrictions on combinations of the number of teeth.
7. 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, a hard material is generally used for the worm and a soft material is used for the worm wheel. Although the efficiency is low due to the sliding contact, the rotation is smooth and quiet. When the helix angle of the screw is small, a self-locking function is created.
8. Internal gear
Internal gears have teeth cut on the inside of cylinders or cones and are paired with external gears. Internal gears are mainly used for planetary gears and gear shaft couplings. The number of tooth differences between internal and external gears is limited due to involute, trochoidal, and trimming problems. The directions of rotation of the inner and outer gears in mesh are the same, while they are opposite when two outer gears are in mesh.
Advantages of Gear
- It is positive drive hence velocity remains constant
- Provisions for changing velocity ratios can be made with the help of gear box
- Its efficiency is very high
- It can be used even for low speeds
- It can transmit high torque values
- It is compact in construction
Disadvantages of Gear
- They are not suitable when shafts are distant
- At high speeds noise and vibration happens
- It requires lubrication
- It has no flexibility
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. A gear may also be known informally as a cog. An advantage of gears is that the teeth of a gear prevent slippage.
Parts of a Gear:
1. Axis: The axis of revolution of the gear, where the shaft passes through.
2. Teeth: The jagged faces projecting outward from the circumference of the gear, used to transmit rotation to other gears. …
3. Pitch Circle: The circle that defines the “size” of the gear.
There are many types of gears such as:
• Spur Gear
• Helical Gear.
• Double Helical Gear.
• Herringbone Gear.
• Bevel Gear.
• Worm Gear.
• Rack and Pinion Gears