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.
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.
Different Types of Gear
There are many different types of gears such as:
- Spur Gear.
- Helical Gear.
- Gear Rack.
- Bevel Gear.
- Spiral Bevel Gear.
- Screw Gear.
- Double Helical Gear
- Herringbone Gear
- Hypoid Gear
- Miter Gear.
- Worm Gear.
- 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
Spur gears are one of the most popular types of precision cylindrical gears. These gears feature a simple design of straight, parallel teeth positioned around the circumference of a cylinder body with a central bore that fits over a shaft.
In many variants, the gear is machined with a hub that thickens the gear body around the bore without changing the gear face. The central bore can also be broached to allow the spur gear to fit onto a spline or keyed shaft.
Spur gears are used in mechanical applications to increase or decrease the speed of a device or multiply torque by transmitting motion and power from one shaft to another through a series of mated gears.
Spur gears are used to transfer motion and power from one shaft to another in a mechanical setup. This transference can alter machinery’s operating speed, multiply torque, and allow for the fine-tuned control of positioning systems. Their design makes them suitable for lower-speed operations or operational environments with a higher noise tolerance.
MORE: What is Spur Gear?
2. Helical Gear
Helical gears are one type of cylindrical gear with a slanted tooth trace. Compared to spur gears, they have a larger contact ratio and excel in quietness and less vibration, and are able to transmit large force. A pair of helical gears have the same helix angle but the helix hand is opposite.
Helical gears and spur gears are two of the most common gear types and can be used in many of the same applications. Spur gears are simple and inexpensive to manufacture, but helical gears offer some important advantages over spur gears.
The teeth of a helical gear are set at an angle (relative to the axis of the gear) and take the shape of a helix. This allows the teeth to mesh gradually, starting as point contact and developing into line contact as the engagement progresses.
One of the most noticeable benefits of helical gears over spur gears is less noise, especially at medium- to high speeds. Also, with helical gears, multiple teeth are always in mesh, which means less load on each individual tooth. This results in a smoother transition of forces from one tooth to the next, so that vibrations, shock loads, and wear are reduced.
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
A bevel gear is a toothed rotating machine element used to transfer mechanical energy or shaft power between shafts that are intersecting, either perpendicular or at an angle. This results in a change in the axis of rotation of the shaft power. Aside from this function, bevel gears can also increase or decrease torque while producing the opposite effect on the angular speed.
A bevel gear can be imagined as a truncated cone. At its lateral side, teeth are milled which interlock to other gears with its own set of teeth. The gear transmitting the shaft power is called the driver gear, while the gear where power is being transmitted is called the driven gear.
The number of teeth of the driver and driven gear are usually different to produce a mechanical advantage. The ratio between the number of teeth of the driven to the driver gear is known as the gear ratio, while the mechanical advantage is the ratio of the output torque to the input torque.
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. Double Helical Gear
Double helical gears are a variation of helical gears in which two helical faces are placed next to each other with a gap separating them. Each face has identical, but opposite, helix angles.
Employing a double-helical set of gears eliminates thrust loads and offers the possibility of even greater tooth overlap and smoother operation. As the helical gear, double helical gears are commonly used in enclosed gear drives.
8. Herringbone Gear
Herringbone gears are very similar to the double-helical gear, but they do not have a gap separating the two helical faces. Herringbone gears are typically smaller than the comparable double helical and are ideally suited for high shock and vibration applications. Herringbone gearing is not used very often due to its manufacturing difficulties and high cost.
9. Hypoid Gear
Hypoid gears look very much like spiral bevel gear, but unlike spiral bevel gears, they operate on shafts that do not intersect. In the hypoid arrangement, because the pinion is set on a different plane than the gear, the shafts are supported by the bearings on either end of the shaft.
10. 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.
11. 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.
12. 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
- Gear drives provides large range of speed and torque for same input power, with more accurate timing than a chain system does, less friction loss and noise.
- Gear is positive drive; hence 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.