Blog 19: Gear Basics: Types of Gears


A gear is a rotating, circular mechanical part with equally spaced teeth around a cylindrical or conical surface. In the case of gears and cogs, an inserted tooth (called a gear) meshes with another toothed part to transmit torque. Gears are used in machines to transmit motion, and change speed, and direction. Different gear combinations can be used to achieve different speeds. It can be used to increase torque, adjust speed, change the direction of travel, etc. Two or more gears that move alternately are called gears. The advantage of gears is that the gear teeth prevent slippage. By connecting pairs of these elements, they are used to transmit rotation and force from the drive shaft to the driven shaft. There are multiple types of gear available depending upon different puporses.

There are many other power transmissions such as belt drives, chain drives, cable drives, etc., but the main advantage of gear systems is that there is little or no slippage between the driving and driven elements. Gears are operated in pairs. The gear or toothed component attached to the shaft or base component of the machine is called the driving gear (i.e. the gear that provides the initial rotational input) and the gear that rotates with that shaft component is called the driven gear (i.e., the gear or geared component that is acted upon by the driving gear and exerts its final performance). Depending on the gear pair design and construction, the direction of rotation or motion can change as a result of motion transmission between the input and output shafts. Additionally, if the gear sizes are unequal, the machine or system can experience a mechanical advantage and vary the output speed and torque.

In the last blog, we learned about the characteristics of Gears.

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, the 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 spur gears. These gears feature a simple design of straight, parallel teeth arranged around a cylindrical body with a central hole to fit the shaft.
In many variations, 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 reamed so that the spur gear fits a splined or keyed shaft.
A spur gear is used in mechanical applications, through a series of meshing gears he transfers motion and power from one shaft to another, thereby increasing or decreasing the speed or increasing the torque of equipment. increase. Spur gears are used to transmit motion and power from one shaft to another within a mechanical assembly. This transmission can change the operating speed of the machine, increase the torque and fine-tune the positioning system. Their design makes them suitable for slow-speed or noise-tolerant operating environments.

2. Helical Gear

A helical gear is a type of spur gear with slanted tooth flanks. Compared to spur gears, gears have a larger contact ratio, smoother movement, and less vibration, and can transmit a large amount of force. The pitch angle of the helical gear is the same, but the pitch needle is the opposite.
Helical and spur gears are two of the most common gear types and can be used in many of the same applications. Although spur gears are easier and cheaper to manufacture, helical gears have some crucial advantages over spur gears. Helical gear teeth are inclined (relative to the gear axis) and have the shape of a spiral. This causes the teeth to gradually mesh, starting with point contact and progressing to line contact as meshing progresses.
One of the most significant advantages of helical gears over spur gears is their low noise, especially at medium to high speeds. In addition, helical gears always have more than one tooth in mesh, resulting in less stress on individual teeth. This results in smoother power transmission from one tooth to the next, reducing vibration, shock loads, and wear.

3. Gear Rack

Teeth of the same size and shape cut at regular intervals along a flat face or straight bar are called racks. A rack is a cylindrical gear with an infinite pitch cylindrical radius. Converts rotary motion to linear motion by meshing with spur gears.
Racks can be broadly classified into straight racks and helical racks, both of which have straight tooth lines. Racks can be connected vertically by processing the ends of the racks.

4. Bevel Gear

A bevel gear is a geared rotating mechanical element used to transmit mechanical energy or shaft power between vertically or obliquely intersecting shafts. This changes the axis of rotation of the shaft power. Apart from this function, bevel gears can also increase or decrease torque while having an opposite effect on the angular velocity.
A bevel gear can be thought of as a truncated cone. The teeth are milled on the sides and mesh with other gears with their own teeth. The gear that transmits the power of the shaft is called the input gear, and the gear that transmits the power is called the output gear.
The number of teeth on the drive and driven gears are usually different to create mechanical advantages. The ratio of the number of teeth between the driven gear and the drive gear is called the gear ratio, and the mechanical gear ratio is the ratio of output torque to the input torque.

5. Spiral Bevel Gear

Spiral bevel gears are beveled 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 zero bevel gear.

6. Screw Gear

A helical gear is a pair of identical helical gears with a 45° helix angle on non-parallel, non-intersecting shafts. Since the tooth contact is one point, the repulsion is low and it is not suitable for large power transmission.
Since power transmission is performed by slippage on the tooth flanks, attention must be paid to lubrication when using helical gears. There are no restrictions on multiple tooth combinations.

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 double helical and 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 gears, but unlike spiral bevel gears, they operate on shafts that do not intersect. In the hybrid 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 beveled 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 hourglass 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.

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