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Everything you need to know about gears

Thinking of power transmission today, its common form is through gear. Gears are wheels with teeth and can also be called toothed wheels.  A gear is a rotating circular machine part having cut teeth, in the case of a gearwheel, inserted teeth are called cogs, which mesh with another toothed part so that torque can be transmitted between them.

Gears are employed in many mechanical devices, serving various purposes depending on their requirement. Gears can be used to change the speed, torque, and direction of a power source. One of the different sizes will create a change in torque, offering mechanical benefits through their gear ratio.

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gear definition, applications, function, components, diagram, working, types

Today you’ll get to know the definition, applications, function, components, diagram, characteristics, classification, types, and working of a gear. You’ll also get to know the advantages and disadvantages of gear in its various applications.

What is gear?

gears are mechanical devices that transmit rotation and power from one shaft to another if they both possess appropriately shaped projections (teeth). These gears are equally spaced around their circumference and the tooth goes into the space between the teeth of the other shaft. In this case, a gear is a machine component in which a rotary power is transmitted by the prime mover’s tooth surface pushing the tooth surface of the driven shaft.

When two or more meshing gears are working in sequence, it’s called a gear train or a transmission. There are many ways to transmit rotation and power from one shaft to another such as through rolling friction, wrapping transmission, etc. well will be further explained!

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The gear ratio in the transmission is the ratio between the rotational speeds of two meshing gears. Because each gear has a different diameter, each of the axes rotates at a different speed when they are both engaged. Modification of the gear ratio is the equivalent of modifying the torque that is applied. The gear ratio can be calculated by dividing the output speed by the input speed or by dividing the number of teeth of the driven gears by the number of teeth of the driven gears.

Finally, on the definition of gear, it is toothed, mechanical transmission elements used to transfer motion and power between machine components.

Applications of a gear

The applications of gear are so vast in the mechanical industry since it solves the solution of motion and power transmission. Even as there are various configurations and types of gears, they still find a way to fit in specific applications. Their mechanical characteristics are widely used throughout industry in a variety of mechanical devices such as equipment, instrumentation, clocks, etc. They are often used in various motorized devices such as automobiles, machines, motorcycles, etc. to increase or reduce speed and torque.

applications of gear

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The table below shows the applications of gears by type:

Type of Gear Common Industries and Applications
Spur
  • Clocks
  • Pumps
  • Watering systems
  • Household appliances
  • Clothes washing and drying machines
  • Power plants
  • Material handling systems
  • Aerospace and aircrafts
  • Railways and trains
Helical
  • Same as spur gears but with greater loads and higher speeds (see above)
  • Automobiles (transmission systems)
Bevel
  • Pumps
  • Power plants
  • Material handling systems
  • Aerospace and aircrafts
  • Railways and trains
  • Automobiles
Worm
  • Instruments
  • Lifts and elevators
  • Material handling systems
  • Automobiles (steering systems)
Rack and Pinion
  • Weighing scales
  • Material handling and transfer systems
  • Railways and trains
  • Automobiles (steering systems)

Components of a gear

Diagram of a gear:

components and diagram of gears

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The primary function of gear is power transmission, also, increase and decrease of torque are another important. Below are some important terms you should understand about gears.

Driven gear – is the gear closest to the power source, which is attached to a driving shaft that provides the rotational input.

Driven gear – is a gear or toothed component that is mounted to the driven shaft, it is imparted by the driving gear and provides the final output.

Idler gear – this is a gear located between the driving gear and driven gear. It is typically employed so that there won’t be a change in direction of rotation during transmission.

Gear ratio – is the ratio between the output value to the input value. This is typically expressed as the number of teeth of the driven gear (output) to the number of teeth of the driven gear (input).

Tooth profile – is the cross-sectional shape of the gear’s teeth.

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Torque – can also be referred to as a moment of force. This is the measure of the rotational or twisting force that causes an object to move.

axes configuration – is the orientation of the axes along which the gear shafts lay and around which the gears rotate to each other. Finally,

Efficiency – is the percentage value of the ratio of output power to the input power.

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So, the function of gear include:

  • Changing the direction of rotation
  • Increasing or decreasing the rotation speed
  • Increasing force
  • Keeping the rotation of two-axis synchronized
  • Measuring time

Characteristics of a gear

Gears can be characterized by the following:

Gear shape:

Gears are often circular in shape, that is, their teeth are arranged around a cylindrical gear body with a circular face.  Although some non-circular gears can feature elliptical, triangular, and square-shaped faces. Applications that use a circular gear experience constancy in their gear ratios, that is, the ratio of the output to the input. Whereas, systems that use non-circular gear experience variable speed and torque ratios. This help to fulfill special or irregular motion requirement, such as multi-speed, reversing motion, and increasing and decreasing output speed.

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Gear tooth design:

Gear teeth are also known as cogs, thus making a gear to be also called cogwheel. Gears are characterized here by their tooth design and construction which can occur in three forms:

  • Gear teeth structure – this depends on the gear structure, gear teeth can be cut directly into the gear blank or inserted as separate, shaped components into the gear blank. For most applications, the entire gears are replaced once succumb to fatigue. This is why employing gear with separate tooth components is beneficial. This is because of the individual replacement of the teeth as each becomes fatigued rather than replacing the whole component.
  • Gear teeth placement – are cut or fit into the outer or inner portion of the gear body. The teeth are placed on the outer surface of the gear body in external gears. On the other hand, in internal gears, the teeth are placed on an inner surface of the gear body, pointing inward towards the gear center. The placement of the gear teeth on each of the gear bodies largely determines the motion of the driven gear.

gear tooth design

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  • Gear tooth profile – refers to the cross-sectional shape of the gear’s teeth and influences a variety of its performance characteristics such as the speed ratio and experienced friction. There is a large number of tooth profiles available for the construction and design of gears. Involute, cycloid, and trochoid are the most common types of tooth profiles used.

Gear axes configuration

Just as earlier stated, axes configuration of a gear is the orientation of the axes along which the gear shafts lay and around which the gear rotates to each other. Parallel, intersecting, non-parallel and non-intersecting are the three principal axes configurations used in gears.

  • Parallel gear configurations – involve gears mounted to rotating shafts on parallel axes within the same plane. Different types of gears which employ parallel configurations include spur gears, helical gears, internal gears, and some variants of rack and pinion gear. In the working of parallel gear configuration, the rotation of the driving shaft (and the driving gear) is in opposite direction to the driven shaft and driven gear. The power and motion transmission here is of high efficiency.

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  • Intersecting gear configurations – are on intersecting axes with the same plane. This gear configuration also offers high transmission efficiencies like the parallel types. Bevel gears such as miter, straight, and spiral bevel gears are categorized to employ intersecting configurations. Changing the direction of motion within power transmission systems is the purpose of these configurations on typical applications.

  • Non-parallel, non-intersecting gear configurations – have their shafts on cross axes but not on the same plane, that is, are not parallel and do not intersect. These configurations generally produce low motion and power transmission efficiencies unlike those of parallel and intersecting configurations. Screw gears, worm gears, and hypoid gears are examples of non-parallel and non-intersecting gears.

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Additional gear design characteristics

Apart from the above-explained options of gear design characteristics, many other available options can be considered when selecting and designing gear for applications. Numbers of teeth, tooth angle, construction material, lubricant type, and lubrication method are some characteristics that may be considered.

The table below shows the characteristics of gears by type:

Type of Gear Characteristics
Spur
  • The most common type of gear
  • Circular gear body
  • Straight teeth cut or inserted parallel to the gear’s shaft
  • Used for parallel axes configuration
  • Mated with spur gears, internal gears, or gear racks
  • High precision and efficiency (A)
  • Easy to manufacture (A)
  • Does not produce thrust force (A)
  • Capable of handling high speed and high loads (A)
  • Gear teeth experience high stress due to tooth design (D)
  • Noise production during high speeds (D)
Helical
  • Circular gear body
  • Teeth twisted at an angle around gear body
  • Used for parallel axes configuration
  • Available in right-hand and left-hand designs
  • Available in single and double helical designs
  • Gradual tooth engagement and less impact loading (A)
  • Quieter, smoother operation (A)
  • Capable of handling greater loads (A)
  • Lower efficiency (D)
  • Higher design complexity, the greater cost of manufacturing (D)
  • Single helical design products thrust force (D), double-helical does not (A)
Bevel
  • Cone-shaped gear body
  • Used for intersecting axes configuration
  • Available in straight, spiral, and Zerol® bevel tooth designs
  • Straight: simplest bevel gear design and easiest to manufacture (A); high impact, noise level, and stress (D)
  • Spiral: gradual tooth engagement and less impact loading, noise, and vibration (A); higher design complexity and greater cost of manufacturing (D)
  • Zerol®: Quieter and smoother than straight bevel, able to rotate in both directions unlike spiral bevel (A)
Worm
  • Pair comprised of a circular gear and a screw-shaped gear
  • Used for non-parallel, non-intersecting axes configuration
  • Large gear ratios and gear reduction (A)
  • Quiet, smooth operation (A)
  • Self-locking mechanism (A)
  • Low transmission efficiency (D)
  • Large amounts of friction (D)
Rack and Pinion
  • Pair comprised of a gear rack and cylindrical gear
  • Used for parallel axes configuration
  • Rack mated with spur or helical gear
  • Converts rotational motion to linear motion or vice versa
  • Simple design, easy to manufacture (A)
  • Capable of handling greater loads (A)
  • Transmission cannot continue infinitely in one direction (D)
  • A large amount of backlash between mated teeth (D)
  • Gear teeth experience high friction and stress due to tooth design (D)

Things to consider before selecting a gear

Below is the thing you must consider before selecting a gear:

  1. Operational and environmental conditions include the construction material of the gear, surface treatment, and lubrication of the gear.
  2. Transmission requirement, that is, if the gear system will feature change in direction and change in torque or speed.
  3. Dimensional restrictions
  4. Design standards, and finally
  5. Costs

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Types of gear

Below are the various types of gear and their uses:

Spur gear

These types of gear have cylindrical pitch surfaces making them be called cylindrical gears. A spur gear is categorized as a parallel shaft gear group, having a tooth line on cylindrical surfaces which is straight or parallel to the shaft. These gear types are the most widely used gears because they can achieve high accuracy with relatively easy production processes. They have no load in the axial direction (thrust load).  A smaller meshing pair is called pinion while the larger ones are called gears.

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Helical Gear

Helical gear types are used with parallel shafts, they are cylindrical gears with winding tooth lines, making them similar to parallel shafts. Helical gears have better meshing than spur gears and have superior quietness and can transmit higher loads. This is why they are suitable for high-speed applications. These types of gear create thrust force in the axial direction, making use of thrust bearings.  This gear can be design to be a right-hand and left-hand twist requiring opposite hand gears for a meshing pair.

Gear Rack

Gear racks are types of gears that are shaped teeth cut at equal distances along a flat surface or a straight rod. A gear rack is a cylindrical gear with the radius of the pitch cylinder being infinite. Rotational motion is converted into linear motion using a cylindrical gear pinion. Gear rack types are broadly divided into straight tooth racks and helical tooth racks. They both have straight tooth lines. Gear racks can be connected end to end when machined.

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Bevel Gear

Bevel gear types have cone-shaped in their appearance, used in transmitting force between two shafts that intersect at one point. The cone shape is at their pitch surface and its teeth. Types of bevel gears are straight bevel gears, helical bevel gears, spiral bevel gears, angular bevel, crown bevel, zerol bevel, miter bevel, and hypoid bevel gears.

Spiral Bevel Gear

Spiral bevel gear types are beveled gears with curved tooth lines. They are superior to straight bevel gears in efficiency, strength, vibration, and noise due to the high tooth contact ratio. However, they are complex in design and more difficult to produce. Also, they cause thrust forces in the axial direction because their teeth are curved.

Screw Gear

Screw gear types are pairs of the same hand helical gears with the twist angle of 45o on non-parallel, non-intersecting shafts. Their load carrying capacity is low due to the tooth contact is a point. Also, they are not suitable for large power transmission. Lubrication is important in screw gear since power is transmitted by the sliding of the tooth surfaces. There are no restrictions as far as the combinations of several teeth.

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Miter Gear

Miter gears are types of bevel gears with a speed ratio of 1. They are used to change the direction of power transmission without changing speed. Miter gears can be straight miter and spiral miter gears.  Thrust bearings must be employ when using spiral miter gears because they produce thrust force in the axial direction. Besides the usual miter gears with 90o shaft angles. They are known as angular miter gears when used with any other shaft angles.

Worm Gear

Worm types of gears are in screw shape cut on a shaft which forms a worm. The mating gear is the worm wheel and altogether on non-intersecting shafts are called a worm gear. The worms and worm wheels are not limited to cylindrical shapes. Friction must be reduced in worm gear because of the sliding contact of the gear surfaces. Also, the worm must be made of hard material, and the worm wheel should be of soft material. Efficiency is low due to the sliding contact, but the rotation is smooth and quiet. If the lead angle of the worm is small, a self-locking feature is obtained.

Internal gear

Internal gear types have their teeth on the inside of the cylinder or cone, paired with external gears. They are used for planetary gear drives and gear shaft couplings. There are drawbacks 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. However, when two external gears are in mesh, they are opposite.

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Working principles of a gear

The working of gear is less complex and can be easily understood. in simple explanation, when a big cog is connected with a small one and it’s turned slowly. The big wheel is powerful enough to make the small cog rotate quickly. Less energy will be required for tuning the big cog slowly than it would rotate the small one quickly. This is to say, there is saving of energy using cogs, and this makes work easier.

Using cogs to increase the speed of a machine, there must be a different number of teeth. To achieve this, the big wheel is turned, then the small wheel rotate must faster to keep up with the big one in less force. In some other variation of gears, when two cogs slot into each other, they turn in the opposite direction. in other words, one wheel rotates clockwise and the other turns anticlockwise. This arrangement is used to turn the power of a machine through an angle.

Watch the video below to learn more about the working of a gear:

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Advantages and disadvantages of a gear

Advantages:

Below are the benefits of gear in their various applications:

  • With gear trains, a large velocity ratio is obtained even with minimum space.
  • Gear is used for power transmission of large H.F.
  • Transmission of motion over a small center distance of shafts.
  • Mechanically strong enough to lift higher loads.
  • Speed reduction and transmission of torque are possible.
  • Less maintenance is required, except for lubrication.
  • Motion can be transmitted between non-parallel, intersecting shafts.
  • Serve for longer than other sources of power transmission.
  • The velocity ratio remains constant.

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Disadvantages:

Despite the good benefits of gear, some drawbacks still occur. Below are the disadvantages of a gear system in its various applications:

  • Not suitable for large velocities.
  • Because of tooth engagement of gears, the machine parts may permanently damage due to excessive loading.
  • Transmitting motion over a larger distance is impossible.
  • The system is flexible.
  • An operation might be noisy.
  • Lubrication is required.

Conclusion

Gears are mechanical devices that transmit rotation and power from one shaft to another if they both possess appropriately shaped projections (teeth). That is all for this in-depth article on a gear, where the definition, applications, function, component, diagram, characteristics, classification, types, and working of gear is explained. We’ve also discussed the advantages and disadvantages of gear in their various applications.

I hope you get a lot from this post, if so, kindly share with other students. Thanks for reading, see you next time!

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