What is Spur Gear?- Design, Types, Parts, Application

The spur gear is a cylindrically shaped tooth component that is used in industrial plants to transmit mechanical movements and to control speed, power, and torque. These simple gears are inexpensive, durable, reliable, and provide constant speed positive drive to facilitate day-to-day industrial operations.

What is a Spur Gear?

Spur gear or straight-cut gears are the simplest types of gear. They consist of a cylinder or disk with teeth projecting radially. Although the teeth are not straight, usually having a special shape to achieve a constant drive ratio that is mainly involute but less often cycloidal, the edge of each tooth is straight and parallel to the axis of rotation.

Spur Gear

Spur gears are the most easily visualized common gears that transmit motion between two parallel and coplanar and teeth that are straight and parallel to the shafts. Spur gears are one of the most popular types of precision cylinders.

These gears feature a simple construction of straight, parallel teeth that are 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 surface. The central hole can also be drilled so that the spur gear fits onto a spline or a keyed shaft.

Because of their shape, they are classified as cylindrical gears. Since the tooth surfaces of the gears are parallel to the axes of the mounted shafts, no thrust is generated in the axial direction.

Due to simple manufacturing, these gears can also be manufactured with a high degree of precision. On the other hand, spur gears have a disadvantage if the teeth of a spur gear have an involute profile and mesh one tooth at a time.

The involute shape means that spur gears only generate radial forces and not axial forces, but the method of tooth combing causes a high load on the gear teeth and a high level of noise. The gear with more teeth is called “gear” and the gear with fewer teeth is called “pinion”. For this reason, spur gears are typically used for lower speed applications, although they can be used at almost any speed.

Types of Spur Gear

There are two primary types of spur gears as describe below:

  1. External gears are gears with teeth cut outside the cylinder. Two external gears mesh and rotate in opposite directions.
  2. Internal gears are gears with teeth cut on the inside of the cylinder. An external gear sits in the internal gear and the gears rotate in the same direction. Because the shafts are closer together, internal gear assemblies are more compact than external gear assemblies. Internal gears are mainly used for planetary gears.
Types of Spur Gears
Internal Gear

Spur Gear Terminology

The following terms are related to spur gears:

Spur Gear Terminology
  1. Addendum: The height of the tooth projects beyond the pitch circle.
  2. Backlash: The clearance between two mating teeth of separate gears.
  3. Chordal thickness: Tooth thickness measured along a chord that runs through the points where the pitch circle crosses the tooth profile.
  4. Chordal addendum: The distance between a chord that passes through the points where the pitch circle crosses the tooth profile and the tooth tip.
  5. Base circle: A theoretical circle used to generate the involute curve when creating tooth profiles.
  6. Center distance: The distance between two gears, measured from the center on the shaft of one gear to the center shaft of the mating gear. This can be roughly determined by taking the radius of each spur gear of the spur gears and adding them up.
  7. Circular pitch: Measurement of the pitch circle arc length from one point on a tooth to the same point on the adjacent tooth.
  8. Circular thickness: The thickness of the tooth at the pitch circle.
  9. Clearance: The space between one gear minor diameter and the mating gears major diameter.
  10. Dedendum: Depth of the tooth between the pitch circle and the minor diameter. Generally greater than the addendum of the mating gear to provide clearance.
  11. Diametral pitch: Ratio of the number of teeth to the pitch diameter.
  12. Fillet: The small radius that connects the tooth profile to the root circle.
  13. Module: The ratio of the reference diameter of the gear divided by the number of teeth. The module is the metric equivalent to diametral pitch.
  14. Outside diameter: The diameter of the addendum circle of the circle along with the outermost points of the teeth of a spur gear. This measurement is the major diameter of the gear.
  15. Pinion: The smaller-sized gear in any meshed pair.
  16. Pitch circle: The circle is derived from a series of teeth and a certain diametrical division. The circle in which tooth spacing or profiles are set from which tooth proportions are created. The speed of the gear is measured here.
  17. Pitch diameter: Is the diameter of the pitch circle. The angular speed of spur gear is measured here. This is also a critical component in determining the center distances between mating spur gears.
  18. Pitch point: The point of tangency of the pitch circles of a pair of mating gears.
  19. Pressure angle: The angle at a division point between the pressure line, which is the line of action, and a line perpendicular to the centerline. And the plane is tangent to the division surface.
  20. Root Diameter: The diameter at the base of the tooth space.
  21. Ratio: Ratio of the numbers of teeth on mating gears.
  22. Root Circle: The circle that passes through the bottom of the tooth spaces.
  23. Velocity ratio: Ratio of input gear revolutions to output gear revolutions within a specified amount of time.
  24. Whole depth: The height of the tooth from major diameter to the minor diameter of a gear.
  25. Working Depth: The depth to which a tooth extends into the space between teeth on the mating gear.

Spur Gear Design

Since the teeth are parallel to the shaft axis, spur gears are only used when the shafts are parallel. The profile of the gear tooth is in the form of an involute curve and remains identical over the entire width of the gear. Spur gears exert radial loads on the shafts.

The design and construction of a spur gear unit significantly affect its performance. To do their job effectively and efficiently, they must be made from high-quality materials and with precise dimensions. The dimensions of each feature are an integral part of how a particular gear works.

As such, when an industry professional needs a new spur gear design or replacement spur gear. It is imperative that you are familiar with the terms for each transmission part and their respective dimensions to ensure clarity and accuracy when manufacturing or ordering.

Design Procedure of Spur Gear:

  • Step 1: – Note down peripheral speed (V= πDN/60) Peripheral speed will be the same for a gear & pinion o If peripheral speed is not able to find we can decide any value in between 3 to 15
  • Step 2: – Note down the material o for low velocity decide cast iron & low-grade steel for high-velocity o Note down & BHN from a given material
  • Step 3: – Design transmission load F=1000*P*C/V
  • Step 4: – Lewi’s Equation, we need to calculate module from above equation o First from gear & pinion whichever is having a small value of for that only we will apply Lewi’s equation. In the case where the number of teeth is not given
  • Y = 0.29(20 degree) Y = 0.35(20degree stup) Y = 0.25(14.5 degree) f = Face width, Lewi’s Equation for Dynamic condition
  • Step 5: – Calculate gear tooth properties if asked
  • Step 6: – Calculate dynamic load using Buckingham equation  
  • Step 7: – Find out a weaker element from gear and pinion
  • Step 8: – Calculate the beam strength for a weaker element
  • Step 9: – Calculate wear strength (FW)

Application of Spur Gear

Spur gear has a wide range of applications:

  • Spur gears are also used to increase or decrease the speed of an object.
  • Spur gears can be used to increase or decrease the torque or power of a specific object.
  • Spur gears are used to transfer motion and force from one shaft to another in a mechanical structure. For this purpose, spur gears are used in washing machines, mixers, tumble dryers, construction machinery, fuel pumps, and mills.
  • In power plants, so-called “trains” of spur gears are used to convert a form of energy such as wind or water power into electrical energy.
  • Spur gears are widely used in aircraft engines, trains, bicycles, ball mills, and crushers where noise is not an issue.
  • Some of the typical industrial applications include gearboxes, conveyor systems, speed reducers, motors, mechanical transport systems, gear pumps, motors, and machining tools.

Advantages of Spur Gear

The Spur gear has various Advantages:

  • Simplicity: Spur gears have a simple, compact design that makes them easy to construct and install even in confined or cramped spaces.
  • Constant Speed Drive: These gears increase or decrease the shaft speed with high precision at a constant speed.
  • Reliability: Spur gears are unlikely to slip during operation and there is a risk of premature failure.
  • Cost Efficiency: The simplicity of their design allows for greater manufacturability, making them more cost-effective to manufacture and purchase.
  • Efficiency: Spur gear systems have a power transmission efficiency between 95% and 99% and can transmit large amounts of power with minimal power loss over several gears.
  • These gears are suitable for drive systems because they have higher power transmission efficiency.
  • A spur gear is built with teeth that are straight and parallel to the axis of the gear. Eliminate axial thrust problems when installing ball bearings.
  • Spur gears are more efficient than helical gears of the same size.
  • They are very reliable and offer a constant speed.
  • Spur gears are also considered a member of the positive gear because they have no slip.

FAQs.

What is spur gear?

A spur gear is a gear with teeth that project outwards from a cylindrical surface. Two spur gears are used to transmit power between parallel shafts. In spur gears, the edge of each tooth is parallel to the axis of rotation and they mesh together when they are fitted to parallel shafts.

What are the types of spur gear?

There are two primary types of spur gears: external and internal. External gears have teeth that are cut on the outside surface of the cylinder. Two external gears mesh together and rotate in opposite directions. Internal gears, in contrast, have teeth that are cut on the inside surface of the cylinder.

What is spur gear terminology?

The following terms are related to spur gears: Addendum: The height of the tooth above the pitch circle. Backlash: The clearance between two mating teeth of separate gears. Base circle: A theoretical circle used to generate the involute curve when creating tooth profiles.

What is the application of Spur Gear?

Spur gears are generally seen as best for applications that require speed reduction and torque multiplication, such as ball mills and crushing equipment. Examples of high-speed applications that use spur gears despite their high noise levels include consumer appliances such as washing machines and blenders.

What is a spur gear used for?

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.

What is a spur gear examples?

Spur gears are generally seen as best for applications that require speed reduction and torque multiplication, such as ball mills and crushing equipment. Examples of high-speed applications that use spur gears – despite their high noise levels – include consumer appliances such as washing machines and blenders.

Which is an advantage of a spur gear?

Spur gears are more efficient compared to helical gears with the same size. They are quite reliable and offer constant velocity. Spur gears are also considered a member of positive transmission because they don’t have any slippage.

Which is better spur or helical gear?

Helical gears are capable of holding more load compared with spur gears, because the load is distributed across more teeth. Helical gears are more durable than spur gears because the load gets distributed across more teeth.

How does a spur gearbox work?

The most commonly used type of gears is spur gears. Also called straight-cut gears, they have straight teeth that are cut or inserted parallel to the gear’s shaft on a circular gear body. In mated pairs, these gears employ the parallel axes configuration to transmit motion and power.

What is module of spur gear?

Also, module is defined as the pitch diameter divided by the number of teeth, and the distance from the pitch circle to the outside circle (addendum) is equal to the module in spur gears.

What are spur gears used for kids?

For this reason, spur gears are used in low-speed applications and in situations where noise control is not a problem, and helical gears are used in high-speed applications, large power transmission, or where noise abatement is important. The speed is considered high when the pitch line velocity exceeds 25 m/s.

How do you identify a spur gear?

Steps

  • Count how many teeth a sample spur gear has z =
  • Measure its tip diameter da =
  • Estimate an approximation of its module, assuming that it has an unshifted standard full depth tooth.
  • Measure the span measurement of k and the span number of teeth.
  • This difference represents pb = πm cos α

Who invented spur gears?

The next major step forward was in 1897, when Herman Pfauter invented a machine that could cut both traditional “spur” gears and helical gears, driving production further forward.

What are the disadvantages of spur gear?

They cannot transfer power between non-parallel axes. Spur gears generate too much noise when operated at high speeds. The wheel’s teeth experience a great amount of stress. They cannot be used for transferring energy over long distances.

Why are helical gears better than spur gears?

Helical Gears have teeth that are set on an angle to the gear axis. Since the teeth engage more gradually, they have a smoother and quieter operation than spur gears. Helical gears also have greater tooth strength and a higher load carrying capacity.

Can a spur gear mesh with a worm gear?

Conclusion. Orthogonal worm gear pairs may be designed so as to engage a spur gear in mesh with a mating involute worm under a right shaft angle in the worm gear pair. The same is also valid with respect to two-starts worm gear pairs. Axial thrust in the spur gear is eliminated under such a scenario.

How are spur gears made?

Most spur gears and helical gears are manufactured with the help of cutting. Yet, there are many other methods like casting, rolling and forging. In the case of plastic spur gears, in addition to tooth cutting such as with metal gears, injection molding is used depending on the quantity of production.

What is the minimum number of teeth on spur gear?

In theory, the minimum number of teeth of a standard involute spur gear is not less than 7, and the minimum number of teeth with a coincidence of 2 is not less than 33.

What are the different types of gearboxes?

In general, there are three different types of gearboxes: concentric, parallel, and right angle.

What is the purpose of a rack and pinion gear?

The rack and pinion, also known as the steering rack, is an assembly in your vehicle that allows your wheels to rotate from side to side when you turn your steering wheel. The rack and pinion got it’s from the type of gears used in the assembly.

How efficient are spur gears?

Spur gearing is a parallel shaft arrangement, and these gears can achieve much higher efficiencies compared to other gears types. Its efficiency varies from 94% to 98% with lower gears ratios.

What is the difference between straight bevel and spur gears?

Spur gear— Teeth are straight and parallel to the axis of the gear. Bevel gear— Teeth are straight but not parallel to the gear axis; however, teeth are orientated along the gear axis (in case of straight teeth bevel gears).

What is the main difference between spur wheel and helical wheel?

In spur gear, the teeth are parallel to the axis of the gear. In helical gear, teeth are inclined at an angle (called helix angle) with the gear axis. Spur gear imposes only radial load on bearings. Helical gear imposes radial load and axial thrust load on bearings.

Why are spur gears used in reverse?

The spur cut is used in reverse simply because you don’t spend that much time in reverse, so they can “cheap out” on that one set of gears typically, it’s three gears – main shaft, countershaft, and reverse idler to change direction.

Which machine is used to manufacture spur gear?

Hobbing is the most common method that is utilized in the cutting process due to its versatility and efficiency. A hobbing machine is used in order to create the gear teeth in a quick and accurate manner.

What materials are spur gears made of?

Spur gears can be made from metals such as steel or brass, or from plastics such as nylon or polycarbonate. Gears made of plastic produce less noise, but at the expense of strength and loading capability.

Which gearbox is used in spur gears?

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.