Spur Gear: Design, Application, and Terminology

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 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 have 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 have various of 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.

Spur gears are used in mechanical applications to increase or decrease the speed of a device or to multiply torque by transferring motion and power from one shaft to another through a series of joined gears.

Spur gears tend to be louder than other types of gears, so they are generally not found in automobiles but are often used in aircraft engines. Every time the tooth of one spur gear meshes with a tooth of another gear, the teeth collide, and this impact makes a noise.

Spur gears are generally considered best for applications that require speed reduction and torque multiplication, such as ball mills and crushers. Examples of high-speed applications that use spur gears despite their high noise levels are consumer devices such as washing machines and mixers.

A spur gear can be recognized by its teeth. The teeth protrude radially and are parallel to the gear axis. The teeth of a spur gear are exactly perpendicular to its flat surfaces. However, the teeth of helical gears are cut at an angle to the face of the gear.

Therefore, a motor with a higher speed needs a combination of a larger pinion and a smaller spur gear to take advantage of all of this low torque. Lower rotation motors such as 5.5 t or 8.5 t generally need to use a combination of a small pinion and a large spur.

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