Gear Terminologies

By Mohit Uniyal|Updated : August 26th, 2022

When designing a gear, understanding the gear terminology is critical. So, before we get into the specifics of gear terminology, let us first grasp what gear is. Gears are among the earliest pieces of equipment known to humans. In the 4th century B.C., Aristotle is credited with delivering the first description of gears. According to his definition, when one gear wheel drives another gear wheel, the direction of rotation is reversed.

The Greek inventors utilized gears in their water wheels and clocks. Leonardo da Vinci's notebooks contain sketches of multiple gears from the historical period. The technical nomenclature used by machine designers is known as gear terminology. Let's take a closer look at the gear terminology used in the gear industry, its diagram, and the gear terminology formula for each in the coming sections.

Table of Content

What is Gear Terminology?

In mechanical industries, gears are commonly employed for power transmission. It's used when the distance between the driver and the driven shaft isn't too great. Because it is the only positive drive that can transfer an exact velocity ratio to the driven shaft, it is widely used in various machines and mechanical devices.

Gear Terminology Definition

"Gear terminology" refers to all definitions used to define a gear or the terminologies needed to make a gear. The most common explanations are as follows:

Pitch Surface

Dedendum

The flank of the Tooth

Pitch Circle

Addendum Circle

Fillet Radius

Pitch Circle Diameter

Dedendum Circle

Clearance

Pitch Point

Base Circle

Backlash

Circular Pitch

Whole Depth

Gear Ratio

Diametral Pitch

Working Depth

Arc of Contact

Module

Tooth Thickness

Contact Ratio

Top-land

Tooth Space

Pressure Angle

Addendum

The face of the Tooth

Gear Terminology Diagram

The diagram shown below carries all the essential gear terminology:

Gear Terminologies Diagram

Figure: Gear Terminology Diagram

Pitch surface

The outer surface of a hypothetical friction wheel that acts as a reference surface for setting the gear teeth is known as a "pitch surface" in gear terminology.

Pitch Circle

A pitch circle is an imaginary circle that would produce the same motion as the actual gear through pure rolling motion. The size of pitch circles for a given pair of mated gears fluctuates with centre distance because they are imaginary by nature.

Pitch Circle Diameter

The pitch circle diameter is the diameter of the pitch circle. The pitch circle diameter is mainly used to determine the gear size. Pitch diameter is another term for it. The diameter of the pitch circle is

d = nm

(where n is the number of teeth and m is the module). The pitch circle sizes are crucial when identifying the shafts of two mating gears.

Pitch Point

The common point of both meshing gear’s pitch circles is known as the "pitch point" when they are in contact. Or, to put it another way, the pitch point is the point of tangency between the two pitch circles.

Circular Pitch

Circular pitch is the distance between a location on one tooth and the equivalent position on the next tooth, measured around the circumference of the pitch circle. Pc is the symbol for it.

Pc = πD/T

  • D = diameter of the pitch circle
  • T = No. of teeth on the wheel

Diametral Pitch

It is the ratio of the number of teeth to the diameter of the pitch circle. It is indicated by Pd.

Pd = T/D = π/Pc

Where,

  • T= No. of teeth,
  • D= Pitch circle diameter.

Module

A "Module" in gear terminology is a measurement unit that shows the size of the gear. Modules of two mating gears must be the same. It is calculated by dividing the gear's reference diameter by the number of teeth.

m= D/T

Top-land

The top land of a gear tooth is its top surface.

Addendum

The addendum of the gear is the distance between the pitch circle and the tooth tip circle.

Dedendum

The dedendum of a gear is the distance between the pitch radius and the root radius at the centre of one gear tooth.

Addendum Circle

The addendum circle in gear terminology is concentric with the standard (reference) pitch circle and radially distant from it by the addendum amount. It coincides with the tops of the teeth of a gear. The addendum circle for external gears is on the outside cylinder, whereas the addendum circle for internal gears is on the inside cylinder.

Dedendum Circle

In gear terminology, a dedendum circle is a tangent to the bottom of a gear wheel's gaps between teeth.

Base Circle

The base circle of the involute gear is the circle from which the tooth profiles of the involute gear are constructed.

Whole Depth

The whole depth of a gear is the radial distance between the top land and the tooth's root. It's the total of the addendum and dedendum. It is termed total depth as well.

Working Depth

The depth of engagement of two gears, or the sum of their addendums, is known as working depth. When the centre distance is standard, the standard working distance is the amount of tooth that extends into the tooth space of a mating gear.

Tooth Thickness

The arc length between opposite faces of a tooth, measured around the standard pitch circle, is the tooth thickness of a gear. Because this length cannot be measured directly, a different gear dimension is measured instead, and the tooth thickness is calculated from there.

Tooth Space

The tooth spacing is the distance between two consecutive teeth measured around the circumference of the pitch circle.

The face of the Tooth

The working surface of the tooth above the pitch circle is referred to as the face of the tooth.

The flank of the Tooth

The working surface between the pitch circle and the bottom land, including the gear tooth fillet, is known as the tooth flank.

Fillet Radius

The circle's radius connects the root circle to the tooth's flank part.

Clearance

The space between the gear's outside diameter and its mate's root diameter is known as clearance. This margin compensates for thermal expansion during operation and avoids a gear tooth's top from interfering with the root of its mate gear tooth.

Backlash

This is the amount by which the width of the tooth spacing exceeds the mating gear's tooth thickness.

Gear Ratio

It is the ratio of the larger to the smaller number of teeth in a pair of mating gears. The involute gear's path of contact is a straight line parallel to the base circle traced by the point of contact of two teeth from the beginning to the end of the engagement.

Arc of Contact

The arc of contact is the path traced by a point on a pitch circle from the beginning to the end of the engagement of a specific tooth pair.

Contact Ratio

More than one pair of teeth should always stay in contact to lessen the average load shared by a pair of teeth. The average number of pairs of teeth in contact is known as the contact ratio.

Pressure Angle

The angle of obliquity in gear terminology, also known as the pressure angle to gear teeth, is the angle formed by the tooth face and the tangent of the gear wheel. It's the angle formed by the pressure line (which runs parallel to the tooth surface) and the plane tangent to the pitch surface at a pitch point.

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FAQs on Gear Terminologies

  • The gear terminology refers to defining the essential parts of the gear. These terminologies are defined to have perfect gear tooth and gear design. For example a few gear terminologies are:

    • Dedendum
    • The flank of the Tooth
    • Pitch Circle
    • Addendum Circle
    • Fillet Radius
    • Pitch Circle Diameter
    • Dedendum Circle
    • Clearance
  • Diametral Pitch is abbreviated as DP. The Diametral Pitch (DP) refers to the number of teeth per inch of pitch circle diameter. Inverse inches (1/in) are the units of DP. The unit millimetre (mm) is assigned to express length by ISO standards, although the unit inch is used in the United States, the United Kingdom, and other countries.

  • The module is the unit of measurement for gear tooth size in the metric system. It's comparable to the Diametral Pitch (DP), which is often used for UK system (inch-measure) gears, but the units are different, and they're not reciprocal. The pitch circle diameter is divided by the number of teeth to get the module. Modules are generally measured in millimeters (mm).

  • The gear ratio aids in estimating how many teeth each gear has to achieve the necessary output speed/angular velocity, or torque. When only the diameters or radii of the gears are examined, we can derive the gear ratio using the following equation: 

    gear ratio = (diameter of input gear)/(diameter of output gear)

  • The smaller of two gears is called a pinion when they are meshing together. The drive gear is the one that transmits the force, while the driven gear is the one that receives it. When the pinion is the driver, the output speed slows while the torque increases, resulting in a step-down drive.

  • The Lead is the axial travel (length along the axle) for one complete helical revolution about the pitch diameter of the gear during one complete turn (360°). The lead angle between the helical tooth face and an analogous spur tooth face is 90 degrees. The lead angle is higher with smaller gear diameters for the same lead. Unless otherwise noted, it is assumed to be measured at the standard pitch diameter.

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