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Vocabulary Term Definition

coarse-pitch gear

A gear with a diametral pitch of 19 or less. The teeth

of coarse-pitch gears are larger and have greater

spaces between them than the teeth of fine-pitchgears.

conjugateIntersecting with two involute profiles that transmit

motion uniformly from one gear to the next.

contact ratio

A ratio that represents the average number of gear

tooth pairs in contact on a pair of meshing gears.Greater contact ratios can create smoother operation.

dedendumThe distance between the bottom land of the gear

tooth and the pitch circle.

dedendum circleAn imaginary circle that passes through the

dedendums of the gear teeth.

diametral pitch

The number of teeth given per inch of a gear's pitch

diameter. Diametral pitch is the most common

method of classifying gears.

drive gear

The gear that receives energy from a power source,

such as an electric motor. A drive gear transmits

power to a meshing driven gear to perform work.

driven gear

The gear that receives motion from the drive gear on

a machine. Driven gears often turn tools or

components.

efficiencyA measure of the work output of a system versus the

total work input.

faceThe surface of a gear tooth located between the pitch

circle and the addendum circle.

fine-pitch gear

A gear with a diametral pitch of 20 or greater. The

teeth of fine-pitch gears are smaller and closer

together than the teeth of coarse-pitch gears.

flankThe surface of a gear tooth located between the pitchcircle and the dedendum circle.

frictionA force that resists motion between two objects thatare in contact with each other.

gear

A round or cylindrical mechanical component with

teeth, used to transmit power. Gears are designed tointersect with one another and can alter the speed,

torque, or direction of mechanical energy.

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Vocabulary Term Definition

gear ratio

The relationship between the number of teeth on two

meshing gears used to transmit motion. The gear

ratio reflects the change in mechanical advantageresulting from the gears.

gear reduction

A gear ratio used to decrease the speed and increase

the torque of mechanical energy. A 1:2 gear ratio isan example of a gear reduction.

herringbone gearA type of gear that has angled teeth in the shape of aletter "V." Herringbone gears resist side loading but

are expensive to produce.

idler gearA gear that is used to keep the direction of motionconsistent between a drive gear and a driven gear.

involute curve The path determined by tracing a point on a line thatis unwound from a circle.

involute gear

A gear that is designed with an involute profile to

transmit motion uniformly. Most gears used todayare involute gears.

linear motion

Motion that takes place along a straight line.

Mechanical energy can take the form of rotary orlinear motion.

meshing

The action of interlocking with another object.

Meshing gears are used to transmit mechanical

energy.

nomenclatureA system of naming objects in a given category.Gear nomenclature refers to the terminology used todescribe and specify gears.

pinionA circular gear used in a rack and pinion system toproduce linear motion. As the pinion turns, the flat

rack slides in a linear direction.

pitchA property used to classify gears. Gears can beclassified by circular pitch or diametral pitch, but the

diametral pitch method is the most common.

pitch circle

An imaginary circle on a gear that divides the gearteeth into top lands and bottom lands, and into

addendums and dedendums. The pitch circles of two

gears in correct mesh contact each other at the pitchpoint.

pitch diameter The diameter of a gear's pitch circle. A gear's pitch

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Vocabulary Term Definition

diameter can be determined by measuring from the

top of one gear tooth to the bottom of the opposite

gear tooth.

pitch point The point at which the pitch circles of two gears arein contact with each other at a single point.

pressure angleThe angle made by the sides of a gear tooth as they

incline toward the top of the gear.

rack

A flat bar with teeth used in a rack and pinion

system to produce linear motion. As the circular

pinion turns, the rack slides in a linear direction.

rack and pinion

A pair of gears used to convert rotary motion into

linear motion. A rack and pinion consists of a

circular gear, or pinion, that meshes with a flat-toothed bar, or rack.

rotary motionSpinning or turning motion that takes place around

an axis, without a change in linear position.

shaft

A cylindrical bar used to support rotating

components or to transmit rotation in a mechanical

system.

speed

The amount of distance an object travels in a given

period of time. Speed is used to measure both linear

and rotational movement.

spur gear

A type of gear that has straight, flat-topped teeth set

parallel to the shaft. Spur gears are the mostcommon industrial gears.

tangent

In gear geometry, the point at which the pitch circles

of two gears are in contact with each other at asingle point. The tangent is also called the pitch

point.

top landThe top part of a gear tooth, which extends abovethe pitch circle.

torqueA force that produces rotation. Power transmissioncomponents are often used to increase or decreasetorque.

wearThe gradual removal of material on a surface caused

by contact between surface asperities.

whole depth Either the total height of a gear tooth, or the total

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Vocabulary Term Definition

depth of the tooth space.

working depthA measurement of how deeply a gear tooth extendsinto the tooth space of the mating gear.

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