Unit 18Gears, Splines, and Serrations

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Learning Objectives

• Identify spur gears and their representations and specifications.

• Identify bevel gears and their representations and specifications.

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Learning Objectives

• Identify worm gears and their representations and specifications.

• Identify splines and serrations and their representations and specifications.

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Gears, Splines, and Serrations

• Gear: A wheel-like machine part with a number of equally spaced teeth– Teeth mesh with the teeth on another gear– Purpose is to transmit power in a mechanism

• Splines: A series of ridges on a shaft– Match to serrations on a mating part– Purpose is to maintain the position of the parts

Spur Gear Terminology

Spur Gear Terminology

• Diametral pitch (Pd): Number of teeth in a gear per inch of pitch diameter– Mating gears must have identical diametral pitches

• Pitch diameter (D): Diameter of an imaginary circle located approximately halfway between the roots and tops of gear teeth– Used to calculate the gear ratio

Spur Gear Terminology

• Circular pitch (Pc): Length of the arc along the pitch circle between one point on a tooth to the same point on the next tooth– Calculated by πD ÷ n

• Circular tooth thickness: Thickness of a gear tooth as measured along the arc– Calculated by Pc ÷ 2

Spur Gear Terminology

• Addendum (a): Radial distance between the pitch circle and the top of the tooth– Calculated as 1 ÷ Pd

• Dedendum (b): Radial distance between the pitch circle and the bottom of the tooth– Calculated as 1.25 ÷ Pd

Spur Gear Terminology

• Clearance: Radial distance between the top of a tooth on one gear and the bottom of the tooth space on the mating gear– Calculated as b – a

• Working depth: Overlap of two mating spur gears– Represented by the sum of the addendums (a + a)

Spur Gear Terminology

• Whole depth: Total depth of a tooth space on one gear– Calculated as a + b

• Pressure angle: Angle between the tooth profile and a radial line at the intersection on the pitch circle– Pressure angles of mating gears must be the same– Standard angles include 14.5° (former), 20°, and 25°

Gear Ratios Using the Pitch Circle• Pitch circles of mating gears are equivalent to theoretical

friction roller diameters and used to calculate gear ratios

Measuring with a Gear-Tooth Caliper• A gear-tooth caliper measures straight distances, as

reflected in this diagram

Bevel Gear Terminology

Worm Gear Terminology

Splines and Serrations

Photo: Daimler

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