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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