Machinery's Handbook, 31st Edition
Friction 173 the most common machine elements when carefully made are: ordinary bearings, 95 to 98; roller bearings, 98; ball bearings, 99; spur gears with cut teeth, including bearings, 99; bevel gears with cut teeth, including bearings, 98; belting, 96 to 98; high-class silent power transmission chain, 97 to 99; roller chains, 95 to 97. Coefficients of Friction.— Table 1 and Table 2 provide representative values of static fric- tion for various combinations of materials with dry (clean, unlubricated) and lubricated surfaces. The values for static, or breakaway, friction shown in these tables will generally be higher than the subsequent sliding friction. Typically, the steel-on-steel static coeffi cient of 0.8 unlubricated will drop to 0.4 when sliding has been initiated; with oil lubrica tion, the value will drop from 0.16 to 0.03. Many factors affect friction, and even slight deviations from normal or test conditions can produce wide variations. Accordingly, when friction coefficients are used in design calculations, due allowance or factors of safety should be considered, and in critical applications, specific tests should be conducted to provide specific coefficients for mate- rial, geometry, and/or lubricant combinations. Table 1. Coefficients of Static Friction for Steel on Various Materials
Coefficient of Friction, μ Clean Lubricated
Coefficient of Friction, μ Clean Lubricated
Material
Material
Steel
0.8 0.22 0.35 0.45 0.35 0.4
0.16
Hard carbon Graphite Plexiglas Polystyrene Polythene Tungsten carbide
0.14 0.1
0.11–0.14
Copper-lead alloy Phosphor-bronze Aluminum-bronze
0.1
0.4–0.6 0.4–0.5
0.1–0.2 0.4–0.5
Brass
0.19 0.21 0.16
0.3–0.35 0.3–0.35
Cast iron Bronze
0.2 0.04
0.2 0.04
Teflon
Sintered bronze 0.13 Table 1 and Table 2 used with permission from The Friction and Lubrication of Solids , Vol. 1, by Bowden and Tabor, Clarendon Press, Oxford, 1950. Table 2. Coefficients of Static Friction for Various Materials Combinations
Coefficient of Friction, μ Clean Lubricated
Coefficient of Friction, μ Clean Lubricated
Material Combination Aluminum-aluminum Cadmium-cadmium Chromium-chromium Copper-copper Magnesium-magnesium Nickel-nickel Platinum-platinum Iron-iron
Material Combination Tungsten carbide-tungsten carbide Plexiglas-Plexiglas Polystyrene-polystyrene Solids on rubber Wood on wood (clean) Wood on wood (wet) Wood on metals (clean) Teflon-Teflon Wood on metals (wet)
1.35 0.5 0.41 1.0 1.0
0.30 0.05 0.34 0.08 0.08 0.28 0.25 0.55 0.04
0.2–0.25
0.12
0.8 0.5
0.8 0.5
0.04
0.04
0.15–0.20 Nylon-nylon
0.15–0.25
0.6 0.7 1.2 1.4 0.6
1–4
0.25–0.5
0.2
0.2–0.6
Silver-silver Zinc-zinc Glass-glass Glass-metal
0.2 0.6 0.6 0.4 0.2 0.4 0.2
0.9–1.0 0.5–0.7
0.1–0.6 Brick on wood 0.2–0.3 Leather on wood 0.05–0.1 Leather on metal (clean)
0.3–0.4
Diamond-diamond Diamond-metal Sapphire-sapphire Hard carbon on carbon Graphite-graphite (in vacuum) Graphite-graphite
0.1
0.1–0.15
0.1 0.2
Leather on metal (wet) Leather on metal (greasy) Brake material on cast iron (wet)
0.2 0.16
0.12–0.14 Brake material on cast iron
0.5–0.8
0.1
0.1
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online