Machinery's Handbook, 31st Edition
Non-metallic Gears 2325 impact and abrasion to a degree that might result in excessive wear of cast-iron teeth. Thus, composition gearing of impregnated canvas has often proved to be more durable than cast iron. Application of Non-Metallic Gears.— The most effective field of use for these non-metallic materials is for high-speed duty. At low speeds, when the starting torque may be high, or when the load may fluctuate widely, or when high shock loads may be encountered, these non-metallic materials do not always prove satisfactory. In general, non-metallic materi- als should not be used for pitch-line velocities below 600 feet per minute (3.05 m/s). Tooth Form: The best tooth form for non-metallic materials is the 20-degree stub-tooth system. When only a single pair of gears is involved and the center distance can be varied, the best results will be obtained by making the non-metallic driving pinion of all-adden dum form, and the driven metal gear with standard tooth proportions. Such a drive will carry from 50 to 75 percent greater loads than one of standard tooth proportions. Material for Mating Gear: For durability under load, the use of hardened steel (over 400 BHN or Brinell Hardness Number) for the mating metal gear appears to give the best results. A good second choice for the material of the mating member is cast iron. The use of brass, bronze, or soft steel (under 400 BHN) as a material for the mating member of phenolic laminated gears leads to excessive abrasive wear. Power-Transmitting Capacity of Non-Metallic Gears.— The characteristics of gears made of phenolic laminated materials are so different from those of metal gears that they should be considered in a class by themselves. Because of the low modulus of elasticity, most of the effects of small errors in tooth form and spacing are absorbed at the tooth surfaces by the elastic deformation, and have but little effect on the strength of the gears. If S = safe working stress for a given velocity lb/in 2 (MPa) S s = allowable static stress lb/in 2 (MPa) V = pitch-line velocity in feet per minute (meter/s) then, the recommended practice of the American Gear Manufacturers’ Association, . S S V 200 150 025 US Units s # = + + a k . . . S S V 1016 0 76 0 25 SI Units s × = + + a k The value of S s for phenolic laminated materials is given as 6000 lb/in 2 (41.36 MPa). The accompanying table gives the safe working stresses S for different pitch-line velocities. When the value of S is known, the horsepower capacity is determined by substituting the value of S for S s in the appropriate equations in the section on power-transmitting capacity of plastics gears starting on page 606 . Safe Working Stresses for Non-Metallic Gears
Pitch-Line Velocity, V fpm m/s 600 3.05 700 3.56 800 4.06 900 4.57 1000 5.08 1200 6.10 1400 7.11 1600 8.13
Safe Working Stress, S
Pitch-Line Velocity, V
Safe Working Stress, S
Pitch-Line Velocity, V
Safe Working Stress, S
lb/in 2 2625 2500 2400 2318 2250 2143 2063 2000
MPa fpm m/s
lb/in 2 1950 1909 1875 1846 1821 1800 1781 1743
MPa 13.44 13.16 12.93 12.73 12.56 12.41 12.28 12.02
fpm m/s
lb/in 2 1714 1691 1673 1653 1645 1634 1622 1617
MPa 11.82 11.66 11.53 11.40 11.34 11.27 11.18
1800 2000 2200 2400 2600 2800 3000 3500
4000 4500 5000 5500 6000 6500 7000 7500
18.10 17.24 16.54 15.98 15.51 14.78 14.22 13.79
9.14
20.32 22.86 25.40 27.94 30.48 33.02 35.56 38.10
10.16 11.18 12.19 13.20 14.22 15.24 17.78
11.15 The tensile strength of the phenolic laminated materials used for gears is slightly less than that of cast iron. These materials are far softer than any metal, and the modulus of
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