Machinery's Handbook, 31st Edition
2326 Non-metallic Gears elasticity is about one-thirtieth that of steel. In other words, if the tooth load on a steel gear that causes a deformation of 0.001 inch (0.025 mm) were applied to the tooth of a similar gear made of phenolic laminated material, the tooth of the non-metallic gear would be deformed about 1 ∕ 32 inch (0.794 mm). Under these conditions, several things will happen. With all gears, regardless of the theoretical duration of contact, one tooth only will carry the load until the load is sufficient to deform the tooth the amount of the error that may be present. On metal gears, when the tooth has been deformed the amount of the error, the stresses set up in the materials may approach or exceed the elastic limit of the material. Hence, for standard tooth forms and those generated from standard basic racks, it is dangerous to calculate their strength as very much greater than that which can safely be carried on a single tooth. On gears made of phenolic laminated materials, on the other hand, the teeth will be deformed the amount of this normal error without setting up any appreciable stresses in the material, so that the load is actually supported by several teeth. All materials have their own peculiar and distinct characteristics, so that under certain specific conditions, each material has a field of its own where it is superior to any other. Such fields may overlap to some extent, and only in such overlapping fields are different materials directly competitive. For example, steel is more or less ductile, has a high tensile strength, and a high modulus of elasticity. Cast iron, on the other hand, is not ductile, has a low tensile strength, but a high compressive strength, and a low modulus of elasticity. Hence, when stiffness and high tensile strength are essential, steel is far superior to cast iron. On the other hand, when these two characteristics are unimportant, but high compres sive strength and a moderate amount of elasticity are essential, cast iron is superior to steel. Preferred Pitch for Non-Metallic Gears.— The pitch of the gear or pinion should bear a reasonable relation either to the power or speed or to the applied torque, as shown by the accompanying table. The upper half of this table is based upon horsepower (kw) transmit ted at a given pitch-line velocity. The lower half gives the torque in pounds-feet (N-m) or the torque at a 1-foot (meter) radius. This torque T for any given horsepower (kw) and speed can be obtained from the following formulas: T T 5252 9550 rpm hp pound-feet rpm kw N-m # # = = Bore Sizes for Non-Metallic Gears.— For plain phenolic laminated pinions, that is, pinions without metal end plates, a drive fit of 0.001 inch per inch (or mm/mm) of shaft diameter should be used. For shafts above 2.5 inches (63.5 mm) in diameter, the fit should be con- stant at 0.0025 to 0.003 inch (0.064–0.076 mm). When metal reinforcing end plates are used, the drive fit should conform to the same standards as used for metal. The root diameter of a pinion of phenolic laminated type should be such that the mini mum distance from the edge of the keyway to the root diameter will be at least equal to the depth of tooth. Keyway Stresses for Non-Metallic Gears.— The keyway stress should not exceed 3000 psi (20.68 MPa) on a plain phenolic laminated gear or pinion. The keyway stress is calculated by the formulas: S V A S V A 33000 1000 hp psi kw MPa # # # # = = where S = unit stress in pounds per square inch (newton per square meter) hp = horsepower transmitted kw = kilowatt power transmitted V = peripheral speed of shaft in feet per minute (meter/sec) A = square inch (square meter) area of keyway in pinion (length × height) If the keyway stress formula is expressed in terms of shaft radius r (inch or meter) and revolutions per minute, it will read:
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