Machinery's Handbook, 31st Edition
PLASTICS GEAR DESIGN 607 because of superior sliding properties with reduced noise and less need for lubrication, chemical or electrical properties, or resistance to wear. However, plastics gear teeth slide more smoothly and easily against metal teeth than do plastics against plastics, and wear is less. For power transmission, plastics gear teeth are usually of involute form. See also Non-Metallic Gearing on page 2324. Most plastics gears are made from nylons and acetals, although acrylonitrile-butadiene- styrenes (ABS), polycarbonates, polysulfones, phenylene oxides, polyurethanes, liquid crystal polymers, and thermoplastic polyesters have also been used. Gears can be made from virtually any plastic, including polypropylene and polyethylene. But care must be taken to manage deflection and stresses, even in low-load and low-speed applications. Additives used in plastics for gears include glass and carbon fiber for added strength and stiffness, and fibers, beads, and powders for reduced thermal expansion and better dimensional stability. Other materials, such as molybdenum disulfide, polytetrafluoroethylene powder (PTFE), and silicones may be incorporated as integral lubricants to reduce friction and wear. Choice of plastics gear material depends on requirements for size and nature of loads to be transmitted, designated speeds, required life, working environment, type of cool - ing, lubrication, and operating precision. Because of cost, plastics gears are sometimes not enclosed in sealed housings and are often given only a single coating of lubricant grease. Overloading of lubricated plastics gear teeth will usually cause tooth fracture, and unlubri cated teeth often suffer excessive wear. Thermoplastics strength varies with temperature, with higher temperatures reducing root stress and permitting tooth deformation. In calcu lating power to be transmitted by spur, helical, and straight bevel gearing, the following formulas should be used with the factors given in Table 13, Table 14, and Table 15: US Customary Units Metric Units Internal and External Spur Gears (33a) (33b) Internal and External Helical Gears (34a) (34b) Straight Bevel Gears (35a) (35b) S s = safe stress in bending (from Table 14); F = face width in inches (mm); Y = tooth form factor (from Table 13); m = module, mm; C = pitch cone distance in inches (mm); C s = service factor (from Table 15); P = diametral pitch; P n = normal diametral pitch; and V = velocity at pitch circle diameter in ft/min (m/s). Example: As an example, assume that a material is to be selected for a spur gear that must transmit 1 ⁄ 8 hp at 350 rpm for 8 hours/day under a steady load. The gear is to have 75 teeth, 32 diametral pitch, 20 degree pressure angle, 0.375 inch face width, and a pitch diameter of 2.3438 inches. Using Equation (33a), HP V PC h S FYV s 55 600 ^ s = + . 327 305 ^ KW V C s h FYmSV s = + HP S FYV s = V PC h 423 78 n s + ^ KW FYmSV s = . V C 179 556 s + ^ h V CC FYmSV C F 327 305 s s . + ^ – ^ h h HP V PCC h S FYV C F s − ^ 55 600 ^ s = + h KW =
55 600 ^
V PCHP s h
S FYV s
+
or
HP
S
=
=
s
FYV
55 600 ^
V PC h
+
s
and
. 0125
. 0434
HP
Y
=
=
rpm D # # π
. 12 350 3 1416 2 3438 215 . # # =
ft
min
V
=
=
⁄
12
. 0 375 0 434 215 55 600 215 32 1 00 0 125 5,124 lb / in s 2 # # # # = + = ^ h . . .
therefore,
S
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