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26--Ropes, Chains, and Slings arrangements that have a definite pitch or lay to form a strand Then, the required number of strands are helically laid or formed around the core, which may be sisal rope, a metallic strand, or independent wire rope core The size, number, and arrangement of wires, the number of strands, the lay, and the type of core m a rope are determined largely by the service for which the rope is to be used In general, the greater the number of wires in a strand and the greater the number of strands, the more flexible the rope Hoisting ropes require flexibility and are usually made up of six strands of 19 (16 to 26 wires per strand) and six strands of 37 (27 to 49 wires per strand) wire rope classifications (Fig 26-2) Depending upon the service conditions, six-strand ropes may have a fiber core (FC), a wire strand core (WSC), or an independent wire rope core (IWRC) and may be either regular lay (wires in the strands are laid in the opposite direction from that of the strands m the rope) or lang lay (wires m the strands are laid m the same direction as that of the strands m the rope) Eight-strand hoisting ropes can also be found They are usually 19-wire rope classi fication with regular lay and fiber core Flexibility is not a requirement for guv wires, highway guards, and similar services Therefore, wire rope of six by seven con struction (six strands with seven oi eight wires per strand) is suitable When lope for a particular service is to be selected, it is recommended that engineers of reliable wue rope manufacturers be consulted for the most suitable type Some service conditions lequne rope with special qualities Fiber cores are affected by temperatures above 212 F Under such con ditions, a metallic core provides greater ef ficiency and safety A zinc-coated or stain less steel wire rope effectively lesists some types of corrosion Specific corrosion pioblems should be refeired to a wne lope mnnufactuier Since preformed wire lope does not un ravel, it has advantages for certain services, such as for slings and on constiuction and similar heavy equipment Preformed rope is less likely to set oi kink, and broken wires do not protrude to create a hazard to the TABLE 26-E SAFETY FACTORS FOR WIRE HOISTING ROPES FOR ELECTRIC ELEVATORS Rope Speed (fpm) Minimum Factor of Safety Passenger Freight 50 250 700 1100 1500 760 8 90 11 00 11 70 11 90 665 7 90 9 80 1040 10 55 Excerpted from Table 212 3 ot American National Standard Safety Code tor Elevators, Dumbwaiters, Escalators, and Moving Walks, A17 1-1971 hands of workmen However, closer inspec tion is necessary to detect broken wires It is recommended that hoisting rope have at least the strength of the "improved plow steel" grade For many applications the "extra improved plow steel" grade, which is the greatest in strength, should be used to provide an adequate factor of safety and bet ter service It resists abrasion, shock, vibiation, and fatigue (Table 26-D) Safety factor for hoisting ropes The safety factor for hoisting ropes is calcu lated by dividing the breaking strength of the rope (rated by the manufacturer or de termined by testing) by the sum of the maximum weights to be hoisted (static load) In the case of mine hoisting rope, the max imum weights to be hoisted include the weight of the skip or car and cage, plus the weight TABLE 26-F TREAD DIAMETERS OF SHEAVES AND DRUMS FOR WIRE ROPE Rope Classification Average Recommended (Times rope diameter) Minimum 6x7 72 42 6 x 19 45 30 6X37 27 18 8 x 19 31 21 720