to provide some torque-balancing that manufacturers advertise as "torque- 

 free" or "no twist" cable. This technique to reduce cable twisting 

 attempts to equalize the opposing strain energies stored in the contra- 

 helical lays. However, the strain energies can be equalled only if the 

 amount of torque in each layer is equal. 



The external armoring prevents crushing of the cable wound on a 

 spool. Protection against abrasion to the armor wires is sometimes 

 provided by coating the individual wires and fibers with polyethylene 

 or urethane. Still another variation of the contrahelical external 

 armoring is the jacketing of the entire cable with a layer of poly- 

 ethylene or urethane. Table II-2 lists a number of jacketing materials 

 and their physical properties. This coating intrudes between the armor 

 layers and provides not only abrasion protection, but aids torque- 

 balancing and extends the fatigue life" of the cable. 



A radical departure from external armoring is a central core 

 strength member cable. The breaking strength of this type of cable is 

 simply that of the central core, and electrical breakouts are easily 

 accomplished. 



Synthetics used as strength members are usually braided or con- 

 structed as parallel fibers. Advantages of braided synthetics include 

 reduced elongation and increased abrasion resistance to the core, while 

 one disadvantage is the reduction of cable breaking strength. 



By making the synthetic fibers truly parallel the tensile strength 

 can be increased two to three times. Since each filament is carried 

 entirely and equally in tension, there are no isolated stress concentra- 

 tions that can result from overlapping layers in braided fibers. This 

 means that the cable has a longer expected working life. However, paral- 

 lel filament construction makes it more difficult to provide a satis- 

 factory mechanical termination than contrahelically-wound external armor. 



Combinations of armor materials have been used in the construction 

 of E-M cables. An example of such a cable was manufactured by South Bay 

 Cable y for Bell Laboratories that contained fiberglass and improved plow 

 steel. 



In the construction of double-armored steel E-M cables the cable 

 breaking strength varies directly with the cable diameter. Figure II-5 

 gives approximate amounts for the breaking strength that can be expected 

 for a given cable diameter. (Data for the figure, except where noted, 

 were obtained from Reference 1.) Double-armored construction, however, 

 is not the only construction that is used. Manufacturers-'-'" claim that 

 they can produce E-M cables with more than two armor layers. Breaking 

 strengths for a given diameter can, therefore, be increased but the space 

 for conductors is proportionately decreased. 



Failure Mechanisms 



During the review of the mechanical and material properties, it 

 appears that knowledge gained from wire rope technology is directly 

 applicable to the strength members of E-M cable. Two explicit examples 

 are: (1) the use of steel or steel alloys as the armoring material; and 

 (2) the contrahelical winding of armor. This typical E-M cable construc- 

 tion is subject to two major failure mechanisms, kinking and fatigue. 



10 



