The Bell System Technical Journal 



January, 1933 



Pulp Insulation for Telephone Cables * 



By H. G. WALKER and L. S. FORD 



Pulp insulation is a new type of insulation that has been developed to 

 replace the well-known spirally wrapped ribbon paper insulation in certain 

 kinds of telephone cables. It consists of a continuous pulp sleeving formed 

 directly on the wire by a modified paper making process. The raw material 

 for this insulation is commercial Kraft pulp and its preparatory treatment in 

 the beaters corresponds to that given in the regular paper making process. 



The machine used to apply this pulp to the wire is a modified single 

 cylinder paper machine equipped to insulate 60 wires simultaneously. The 

 wires are taken from the supply spools by means of flyers so as to allow the 

 brazing of the wire on a nearly empty spool to a conveniently located full 

 one. This gives continuous operation. The wires are fed to the machine 

 through an electrolytic cleaner for the removal of residual drawing com- 

 pound. The surface of the mold or paper forming mechanism is divided into 

 60 narrow portions in such a way as to form that many narrow sheets con- 

 tinuously. The wires are brought into contact with the mold in such a way 

 that, as it rotates and forms the sheets, a single wire is embedded in each 

 sheet. These sheets and wires are transferred from the mold to a traveling 

 wool blanket by the pressure of the couch roll. The traveling blanket 

 carries the sheets and wires through the presses for dewatering and con- 

 solidating, and delivers them to the polishers where the sheet is turned down 

 by a rapidly rotating mechanism into a cylindrical wet sleeve surrounding 

 the wire. The moisture is driven from the wet insulation by passage 

 through a box type electric furnace one end of which is maintained at a 

 rather high temperature. The insulated wire is then taken up on spools 

 ready for the twisting operation. The speed of the machine is about 130 

 feet per minute. . 



The major difficulties in the process have been overcome and the basic 

 properties of the insulation have been determined. Equipment for the 

 production of about 225 million conductor feet per week has been provided 

 and the entire output of 24 and 26 A.W.G. cables is being made in pulp. 



These cables are designed to the same size as the ribbon paper cables 

 which they replace and compare favorably with them in their electrical 

 characteristics except that the mutual capacitance is slightly higher. The 

 impairment in transmission efficiency due to the higher capacitance is, how- 

 ever, more than offset by the lower cable first cost. 



Standardized installation practices are followed except that a softer and 

 more lubricating type of boiling-out compound than paraffin wax is required, 

 particularly at low temperatures. A suitable compound has been found by 

 adding paraffin oil to wax in varying proportions depending upon the tem- 

 perature at the point of splicing. 



The anticipated savings have been realized in the operation of the com- 

 mercial units and the further expansion of the uses of this insulation is 

 being studied. 



In this paper a more complete and technical treatment of the pulp 

 insulation development is presented than was given in previous papers.' 



* Presented before A. I. E. E. in Baltimore, Md., October, 1932. Published in 

 abridged form in Electrical Engineering, December, 1932. 



^Bell System Technical Journal, Vol. X, pp. 432-471, "Developments »n the 

 Manufacture of Lead Covered Paper Insulated Telephone Cable" — J. R. Shea. 

 Bell Telephone Quarterly, Vol. X, No. 4, pp. 211-215, "An Important New Insulating 

 Process for Cable Conductors"— H. G. Walker. Bell Laboratories Record, Vol. X, 

 No. 8, pp. 270-278, "Pulp— The New Cable Insulation"— L. S. Ford. 



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