has outlived its usefulness. Full your yarn from the middle,  away  from  the  ends,  or  it  will  get  fouled. STRENGTH OF FIBER LINE Overloading a line poses a serious threat to the safety of personnel, not to mention the heavy losses likely to result through damage to material. To avoid overloading, you must know the strength of the line with  which  you  are  working.  This  involves  three factors:  breaking  strength,  safe  working  load  (swl), and safety factor. Breaking strength refers to the tension at which the line will part when a load is applied. Breaking strength  has  been  determined  through  tests  made  by rope  manufacturers,  who  provide  tables  with  this information. In the absence of manufacturers’ tables, a rule of thumb for finding the breaking strength of manila line using the formula: =  BS.  C equals the circumference in inches, and BS equals the breaking  strength  in  pounds.  To  find  BS,  first  square the  circumference;  you  then  multiply  the  value obtained by 900. With a 3-inch line, for example, you will get a BS of 8,100, or 3 x 3 x 900= 8,100 pounds. The  breaking  strength  of  manila  line  is  higher than that of sisal line. This is caused by the difference in strength of the two fibers. The fiber from which a particular line is constructed has a definite bearing on its  breaking  strength.  The  breaking  strength  of  nylon line is almost three times that of manila line of the same size. The best rule of thumb for the breaking strength of nylon is BS = C² x 2,400. The symbols in the rule are  the  same  as  those  for  fiber  line.  For  2  1/2-inch nylon line, BS = 2.5 x 2.5 x 2,400= 15,000 pounds. Briefly defined, the safe working load of a line is the load that can be applied without damaging the line. Note that the safe working load is considerably less  than  the  breaking  strength.  A  wide  margin  of difference   between   breaking   strength   and   safe working load is necessary. This difference allows for such factors as additional strain imposed on the line by  jerky  movements  in  hoisting  or  bending  over sheaves  in  a  pulley  block. You may not always have a chart available to tell you the safe working load for a particular size line. Here is a rule of thumb that will adequately serve your needs  on  such  an  occasion:  swl  = x 150.  In  this equation,  swl  equals  the  safe  working  load  in  pounds, and C equals the circumference of the line in inches. Simply take the circumference of the line, square it, then multiply by 150. For a 3-inch line, 3 x 3 x 150= 1,350 pounds. Thus, the safe working load of a 3-inch line is equal to 1,350 pounds. If line is in good shape, add 30 percent to the swl arrived at by means of the preceding rule; if it is in bad shape, subtract 30 percent from the swl. In the example  given  above  for  the  3-inch  line,  adding  30 percent to the 1,350 pounds gives you a safe working load of 1,755 pounds. On the other hand, subtracting 30 percent from the 1,350 pounds leaves you with a safe working load of 945 pounds. Remember that the strength of a line decreases with  age,  use,  and  exposure  to  excessive  heat,  boiling water, or sharp bends. Especially  with  used  line, these and other factors affecting strength should be given  careful  consideration  and  proper  adjustment made in determining the breaking strength and safe working load capacity of the line. Manufacturers of line provide tables that show the breaking strength and safe working load capacity of line. You will find such  tables  very  useful  in  your  work.  You  must remember,  however,  that  the  values  given  in manufacturers’  tables  only  apply  to  new  line  being used  under  favorable  conditions.  For  that  reason,  you must  progressively  reduce  the  values  given  in manufacturers’  tables  as  the  line  ages  or  deteriorates with  use. Keep in mind that a strong strain on a kinked or twisted line will put a permanent distortion in the line. Figure  4-4  shows  what  frequently  happens  when pressure is applied to a line with a kink in it. The kink that could have been worked out is now permanent, and the line is ruined. The safety factor of a line is the ratio between the breaking strength and the safe working load. Usually, a safety factor of 4 is acceptable, but this is not always the  case.  In  other  words,  the  safety  factor  varies depending on such things as the condition of the line and  circumstances  under  which  it  is  to  be  used. Although the safety factor should never be less than 3, it often must be well above 4 (possibly as high as 8 or Figure 4-4.—Results of a strong strain on a tine with a kink in it. 4-3