ropes  obtained  commercially  and  tested  to  destruction.
Maximum  and  minimum  values  are  given,  as  well  as  average  ones,,
to  show  the  range  of  ultimate  stress  values,  even  in  new  ropes,

23.  More  than  one-third  of  the  ultimate  load  should  never  be  Working.
pat  on  a  rope,  and  a  larger  factor  of  safety  should  generally  be  loads,
employed,  especially  if  using  worn  ropes  and  with  a  live  load.
For  important  work  a  piece  of  the  cordage  to  be  employed  might
be  tested  to  destruction,  and  the  amount  of  the  working  load  thus
determined.  For  hasty  work  in  the  field  a  rough  rule  is  necessary
which  must  give  enough  margin  to  be  safe  with  the  weakest
variety  of  cordage  likely  to  be  used.  The  safe  working  load  of
all  cordage  in  the  field,  with  the  exception  of  coir,  has  conse-.
quently  been  laid  down  as  follows  : —

C"  cwts.
where  C  is  the  circumference  in  inches.     This  may  be  increased,.
for  good  cordage,  in  good  condition,  up  to  a  maximurn.  of.—

2  C-  cwtii.
The  safe  working  load  on  coir  cordage  can  be  taken  as —
I  C-  cwts.
It  may  be  noted  that  the  expression  C^  cwts.  gives  an  intensity
of  stress  of  about  1,400  lbs.  per  square  inch.
24.  Cordage  stretches  considerably  when  loaded.     In  certain  Stretching.
cases  it  might   be  UocfiU   to   know  approximately  the  amount

of  stretching  that  will  take  place  under  a  given  load.
(B11UU3)  |j>


.'jO


5IATERIALS.
Part  IIIa


Use  of
cordage.
