DOUBLE
SPANISH


Tackles  601

a  factor  of  safety  of  4},  using  the  best  Manila  rope,  we  get  the
equivalent  circumference  from  the  formula

Vtension  x  10  =  V^iTxTo  =  2|",

say  2 J",  as  the  manufactured  sizes  grade  by  quarters.
The  size  of  the  double  blocks  to  take  the  rope  would  be  7  inches,
obtained  by  the  rule  on  p.  394,  and  it  would  require  four  men  to
handle  the  hauling  part.
It  is  desired  to  lift  a  weight  of  12  tons  with  a  ship's  derrick,  and
the  maximum  load  on  the  winch  must  not  exceed  6  tons;  required
the  purchase,  size  of  steel  wire  rope  falls  and  blocks  ?  Owing  to
the  heavy  load  dealt  with  in  this  case,  the  factor  of  safety  need
not  exceed  6.  The  hauling  part  of  falls  to  be  led  through  a  lead-
ing block  at  heel  of  derrick.
lioad  to  be  raised 12     tons

Friction  of  6  sheaves 6        **

Derrick  gear 4    »*

Total  load  to  overcome 18.4    **

As  the  load  on  the  winch  may  not  exceed  6  tons,  the  purchase

18  4
should  be  -j-^  =  four  parts  in  the  falls  —  a  twofold  purchase.
A  factor  of  safety  of  6  having  previously  been  decided  upon,  we
get  for  the  breaking  stress  4.6  x  6=  23  tons,  and  the  equivalent  cir-
cumference of  special  flexible  steel  wire  rope,  per  table  =  3  inches
circ,  which  will  require  two  double  blocks  with  sheaves  13 J
inches  in  diameter.  It  should  be  noted  that  the  maximum
tension  comes  on  the  hauling  part  in  hoisting,  but  on  the  standing
part  in  lowering.
The  stress  on  topping  lift,  allowing  for  friction  of  one  sheave,
and  i)Ower  applied  is  equal  to  0  4  tons,  requiring  special  flexible
steel  wire  rope  of  8 J"  circumference.
