two  metal  framed,  heavy-duty  blocks.  Block  A  is designed for manila line, and block B is for wire rope. BLOCK TO LINE RATIO The size of a fiber line block is designated by the length  in  inches  of  the  shell  or  cheek.  The  size  of standard wire rope block is controlled by the diameter of  the  rope.  With  nonstandard  and  special-purpose wire rope blocks, the size is found by measuring the diameter  of  one  of  its  sheaves  in  inches. Use care in selecting the proper size line or wire for the block to be used. If a fiber line is reeved onto a tackle whose sheaves are below a certain minimum diameter,  the  line  becomes  distorted  which  causes unnecessary wear. A wire rope too large for a sheave tends to be pinched which damages the sheave. Also, the wire will be damaged because the radius of bend is too short. A wire rope too small for a sheave lacks the  necessary  bearing  surface,  puts  the  strain  on  only a few strands, and shortens the life of the wire. With fiber line, the length of the block used should be  about  three  times  the  circumference  of  the  line. However, an inch or so either way does not matter too much; for example, a 3-inch line may be reeved onto an 8-inch block with no ill effects. Normally, you are more likely to know the block size than the sheave diameter;  however,  the  sheave  diameter  should  be about twice the size of the circumference of the line used. Wire rope manufacturers issue tables that give the proper sheave diameters used with the various types and  sizes  of  wire  rope  they  manufacture.  In  the absence of these, a rough rule of thumb is that the sheave  diameter  should  be  about  20  times  the  diameter of  the  wire.  Remember,  with  wire  rope,  it  is  the diameter,  rather  than  circumference,  and  this  rule refers to the diameter of the sheave, rather than to the size of the block, as with line. TYPES OF BLOCKS A  STANDING   BLOCK   is  a  block  that  is connected to a fixed object. A  TRAVELING   BLOCK   is  a  block  that  is connected to the load that is being lifted. It also moves with the load as the load is moved. A SNATCH BLOCK (fig. 6-5) is a single sheave block fabricated so the shell opens on one side at the base of the hook to allow a rope to slip over the sheave without threading the end through the block. Snatch Figure 6-5.—Snatch blocks. blocks are used when it is necessary to change the direction of pull on the line. REEVING BLOCKS To reeve blocks in simple tackle, you must first lay the blocks a few feet apart. The blocks should be placed down with the sheaves at right angles to each other  and  the  becket  bends  pointing  toward  each  other. To start reeving, lead the standing part of the falls through  one  sheave  of  the  block  that  has  the  greatest number  of  sheaves.  Begin  at  the  block  fitted  with  the becket.  Next  pass  the  standing  part  around  the  sheaves from one block to the other, making sure no lines are crossed until all sheaves have a line passing over them. Now secure the standing part of the falls at the becket of  the  block  having  the  fewest  number  of  sheaves, using  a  becket  hitch  for  temporary  securing  or  an  eye splice  for  permanent  securing. When  blocks  have  two  or  more  sheaves,  the standing part of the fall should be led through the sheave closest to the center of the block. This places the strain on the center of the block and prevents the block from toppling and the lines from being chafed and cut through by rubbing against the edges of the block. Falls   are   normally   reeved   through   8-inch   or 10-inch wood or metal blocks, in such away as to have the  lower  block  at  right  angles  to  the  upper.  Two 3-sheave  blocks  are  the  traditional  arrangement,  and the  method  of  reeving  is  shown  in  figure  6-6.  The hauling part has to go through the middle sheave of the upper block or the block will tilt to the side and the falls  will  jam  under  load. 6-3