Home Page T-13 The "S" twist is obtained if we visualize the two strands being held stationary with our left hand on  one  end,  while  a  clockwise  rotation  is  imparted  by  our  right  hand  to  the  two  strands,  thus creating a twisted cord.  The "Z" twist is obtained similarly, if a counterclockwise rotation is imparted to the two strands. Different  types  of  cord  twist  will  cause  side  thrust  in  opposite  directions.    The  "S"  twist  will cause a lateral force direction which will obey the "Right Hand" rule as shown in Figure 17. A  "Z"  type  cord  twist  will  produce  a  direction  of  lateral  force  opposite  to  that  of  "S"  cord. Therefore, in order to produce a belt with minimum lateral force, standard belts are usually made with "S" and "Z" twist construction, in which alternate cords composed of strands twisted in opposite directions are wound in the belt.  This is illustrated in Figure 18. The  lay  of  the  cord  is  standard,  as shown in Figure 18, and it is wound from left to  right  with  the  cord  being  fed  under  the mold.    The  smaller  the  mold  diameter  and the  fewer  the  strands  of  cord  per  inch,  the greater will be the helix angle, and the greater the  tendency  of  the  lay  of  the  cord  to  make the belt move to one side. In  general,  a  standard  belt  of  "S"  and "Z" construction, as shown in Figure 18, will have  a  slight  tendency  to  behave  as  a predominantly  "S"  twist  belt,  and  will  obey the "Right Hand" rule accordingly. 6.3  Factors Contributing To Side Travel The  pulleys  in  a  flat  belt  drive  are  crowned  to  keep  the  belt  running  true.    Since  crowned pulleys are not suitable for a timing belt, the belt will always track to one side.  Factors contributing to this condition include: I. In the Drive 1.    Misalignment  –   A belt (any belt – any construction) will normally climb to the high end (or tight) side. 2.    Tensioning   –   In  general,  lateral  travel  can  be  altered  or  modified  by  changing tension. 3.    Location of plane   –   Vertical drives have a greater tendency to move laterally due to gravity. (a) Clockwise Rotation (b) Counterclockwise Rotation Fig. 17    Right Hand Rule Applicable to "S" Twist Fig. 18  "S" and "Z" Cord Lay of the Mold