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T–3 HANDBOOK OF TIMING BELTS AND PULLEYS SECTION 1    INTRODUCTION Timing belts are parts of synchronous drives which represent an important category of drives. Characteristically,  these  drives  employ  the  positive  engagement  of  two  sets  of  meshing  teeth. Hence, they do not slip and there is no relative motion between the two elements in mesh. Due to this feature, different parts of the drive will maintain a constant speed ratio or even a permanent relative position.  This is extremely important in applications such as automatic machinery in which a definite motion sequence and/or indexing is involved. The  positive  nature  of  these  drives  makes  them  capable  of  transmitting  large  torques  and withstanding large accelerations. Belt  drives  are  particularly  useful  in  applications  where  layout  flexibility  is  important.    They enable  the  designer  to  place  components  in  more  advantageous  locations  at  larger  distances without paying a price penalty.  Motors, which are usually the largest heat source, can be placed away from the rest of the mechanism.  Achieving this with a gear train would represent an expensive solution. Timing  belts  are  basically  flat  belts  with  a  series  of  evenly  spaced  teeth  on  the  inside circumference, thereby combining the advantages of the flat belt with the positive grip features of chains and gears. There is no slippage or creep as with plain flat belts.  Required belt tension is low, therefore producing very small bearing loads.  Synchronous belts will not stretch and do not require lubrication. Speed is transmitted uniformly because there is no chordal rise and fall of the pitch line as in the case of roller chains. The  tooth  profile  of  most  commonly  known  synchronous  belts  is  of  trapezoidal  shape  with sides  being  straight  lines  which  generate  an  involute,  similar  to  that  of  a  spur  gear  tooth.    As  a result,  the  profile  of  the  pulley  teeth  is  involute.    Unlike  the  spur  gear,  however,  the  outside diameter  of  a  timing  pulley  is  smaller  than  its  pitch  diameter,  thus  creating  an  imaginary  pitch diameter which is larger than the pulley itself.  This is illustrated in  Figure 1.  Backlash between pulley and belt teeth is negligible. The trapezoidal shape timing belt was superseded by a curvilinear tooth profile which exhibited some desirable and superior qualities.  Advantages of this type of drive are as follows: Proportionally deeper tooth;  hence tooth jumping or loss of relative position is less probable. Lighter construction, with correspondingly smaller centrifugal loss. Smaller unit pressure on the tooth since area of contact is larger. Fig. 1    Pulley and Belt Geometry NOTE:   Credit  for  portions  of  this  technical  section  are  given  to:    Gates  Rubber  Co.,  Sales  Engineering  Dept., Rubber Manufacturers Association (RMA), International Organization for Standardization (ISO). Pitch (circular pitch) Pulley Pitch Circle Belt Pitch Line Trapezodial Tooth Profile Curvilinear Tooth Profile Pitch (circular pitch) Belt Pitch Line Sprocket Pitch Circle