Home Page T-46 9.9   Registration The three primary factors contributing to belt drive registration (or positioning) errors are belt elongation, backlash, and tooth deflection.  When evaluating the potential registration capabilities of a  synchronous  belt  drive,  the  system  must  first  be  determined  to  be  either  static  or  dynamic  in terms of its registration function and requirements. Static  Registration:    A  static  registration  system  moves  from  its  initial  static  position  to  a secondary static position.  During the process, the designer is concerned only with how accurately and  consistently  the  drive  arrives  at  its  secondary  position.    He/she  is  not  concerned  with  any potential registration errors that occur during transport.  Therefore, the primary factor contributing to registration error in a static registration system is backlash.  The effects of belt elongation and tooth deflection do not have any influence on the registration accuracy of this type of system. Dynamic  Registration:    A  dynamic  registration  system  is  required  to  perform  a  registering function  while  in  motion  with  torque  loads  varying  as  the  system  operates.    In  this  case,  the designer is concerned with the rotational position of the drive pulleys with respect to each other at every point in time.  Therefore, belt elongation, backlash and tooth deflection will all contribute to registrational inaccuracies. Further discussion about each of the factors contributing to registration error is as follows: Belt  Elongation:    Belt  elongation,  or  stretch,  occurs  naturally  when  a  belt  is  placed  under tension.  The total tension exerted within a belt results from installation, as well as working loads. The amount of belt elongation is a function of the belt tensile modulus, which is influenced by the type of tensile cord and the belt construction.  The standard tensile cord used in rubber synchronous belts is fiberglass.  Fiberglass has a high tensile modulus, is dimensionally stable, and has excellent flex-fatigue  characteristics.    If  a  higher  tensile  modulus  is  needed,  aramid  tensile  cords  can  be considered,  although  they  are  generally  used  to  provide  resistance  to  harsh  shock  and  impulse loads.    Aramid  tensile  cords  used  in  small  synchronous  belts  generally  have  only  a  marginally higher  tensile  modulus  in  comparison  to  fiberglass.    When  needed,  belt  tensile  modulus  data  is available from our Application Engineering Department. Backlash:  Backlash in a synchronous belt drive results from clearance between the belt teeth and  the  pulley  grooves.    This  clearance  is  needed  to  allow  the  belt  teeth  to  enter  and  exit  the grooves  smoothly  with  a  minimum  of  interference.    The  amount  of  clearance  necessary  depends upon  the  belt  tooth  profile.    Trapezoidal  Timing  Belt  Drives  are  known  for  having  relatively  little backlash.    PowerGrip  HTD  Drives  have  improved  torque  carrying  capability  and  resist  ratcheting, but  have  a  significant  amount  of  backlash.    PowerGrip  GT  Drives  have  even  further  improved torque carrying capability, and have as little or less backlash than trapezoidal timing belt drives.  In special  cases,  alterations  can  be  made  to  drive  systems  to  further  decrease  backlash.    These alterations  typically  result  in  increased  belt  wear,  increased  drive  noise  and  shorter  drive  life. Contact our Application Engineering Department for additional information. Tooth Deflection:  Tooth deformation in a synchronous belt drive occurs as a torque load is applied to the system, and individual belt teeth are loaded.  The amount of belt tooth deformation depends upon the amount of torque loading, pulley size, installation tension and belt type.  Of the three  primary  contributors  to  registration  error,  tooth  deflection  is  the  most  difficult  to  quantify. Experimentation with a prototype drive system is the best means of obtaining realistic estimations of belt tooth deflection. Additional guidelines that may be useful in designing registration critical drive systems are as follows: •  Select PowerGrip GT or trapezoidal timing belts.