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T–56 the capacity to maintain the necessary accuracy. 4.   Pulleys for fixed center systems should be manufactured with a process that is capable of producing the required O.D. tolerances accurately enough. 5.   The performance capabilities of the drive system should be verified by testing belts produced over their full length tolerance range on drive systems representing the full potential center- distance variation. SECTION 13    IDLER USAGE Idlers in synchronous belt drives are commonly used to take up belt slack, apply installation tension  or  to  clear  obstructions  within  a  system.    While  idlers  cause  additional  belt  bending, resulting in fatigue, this effect is generally not significant as long as proper design procedures are followed.    Synchronous  belts  elongate  very  little  over  time,  making  them  relatively  maintenance free.    All  idlers  should  be  capable  of  being  locked  down  after  being  adjusted  and  should  require little additional attention.  Specific guidelines and recommendations are given below. 13.1   Inside/Outside Inside idlers are generally preferred over backside idlers from a belt fatigue standpoint.  Both are commonly used with good success.  Inside idlers should be pulleys, but can be flat, if the O.D. is  equivalent  to  the  pitch  diameter  of  a  40-groove  pulley.    Backside  idlers  should  be  flat  and uncrowned. 13.2   Tight Side/Slack Side Idlers should be placed on the slack (or nonload-carrying) side, if possible.  Their effect on belt fatigue is less on the slack side than on the tight (or load-carrying) side.  If spring-loaded idlers are used,  they  should  never  be  placed  on  the  tight  side  (see  Spring-Loaded  Idlers).    Also,  note  that drive direction reversals cause the tight and slack spans to reverse, potentially placing the idler on the tight side. 13.3   Idler Placement In synchronous belt drives, idlers can be placed nearly anywhere they are needed.  Synchronous drives  are  much  less  sensitive  to  idler  placement  and  belt  wrap  angles  than  V-belt  drives.    The designer  should  make  sure  that  at  least  6  belt  teeth  are  in  mesh  on  load-carrying  pulleys.    For every  tooth  in  mesh  less  than  this  (with  a  minimum  of  2),  20%  of  the  belt  torque  rating  must  be subtracted.  In order to minimize the potential for belt ratcheting, each loaded pulley in the system should also have a wrap angle of at least 60.  If a loaded pulley has less than 6 teeth in mesh and 60  of  wrap,  idlers  can  often  be  used  to  improve  this  condition.    Nonloaded  idler  pulleys  do  not have tooth meshing or wrap angle restriction. 13.4   Spring-Loaded Idlers Using a spring to apply a predetermined force against a tensioning idler to obtain proper belt installation tension is acceptable as long as the idler can be locked down after belt installation. Dynamic spring-loaded idlers are generally not recommended for synchronous belt drives.  If used, spring-loaded belt idlers should never be used on the tight (or load-carrying) side.  Tight side tensions vary with the magnitude and type of load carried by the system.  High tight side tensions can  overcome  the  idler  spring  force  allowing  the  belt  to  ratchet.    In  order  to  prevent  this  from occurring,  an  excessively  high  spring  force  is  required.    This  high  spring  force  can  result  in  high shaft/bearing loads and accelerated belt wear. If dynamic spring-loaded idlers are to be used, they should be used on the slack (or nonload- carrying)  side  of  the  drive.    Potential  drive  loading  variations  in  the  system  will  have  the  least possible impact on idler movement due to spring compression with the idler placed in this way.  Be sure  to  note  that  the  tight  and  slack  spans  shift  as  the  direction  of  drive  rotation  reverses.    This could place the spring-loaded idler on the tight side.  In some cases, drive vibration and harmonic problems may also be encountered with the use of spring-loaded idlers.