Home Page T-52 pulley grooves before applying final tension.  Serpentine drives with multiple pulleys and drives with large  pulleys  are  particularly  vulnerable  to  belt  tensioning  problems  resulting  from  the  belt  teeth being only partially engaged in the pulleys during installation.  In order to prevent these problems, the belt installation tension should be evenly distributed to all belt spans by rotating the system by hand.    After  confirming  that  belt  teeth  are  properly  engaged  in  the  pulley  grooves,  belt  tension should  be  rechecked  and  verified.    Failure  to  do  this  may  result  in  an  under-tensioned  condition with the potential for belt ratcheting. 12.3   Belt Take-up Synchronous  belt  drives  generally  require  little  if  any  retensioning  when  used  in  accordance with proper design procedures.  A small amount of belt tension decay can be expected within the first several hours of operation.  After this time, the belt tension should remain relatively stable. 12.4   Fixed Center Drives Designers sometimes attempt to design synchronous belt drive systems without any means of belt adjustment or take-up.  This type of system is called a Fixed Center Drive.  While this approach is often viewed as being economical, and is simple for assemblers, it often results in troublesome reliability and performance problems in the long run. The primary pitfall in a fixed center design approach is failure to consider the effects of system tolerance  accumulation.    Belts  and  pulleys  are  manufactured  with  industry  accepted  production tolerances.    There  are  limits  to  the  accuracy  that  the  center  distance  can  be  maintained  on  a production basis as well.  The potential effects of this tolerance accumulation is as follows: Low Tension: Long Belt with Small Pulleys on a Short Center Distance High Tension: Short Belt with Large Pulleys on a Long Center Distance Belt  tension  in  these  two  cases  can  vary  by  a  factor  of  3  or  more  with  a  standard  fiberglass tensile cord.  This potential variation is great enough to overload bearings and shafting, as well as the  belts  themselves.    The  probability  of  these  extremes  occurring  is  a  matter  of  statistics,  but however remote the chances may seem, they will occur in a production setting.  In power transmission drives,  the  appearance  of  either  extreme  is  very  likely  to  impact  drive  system  performance  in  a negative manner. The  most  detrimental  aspect  of  fixed  center  drives  is  generally  the  potentially  high  tension condition.    This  condition  can  be  avoided  by  adjusting  the  design  center  distance.    A  common approach in these designs is to reduce the center distance from the exact calculated value by some small fraction.  This results in a drive system that is inherently loose, but one that has much less probability of yielding excessively high shaft loads.  NOTE:  This approach should not be used  for power transmission drives since the potentially loose operating conditions could result in accelerated wear and belt ratcheting, even under nominal loading. There  are  times  when  fixed  center  drive  designs  can't  be  avoided.    In  these  cases,  the following recommendations will maximize the probability of success. 1.   Do  not  use  a  fixed  center  design  for  power  transmission  drives.  Consider  using  a  fixed center design only for lightly loaded or motion transfer applications. 2.   Do  not  use  a  fixed  center  design  for  drives  requiring  high  motion  quality  or  registration precision. 3.   When considering a fixed center design, the center distance must be held as accurately as possible,  typically  within  0.002"  –  0.003"  (0.05  mm  –  0.08  mm).    This  accuracy  often requires  the  use  of  stamped  steel  framework.    Molding  processes  do  not  generally  have the capacity to maintain the necessary accuracy.