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T–42 SECTION 9    DESIGN AND INSTALLATION SUGGESTIONS There are some general guidelines which are applicable to all timing belts, including miniature and double-sided belts:   1.    Drives  should  always  be  designed  with  ample  reserve  horsepower  capacity.    Use  of overload  service  factors  is  important.    Belts  should  be  rated  at  only  1/15th  of  their respective ultimate strength.   2.    For  MXL  pitch  belts,  the  smallest  recommended  pulley  will  have  10  teeth.    For  other pitches, Table 18, on the previous page, should be used.   3.    The pulley diameter should never be smaller than the width of the belt.   4.    Belts with Fibrex-glass fiber tension members should not be subjected to sharp bends or rough handling, since this could cause breakage of the fibers.   5.    In order to deliver the rated horsepower, a belt must have six or more teeth in mesh with the  grooves  of  the  smaller  pulley.    The  number  of  teeth  in  mesh  may  be  obtained  by formula  given  in  SECTION  24  TIMING  BELT  DRIVE  SELECTION  PROCEDURE.    The shear strength of a single tooth is only a fraction of the belt break strength.   6.    Because of a slight side thrust of synchronous belts in motion, at least one pulley in the drive must be flanged.  When the center distance between the shafts is 8 or more times the diameter of the smaller pulley, or when the drive is operating on vertical shafts, both pulleys should be flanged.   7.    Belt surface speed should not exceed 5500 feet per minute (28 m/s) for larger pitch belts and  10000  feet  per  minute  (50  m/s)  for  minipitch  belts.    For  the  HTD  belts,  a  speed  of 6500 feet per minute (33 m/s) is permitted, whereas for GT2 belts, the maximum permitted speed is 7500 feet per minute (38 m/s).  The maximum allowable operating speed for T series is 4000 feet per minute (20 m/s).   8.    Belts are, in general, rated to yield a minimum of 3000 hours of useful life if all instructions are properly followed.   9.    Belt drives are inherently efficient.  It can be assumed that the efficiency of a synchronous belt drive is greater than 95%. 10.    Belt  drives  are  usually  a  source  of  noise.    The  frequency  of  the  noise  level  increases proportionally  with  the  belt  speed.    The  higher  the  initial  belt  tension,  the  greater  the noise level.  The belt teeth entering the pulleys at high speed act as a compressor and this creates noise.  Some noise is the result of a belt rubbing against the flange, which in turn may be the result of the shafts not being parallel.  As shown in Figure 9 (page T-9), the noise level is substantially reduced if the PowerGrip GT2 belt is being used. 11.    If the drive is part of a sensitive acoustical or electronics sensing or recording device, it is recommended that the back surfaces of the belt be ground to assure absolutely uniform belt thickness. 12.    For some applications, no backlash between the driving and the driven shaft is permitted. For these cases, special profile pulleys can be produced without any clearance between the  belt  tooth  and  pulley.    This  may  shorten  the  belt  life,  but  it  eliminates  backlash. Figure 10 (page T-9) shows the superiority of PowerGrip GT2 profile as far as reduction of backlash is concerned. 13.    Synchronous  belts  are  often  driven  by  stepping  motors.    These  drives  are  subjected  to continuous  and  large  accelerations  and  decelerations.    If  the  belt  reinforcing  fiber,  i.e., tension member, as well as the belt material, have high tensile strength and no elongation, the belt will not be instrumental in absorbing the shock loads.  This will result in sheared belt teeth.  Therefore, take this into account when the size of the smallest pulley and the materials for the belt and tension member are selected. 14.    The choice of the pulley material (metal vs. plastic) is a matter of price, desired precision, inertia,  color,  magnetic  properties  and,  above  all,  personal  preference  based  on experiences.  Plastic pulleys with metal inserts or metal hubs represent a good compromise.