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Page Title: DRUM BRAKES
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TRUCK SERVICE MANUAL TM 5-4210-230-14&P-1 BRAKES-HYDRAULIC DRUM BRAKES INSPECTION OF DRUMS The friction surface of brake drums must be smooth, true  and  concentric.    Make  certain  with  a  visual  check  that drums    are    not    barrel    shaped,    bellmouthed,    scored    or eccentric. Hard   or   chill   spots,   Fig.   1,   in   brake   drum   may produce  pedal  pulsation  and  roughness  or  brake  surge.    If these effects are present, drum should be replaced. Fig.    1    Hard or Chill Spotted Drum A barrel shaped drum (Fig.  2) results from overheating.    If  this  barrel  shaped  condition  is  not  corrected, the braking surface is reduced and uneven lining wear results. Fig.    2      Barrel Shaped Drum Extreme  pressure  which  over  a  period  of  time  will create a bellmouthed drum as shown in Fig. 3.  Brake linings on  a  bellmouthed  brake  drum  will  make  contact  only  on  the inner  surface  of  the  drum.    In  addition  to  cutting  the  braking surface  to  a  minimum,  it  will  also  cause  uneven  and  rapid wear. Scored  drums  are  the  result  of  worn  linings  to  the point    where    the    drum-to-shoe    contact    is    made    or    an accumulation of small steel particles imbed themselves in the brake  lining  (Fig.    4).  The  steel  particles  form  a  tough  scale which is sometimes harder than the drum.  As a result, deep grooves are formed in friction surface of drum. Fig.  3  Bellmouthed  Drum Fig.  4    Scored  Drum Brake  drum  scoring  never  improves  but  continually gets   worse   until   both   lining   and   brake   drum   are   useless. Attempting to reline brakes without turning scored brake drum surface   will   quickly   destroy   new   lining   and   make   effective braking impossible. Brake   lining   in   an   eccentric   or   out-of   round   drum cannot  make  full  contact  with  the  drum  resulting  in  rapid  or uneven  lining  wear  and  could  even  cause  brakes  to  seize  or chatter.      Maximum   allowable   out-of-round   or   eccentricity should be .25 mm (.010 in.) If    inspection    shows    that    any    of    the    preceding conditions   exist,   brake   drum   should   be   either   turned   or replaced.      To   assure   a   balanced   braking   system,   always install turned or new brake drums in pairs on each axle. Any  time  a  new  brake  drum  is  to  be  installed  on  a vehicle,  the  runout  should  be  checked  as  follows.    Place  the new   brake   drum   with   hub   and   wheel   assembled   in   lathe making  certain  drum  is  centered.    Mount  Dial  Indicator  SE- 1848  on  lathe  and  check  runout  about  12.7  mm  (.5  in.)  in from  edge  of  drum  as  shown  in  Fig.    5.    Runout  must  not exceed .25 mm (.010 in.). Before  assembling  drum,  hub  and  wheel,  all  parts must be clean and free of foreign matter. CTS-2779  Page 3 PRINTED IN UNITED STATES OF AMERICA

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