ENGINEMAN  1  &  C Broken or bent crankshafts may be caused by the  improper  functioning  of  a  torsional  vibration damper. Vibration dampers are mounted on the crankshafts of some engines to reduce the tor- sional vibrations set up within the crankshaft and to ensure a smoother running engine. If a damper functions improperly, torsional vibrations may rupture the internal structure of the shaft. The principle of operation is similar in most dampers,   yet   their   construction   and   their component parts vary somewhat. If the engine is equipped with a vibration damper, the engine in- struction  manual  must  be  consulted  for  informa- tion on type, construction, and maintenance of the damper. In most engines, one end of the crankshaft is flanged to receive the damper, the damper being bolted or doweled onto the flange. A damper must be fastened securely to the crankshaft at all times during engine operation; otherwise, the damper will not control the crankshaft vibrations. Small  dampers  are  usually  grease-packed, while larger ones frequently receive lubrication from the main oil system. Dampers that are grease lubricated   must   have   the   grease   changed periodically, as specified in the manufacturer’s instructions. If the assembly is of the elastic type, it must be protected from fuel, lube oil, grease, and excessive heat, all of which are detrimental to the rubber. Excessive rumbling at certain engine speeds may indicate that the damper is not functioning properly.  You  must  learn  to  distinguish  between this and the normal noise usually heard in some engines during the first and last few revolutions when the engine is starting or stopping. This noise is normal, it is due to the large designed clearances in the damper and is not a sign of impending trouble. Crankshaft breakage or bending may be the result of excessive bearing clearances. Excessive clearance in one main bearing may place practic- ally all of the load on another main bearing. Flex- ing of the crankshaft under load may result in fatigue and eventual fracture of the crank web. (See figure 3-19.) Excessive bearing clearance may be caused by the same factors that cause journal bearing  failure.  Furthermore,  off-center  and  out- of-round  journals  tend  to  scrape  off  bearing material. This leads to excessive wear and to the increase of the clearance between the shaft and Figure 3-19.—Cracked crank web. 121.3 bearing. You can minimize the possibility of jour- nal  out-of-roundness  by  taking  measures  to  pre- vent  improper  lubrication,  journal  bearing  failure, overspeeding   or   overloading   of   the   engine, excessive  crankshaft  deflection,  and  misalignment of  parts. Crankshaft   bending   breakage   (out-of- roundness)  may  also  result  from  excessive crankshaft deflection. Excessive shaft deflection, caused by improper alignment between the driven unit and the engine, may result in a broken or bent shaft along with considerable other damage to bearings, connecting rods, and other parts. Ex- cessive crankshaft deflection may also be caused by overspeeding an engine. The amount of deflec- tion of a crankshaft may be determined by the use of a straight gage. The  straight  gage  is  merely  a  dial-reading inside  micrometer  used  to  measure  the  variation in the distance between adjacent crank webs where the engine shaft is barred over. When installing the  gage,  or  indicator,  between  the  webs  of  a crank throw, place the gage as far as possible from the axis of the crankpin. The ends of the indicator should rest in the prick-punch marks in the crank webs. If these marks are not present, you must make them so that the indicator may be placed in its correct position. Consult the manufacturer’s technical manual for the proper location of new marks. 3-32