Propeller  shafts  may  be  solid  or  tubular  type  and require  little  or  no  maintenance. Solid shafts are normally used where high shaft speeds are unnecessary. They   are   used   extensively   to   power   auxiliary equipment, such as winches and hydraulic pumps. The hollow  shaft  is  used  almost  exclusively  to  transmit power  to  the  axles  on  automotive  vehicles.  The  hollow shaft, because it rotates at high speed, must be balanced to prevent vibration and premature bearing failure in the transmission and differential assemblies. A slip joint at one end of the propeller shaft takes care of end play. The driving axle, attached to the springs,  is  free  to  move  up  and  down,  while  the transmission is attached to the frame and cannot move. Any upward or downward movements of the axle causes the suspension springs to flex. This action shortens or lengthens the distance between the axle assembly and the  transmission.    The  slip  joint  makes  up  for  this changing  vertical  distance. The type of slip joint normally used consists of a splined stub shaft, welded to the propeller shaft, that fits into a splined sleeve in the universal joint, as shown in figure 2-12. UNIVERSAL JOINTS A  universal  joint  acts  as  a  flexible  coupling between  two  shafts  and  permits  one  shaft  to  drive another shaft that is at an angle to it. The universal joint is flexible in the sense that it permits power to be transmitted, while the angle of the shaft is being continually   changed. A   conventional   universal   joint   assembly   is composed of three fundamental units: a journal (cross) and two yokes, as shown in figure 2-12. The two yokes are set at right angles to each other and are joined by the journal. This design permits each yoke to pivot on the journal, allowing the transmission of rotary motion from one yoke to the other. As a result, the universal joint can transmit power from the engine through the shaft to the drive axle, even when the engine is mounted in the frame at a higher level than the drive axle, as shown in figure 2-13. Universal joints need little, if any, maintenance other  than  lubrication. Some universal joints have grease fittings and should be lubricated according to the manufacturer’s   specifications. CENTER SUPPORT BEARINGS When  two  or  more  propeller  shafts  are  connected together  in  tandem,  their  alignment  is  maintained  by  a rubber-bushed center support bearing, secured to a cross member of the frame. A typical center support bearing assembly is shown in figure 2-14. The standard bearing is prelubricated and sealed and requires no further lubrication;   however,   some   support   bearings   on heavy-duty vehicles have lubrication fittings. The first indication of support bearing failure is excessive chassis vibration  at  low  speed  caused  by  the  bearing  turning with the shaft in the rubber support. FINAL DRIVES A final drive transmits the power delivered from the propeller shaft to the drive wheels or to sprockets equipped  on  tracklaying  equipment.  Because  it  is located  in  the  rear  axle  housing,  the  final  drive  is usually identified as a part of the rear axle assembly. The final drive consists of two gears, called the ring gear  and  pinion.   These are beveled gears, and they may be worm, spiral, spur, or hypoid, as shown in figure 2-15. The function of the final drive is to change by 90 degrees  the  direction  of  the  power  transmitted  through the propeller shaft to the driving axles. It also provides a fixed reduction between the speed of the propeller shaft and the axles driving the wheels. In passenger Figure  2-14.—Center  support  bearing. 2-10