The torsional vibrations set up by the fluctuations in velocity are transferred down the drive shaft to the next U-joint.
At this joint similar speed fluctuation occurs. Since these speed variations take place at equal and opposite angles to the
first joint, they cancel out each other. To provide for this canceling effect, drive shafts should have at least two U-joint and
their operating angles must be equal to each other. Speed fluctuations can be cancelled if the driven yoke has the same point of rotation, or same plane, as the driving yoke. When the yokes are in the same plane, the joints are said to be "in phase."
On a two-piece drive shaft, you may encounter problems if you are not careful. The center U-joint must be
disassembled to replace the center support bearing. The center driving yoke is spline to the front drive shaft. If
the yoke's position on the drive shaft is. not indicated in some manner, the yoke could be installed in a position
that is out of phase. Manufacturers use different methods of indexing the yoke to the shaft. Some use aligning
arrows. Others machine a master spline that is wider than the others. The yoke and shaft cannot be reassembled
until the master spline is aligned properly. When there are on indexing marks, you should index the yoke to the drive shaft before disassembling the U-joint. This saves time and frustration during reassembly.