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Driveline
Geometry 101 -
Continued
General Application
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TORQUE
FLOW
Viewed in cross-section (Fig.
2), rotating counter-clockwise, the cross is closest to simple circular
motion at 3 and 9 o'clock. Traveling from 3 to 12, each arm of the cross
attached to the driveshaft accelerates toward the driveshaft as well as
maintaining circular movement. It decelerates as it moves from 12 to 9
o'clock and back to center, accelerates from 9 to 6 toward the
transmission, and finally decelerates from 6 to 3 while moving back to
center. Distances A and B are the additional amount the cross arm travels
twice per revolution of the driveshaft, and account for the increased
speed of the cross as it oscillates front to back and back to front. The
resulting path that the cross arms travel is seen as an ellipse when
viewed from the end of the transmission shaft.
decelerating twice per
revolution. It also means that at the other end of the driveshaft the same
action is taking place, and is the explanation of why U-joints are always
used in pairs. You see, the constant speed, circular motion of the
transmission output shaft is changed, through the elliptic path of the
U-joint, to a constantly accelerating and decelerating circular motion in
the driveshaft which must be translated back to a constant speed circular
motion in the rear axle (by the opposite action of the other U-joint) to
avoid surging or vibration.
The key to making all this
happen without setting up destructive harmonics, or torsion (flexing) in
the driveshaft, and/or hammering transmission and rear axle components is
making sure that the U-joints are in equal but opposite positions at any
point in the driveshaft rotation. When the U-joints are in phase (the same
point on both U-joints reaches the same rotation position at the same time
and the angles between the driveshaft and both the transmission output
shaft and pinion shaft are equal, the acceleration / deceleration cycles
tend to cancel out, resulting in smooth and quiet operation. This is known
as cancellation, and is the whole objective.  [more]
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