Driveshaft rpm
Maximum Normal Operating Angles
5000
3°15'
4500
3°40'
4000
4°15'
3500
5°00'
3000
5°50'
2500
7°00'
2000
8°40'
1500
11°30'
maximum operating angle. Driveshaft rpm is the primary factor in determining the
maximum allowable operating angles in a give application.
Figure 8-47
correlates
expected driveshaft rpms with
maximum working angles. You
can see that the faster a
driveshaft turns, the lower the
allowable working angle drops.
Angles are calculated at the U-
joints as the driveshaft angles
downward from the
transmission to connect to the
input yoke on the rear axle
differential carrier yoke. For
example, a 2,100 rpm engine
running through a fast
overdrive transmission and a
fairly slow axle ratio might turn
Figure 8-47 - Maximum U-joint angles.
a driveshaft faster than 3,000 rpm. This would limit the maximum permissible angle to 5
degrees, measured as the truck sits on level pavement.
With equal working angles, the rear U-joint will slow down by the same amount that the
forward joint speeds up during a rotation, resulting in U-joint cancellation. The driving
and driven shafts will turn at constant and identical speeds. If the working angles of two
opposed U-joints vary more than 1 degree, the driveshaft will not rotate smoothly
because it accelerates and decelerates during a cycle. The result is vibration and
ultimate U-joint failure.
U-joint working angles become greater when the vehicle suspension flexes over uneven
road surfaces. If this occurs at slow speeds, it should not be a major problem unless
driveline angles are high to begin with. Driveline angles tend to present fewer problems
these days than when tractors with a short wheelbase were mandated to keep vehicle
overall length within legal limitations. Relaxation of those legal restrictions has led to the
use of a longer wheelbase that allows driveshafts to be longer and eliminates
aggressive U-joint working angles.
The longer the driveshaft, the greater the weight and therefore the greater the radial
forces, especially as driveshaft rpm increases. At high speeds, balance becomes more
critical. This is why manufacturers limit tube length. For example, at 3,000 rpm the
length of any single driveshaft section, measured between the centerline of the U-joints
at either end, should not usually exceed 70 inches.
NAVEDTRA 14050A
8-52