I
came across an interesting and informative technical piece
on the website of Mark Williams Enterprises (www.markwilliams.com)
that discusses driveshafts. I thought it was good information
so I'm sharing here with you.
Operating
Angle
Operating angles in a driveshaft are the angles between
the pinion, driveshaft and transmission centerlines. The
optimal angle for any driveshaft to run at is 0 degrees,
where many vibrational and frictional problems are nonexistent.
In order to minimize power loss and vibration in an offset
configuration, the pinion centerline and the transmission
centerline need to be parallel. In general, the largest
angle for racing applications should 2 degrees and the centerlines
should be parallel within 1/2 degree. With suspension movement
the operating angle will increase, but should not exceed
15 degrees. If the centerlines are off too far, the ujoints
travel at uneven operating velocities, causing vibration
(this is the same problem induced by poorly phased end yokes).
This vibration is hard to distinguish from an unbalanced
driveshaft.
Critical
Speed
Critical speed is the speed at which a spinning shaft will
become unstable. This is one of the single largest factors
in driveshaft selection. When the whirling frequency and
the natural frequency coincide, any vibrations will be multiplied.
So much that the shaft may self destruct. Another way to
think of this is that if a shaft naturally vibrates at 130
times a second, and one point on the shaft passes through
0 degrees 130 times a second (7800 RPM) then the shaft has
hit a critical speed. There are several ways to raise the
critical speed of a driveshaft. You can make it lighter,
stiffer, or increase diameter without increasing weight.
This is the reason carbon fiber makes a good driveshaft,
it is stiff and light and can be made to any diameter or
wall thickness. Aluminum, while it has a very good critical
speed is not quite as strong as steel. Steel, with good
strength characteristics will have a lower critical speed.
