There are five types of drive shaft induced vibrations that are associated with the installation parameters of a drive shaft. We're going to explain all of them in hopes that you can "head-off" a problem before it occurs.
-Transverse Vibrations -Torsional Vibrations -inertial Excitation Vibrations -Secondary Couple Vibrations -Critical Speed Vibrations
Are caused by two things:
A torsional vibration occurs twice per revolution. Torsional vibration will cause the drive shaft, "downstream" of the front U-join, to "speed up" or "slow down" twice per revolution. That means that a power supply producing a constant speed of 3,000 RPM can actually be attached to a drive shaft that is changing speed 6,000 times per minute. the amount of change in speed, called the magnitude, or size of change, is proportional to the size of the angle at the drive end of the drive shaft, or the amount of misalignment between the yokes at tyhe drive and driven end of your drive shaft.
Why are torsional vibrations so serious? Because when you vary the speed of a drive shaft, you not only vary the torque on all of its components, but you vary the torque on all the components that are connected to the drive shaft. Torque is LOAD. When you vary the load, at twice per revolution, you start to bend components. You know what happens then....the same thing that happens when you bend a can lid back and forth. IT BREATS!
When a drive shaft is assembled, its inner components usually consist of a slip yoke on one end and a tube yoke on the other end, and they are usually assembled in relation to each other. This is called PHASING. Most drive shafts are assembled with their yokes in line, or "IN PHASE".
A drive shaft that is "in phase" and has correct operating angles at the drive end of the shaft does not create a torsional vibration. Drive Shafts that are NOT in phase will vibrate with the same twice per revolution as a drive shaft with incorrect operating angles. The easiest way to make sure your drive shaft is in its correct phase is to mark the tube and slip yoke every time you take i tapart so you can put it back in its original orientation when you re-assemble it. Re-assembling a drive shaft out of phase is the #1 cause of torsional vibration.
Critical Speed occurs when a drive shaft rotates too fast for its length. It is a function of its rotating speed and mass and it is the RPM where a drive shaft starts to bend off its normal centerline.
As a drive shaft bends, it does two things:
Drivelines that are oprated at a cruising speed of half the constant running speed that occurs at, or near, half critical speed may experience a continuous vibration that cannot be fixed by balancing or any of the other common vibration remedies. This is a harmonic vibration, all mechanical things have harmonic vibrations. This means your engine, transmission, transfer case, ring & pinion, axles, and bearings all contribute to driveline harmonies and vibrations.
Now let's talk about C.V. (double cardan) drivelines. The benefit of a C.V. driveline is a smoother operations and increased operating angles. It is important to adjust the pinion yoke to point up at the C.V.
See the diagram below:
We have seen, many times, that vibrations issues have been caused by low quality lift kits that don't include adjustable arms. Whenever you are lifting a vehicle you are changing the operating angle of the drive shaft. When you have adjustable arms you have a greater ability to correct the geometry and reduce vibrations. So it is highly recommended, if you are going to lift your vehicle, that you purchase a lift kit with adjustable arms. Doing so will save you a lot of issues down the road and on the trail.