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Vibration Issues

 

Drive Shaft Vibrations

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.

They are

-Transverse Vibrations   -Torsional Vibrations   -inertial Excitation Vibrations   -Secondary Couple Vibrations   -Critical Speed Vibrations

Transverse Vibrations

Are caused by two things:

  1. The U-Joint operating angle at the "drive" end of the drive shaft
  2. The orientation (phasing) of the yokes at each end of the drive shaft

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.

Torsional Vibrations are SERIOUS Vibrations

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".

Phasing affects torsional vibrations

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.

How do you make sure your drive shaft application will not create a torsional vibration?

  1. Make sure the operating at the front of your drive shaft and the operating angle at the rear of your drive shaft are less than three degrees and are not equal within one degree. Make sure these angles are correct. You may need to shim the drive end or the driven end of the application.
  2. To make sure torsional vibration does not enter your drive system, make the angles at each end of the drive shaft equal with each other to cancel out the torsional vibration. However the vibrations will still be there if the angles are too large...so do whatever nevessary to make the operation angles small.
  3. Make sure your drive shaft is in phase...the same phase as it was in when it was manufactured. Do not disassemble your drive shaft slip assembly unless it is absolutely necessary.

Inertial Excitation Vibrations

  • inertial vibrations are also caused by the operating angle at the drive end of your drive shaft.
  • Inertial vibrations are created when you start changing the speed of a heavy drive shaft.
  • Inertial vibrations also create bending on the drive shaft attaching components.
  • There is only ONE WAY to control an inertial vibration...ALWAYS make sure the operating angle at the drive end of your drive shaft is less than three degrees.
  • A large angle even if it is an "equal" angle will still cause inertia problems.

Secondary Couple Vibrations

  • Secondary couple vibrations are also caused by the operating angle at the drive end of your drive shaft.
  • Every U-joint that operates at an angle creates a secondary couple load that traverse down the centerline of the drive shaft.

Critical Speed Vibrations

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:

  1. It gets shorter. If it gets short enough, it can pull out of its slip and drop to the ground.
  2. It starts to "whip" up and down or back and forth like a jump tope. If it whips far enough, it will fracture in the mniddle of the tube.

Half Critical speed

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:

 

Vibration issues caused by lift kits

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.