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 hope 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 imbalance. All drive shafts should be balanced at their application speeds. • Think about this...when was the last time you DID NOT have your tires balanced • Drive shafts are heavy. • Drive shafts rotate much faster than a tire. Common sense says that we should not hesitate to balance an object that rotates faster than our tires...especially if there is a possibility that it can lead to a serious failure. All drive shafts should be inspected for missing balance weights at every service interval. A transverse vibration ALWAYS occurs at drive shaft speed, and occurs once per revolution. If you experience a vibration that is speed sensitive, have your drive shaft balance checked.
Torsional 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-joint, 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 the drive and driven end of your drive shaft.
Torsional Vibrations are SERIOUS Vibrations Why? 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 of 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 BREAKS! 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 the correct operating angels 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 it apart 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 made need to shim the drive end or the driven end if 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 necessary 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 center line. 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 rope. If it whips far enough, it will fracture in the middle of the tube.
Half Critical Speed Drivelines that are operated at a cruising speed of have a constant running speed that occurs at, or near, half critical speed may experience a continuous vibration that cannot be fixed 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, bearings, and yes they all contribute to driveline harmonies and vibrations. Now lets talk about C.V. (double cardon) drivelines. The benefit of a C.V. driveline is smoother operation and increased operating angles. It is important to adjust the pinion yoke to point up at the C.V.
See Diagram Below:
6 degrees maximum operating angle is what a C.V. is supposed to operate at from the manufacturer. In the off road world the use of C.V. is more along the lines of 12° to 20° or more. This means higher maintenance and shorter life due to premature wear. You may need to cut & turn your axles. Each vehicle is its own beast. When you choose to modify your vehicle you are changing the geometry it was intended to operate at. Other changes such as gears and tires all affect the load being put on the vehicle also contributing to changes in the harmonics. Sometimes there is the one in a million that is a problem child. You have chosen to modify your vehicle and it will ride and drive differently.