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 the hope that you can “head-off” a problem before it occurs. They are:
- Transverse vibrations
- Torsional vibrations
- Inertial excitation vibrations
- Secondary couple vibrations, and…
- Critical speed 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…much heavier than a tire
- Drive shafts rotate much faster than a tire.
Common sense says that we should not hesitate to balance an object that is heavier and 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 at once per revolution. If you experience a vibration that is speed sensitive, have your drive shaft balance checked at your closest Machine Service, Inc location.
Are caused by two things:
- The U-joint operating angle at the “drive” end of the drive shaft, and…
- The orientation (phasing) of the yokes at each end of the drive shaft
A torsional vibration is a twice per revolution vibration.
A torsional vibration will cause the drive shaft, “downstream” of the front U-joint, to “speed up” and “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 that change in speed, called the magnitude, or size of the 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.
Here’s another way to explain it
- Picture a drive shaft running at a constant speed and driving a truck or a large roller in a mill.
The front end of the drive shaft is connected to the power source and the torque coming out of the power source is fairly constant.
- The rear end of the drive shaft is connected to the truck’s axle or to the roller and it sees varying loads based on terrain or on how much work it is doing.
- As the front end produces the load, the back end passes it on into the vehicle or stationary application and if all is well, that load is relatively constant and well within the torque carrying capabilities of your drive shaft
- When something happens to alter the operating angle at the front U-joint of the drive shaft a twice per revolution change in speed is introduced into the application.
- The front of the drive shaft keeps going constant, but the back end of the drive shaft starts to see the twice per revolution change in speed, and is always playing “catch-up” with the front.
- This causes a twice per revolution “twist” in the drive shaft
- A twice per revolution “bending” moment is introduced into the drive shaft welds, slip splines, U-joints and into all of the connected components in the application.
- You, in effect, run a torsional fatigue test on the drive shaft and everything used to attach it to your application.
- Drive shaft manufacturers run fatigue tests on the components and welds in their drive shafts by doing the same thing in their test labs. They hold one end of the drive shaft stationary and hook the other end to a rotary actuator. Then they twist it until it fails.
- If you have a torsional vibration problem you will experience drive shaft tube welds that break, splines that wear prematurely and nuts and bolts that start loosening.
- You will also start to experience vibrations.
- If you see a failure that looks like this, you should suspect a torsional vibration problem.
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.
Phasing affects torsional vibrations
A drive shaft that is “in phase” and has the 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 vibration 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 that “all-of-a-sudden appears” in your application. If you suspect that your drive shaft is not in phase, take it to the closest Machine Service location for inspection.
How do you make sure your drive shaft application will not create a torsional vibration?
- Make sure the operating angle 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 equal within one degree. Make sure these angles are correct. Rotate the pinion if the problem is in a vehicle. Shim the driving end or the driven end if the application is a stationary application. Correcting torsional vibration problems is not rocket science. Fix the angles and you will fix the problem, it’s that simple.
- To make sure the 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 vibration will still be there if the angles are too large…so do whatever necessary to make the operation angles small.
- 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.
- If you have a multi piece drive shaft set-up, make sure the operating angle at the front of each of your coupling shaft(s) (the shaft(s) with the bearing(s) or pillow block(s) on it (them)) are less than one and one-half degrees. Also make sure the operating angles on the rear drive shaft (usually the drive shaft with slip in it) are less than three degrees and are equal within one degree.
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 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 traverses 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 of its normal rotating centerline.
As a drive shaft bends, it does two things:
- It gets shorter. If it gets short enough, it can pull out of its slip and drop to the floor or ground.
- 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.
CAUTION: If you ever see a drive shaft with a bent, fractured tube, do not replace it with a new drive shaft of the same construction. It may fail again. Contact Machine Service engineering immediately.
- EVERY drive shaft, no matter what its length and mass, has a critical speed.
The shorter the drive shaft, the higher its critical speed.
Conversely…the longer a drive shaft, the lower its critical speed.
- REMEMBER THIS: When a drive shaft runs at its critical speed, IT ALWAYS FAILS, and the failure is ALWAYS CATASTROPHIC.
- Machine Service engineers will ALWAYS calculate the critical speed of any drive shaft they manufacture.
- Machine Service engineers will ALWAYS make sure that drive shafts installed or “spec’d” by Machine Service will NEVER fail due to critical speed.
- If you are in the business of repairing or altering drive shafts, NEVER lengthen ANY drive shaft, in ANY application, without contacting the engineering department at Machine Service.