Technical 101
Drive shafts are used to transmit power from a power source, be it an engine or electric motor, to a driven member. In a vehicle, that driven member is usually an axle. In a stationary application, the driven member can be any number of devices, from a pump to a generator and everything in between.
A properly “sized” drive shaft should never fail or wear out if it is properly installed and properly maintained.
When a drive shaft or a part of a drive shaft fractures, it is usually the result of some sort of shock load. If the fracture occurs in a vehicle, it can usually be traced back to an irregular action by the driver. If the fracture occurs in a stationary application, it is usually caused by something that “ties’ up in the drive train.
The components of a drive shaft will wear prematurely if they are not properly serviced. All drive shaft manufacturers provide recommended service intervals and recommend the proper lubricant to use for their products. We suggest you consult their recommendations for your particular application.
This program will cover the most common types of failures and analyze the cause
of each.
It will also delve into proper application and “set-up.”
Drive shaft failure descriptions
U-joint cross, broken at a bearing surface
U-joints seldom break off at the bearing surface. It takes a very large shock load to cause this type of failure. It is also very difficult to inspect for this type of failure because they, many times, start as a small crack and progress into a complete failure some time further “down the road”.
U-joint cross, broken thru lube fitting hole
This failure is usually caused by someone who does not install the U-joint in the correct orientation in the drive shaft The front U-joint of a drive shaft MUST be installed so the driving torque compresses the U-joint lube fitting.
Here’s what we mean:
- When you look at any application from the drive end, the drive shaft turns clock-wise when torque is applied.
- In a vehicle, the “driving yoke” is the yoke on the transmission and the “driven yoke” is the yoke on the drive shaft
- In an industrial application, the driving yoke is the yoke attached to the power supply.
- If the U-joint is correctly installed, torque will compress the lube fitting.
How to check the orientation of the lube fitting in an application:
- Look at the drive shaft from the front. Rotate it so the ears of the yoke on the driving member are vertical, (“North” and “South”). The yoke on the drive shaft will be horizontal, (“East” and “West”).
- If the U-joint is correctly installed, the lube fitting will be at a 45 degree angle to the right. (“Northeast”).
Here’s another way to check lube fitting orientation on your drive shaft:
- Lay it on the table with the yokes of the slip yoke horizontal. (East and West).
- Look into the slip yoke and at the position of the zerk fitting. It should be inboard on the yoke (towards the tube) and pointing Northeast.
Lube related failures cause the vast majority of drive shaft related failures.
The most common lube related failure usually occurs in the U-joint kit area.
The second most common failure usually occurs in the “slip” area of the drive shaft
Lack of lubrication or neglecting to follow recommended lube procedures is the #1 cause of drive shaft failures.
A lube related failure might include:
- A U-joint with a completely “burned off” journal.
- A U-joint with needle marks in the bearing surface. (Called brinelling) (Needle marks that can be felt with a thumbnail.)
- A U-joint with “scrape” marks, (called spalling) on the bearing surface.
Spalling occurs when dirt or moisture enters the bearing area of the U-joint and is not “flushed out” during normal service because:
- The service mechanic did not follow service interval recommendations or…
- Did not take care to “purge” all four seals of the U-joint.
A U-joint having one or more “blackened” bearing surfaces.
- A U-joint bearing surface can “burn” if it is operated at a high U-joint angle (more on U-joint angles later) or if it is installed in a drive shaft yoke or end fitting yoke that has ears that are not in correct alignment.
- A key indicator of a defective yoke is a U-joint that has two burned bearing surfaces, 180 degrees from one another.
A lube related failure might be caused by:
Using the incorrect lube.
- Manufacturers recommend using a lube meeting the NLGI Grade 2 spec, with an EP additive.
- It should have a temperature
range of +325 degrees F to a low of –10 degrees F.
If you are in doubt of the correct grease to use, some of our U-joint suppliers use Chevron Ultra Duty EP2 grease in their production drive shafts
Lubricating at incorrect intervals:
U-joints should always
be serviced at the manufacturers recommended intervals:
- Service intervals may vary from 3 months for regular, over the road type vehicles that carry normal highway approved loads, to 500 hours for normal industrial applications, to 200 hours for an industrial application that runs constantly, in severe environmental conditions, or at high loads, or both.
- All manufacturers provide recommendations regarding service intervals for their products.
Not following the correct procedure when you lube a U-joint.
We recommend forcing grease into each U-joint until you see new grease at
each of the four U-joint bearing seals. Use care to make sure you see ONLY
new grease at each seal. This “purging effort” will guarantee that you have
forced any contaminates and moisture out of the U-joint.
If something happens and you cannot purge all four seals, you may be forced to remove a U-joint to inspect it. Make sure you follow manufacturer recommendations on U-joint installation, when you re-install it, because some manufacturers use self locking bolts to retain their U-joints and these bolts may not be re-usable.
Not following the correct procedure when you lube a U-joint.
Most drive shafts have some sort of slip assembly built into them so they
can change length if necessary. Most slip assemblies consist of a slip
yoke (with u-joint in it) and a splined shaft that is usually welded to the
drive shaft tube.
- Manufacturers recommend forcing grease into the slip assembly through the lube fitting until it comes out of the vent hole in the slip yoke plug.
- Then they recommend covering the vent hole with your finger while you force additional grease into the slip assembly until it comes out of the seal on the end of the slip yoke.
- Visible grease at the slip yoke seal insures grease has passed over the slip surfaces inside the assembly.
Improper installation and application techniques come in a close second as the cause of drive shaft failures. An improperly installed drive shaft can generate system-destroying vibrations that can result in catastrophic failures.
DRIVESHAFT 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
Transverse vibrations
Are caused by imbalance.
All drive shafts should be balanced at their application speeds.
- Drive shafts are heavy…much heavier than a tire.
- Drive shafts rotate much faster than a tire.
- Think about this…when was the last time you DID NOT have your tires balanced.
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 location.
Torsional vibrations
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 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.
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 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.
Do what you have to do to 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.
Making the angles equal at each end of the drive shaft will cancel out the torsional, so it does not enter your drive system, but it will still be there and can do drive shaft damage if the angles are too large…. so do whatever is necessary to make operating 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 and 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 it 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.
Drive shaft failure analysis:
Typical U-joint kit failures
Brinelling (needle marks on the surface of the u-joint cross):
Can be caused by excessive torque
If you have changed engines or transmissions, calculate the torque transmitted by the new combination.
Your drive shaft series may be too small.
- Can be caused by operating angles which are too large
- Can be caused by a bent or sprung yoke.
- Overloading a drive shaft can cause yoke
ears to bend. Bearings will not roll in the bearing cap if the yoke ears
are not aligned. If the bearings stop rolling, they remain stationary and
will “beat themselves” into the surface of the cross.
A “frozen” slip assembly will not allow the drive shaft to lengthen or shorten. Every time the drive shaft tries to shorten, the load will be transmitted into the bearings and they will mark the cross trunnion. Unlike brinnell marks caused by torque, brinnell marks that are caused by a frozen slip are always evident on the front and back surfaces of the cross trunnion. - Improper torque
on U-bolt nuts can cause brinelling.
Most manufacturers publish the recommended torque for a U-bolt nut.
Spalling. Looks like the bearing surface
of the U-joint has been “scraped” away.
Spalling is usually by water or dirt contamination
- Check the U-joint kit seals for damage and replace as necessary.
- Check to make sure the service technician is using the proper lube type.
Burned U-joint cross trunnions:
Improper lube procedures, where recommended purging is not accomplished,
can cause one or more bearings to be starved for grease.
Always make sure new, fresh grease is evident at all four U-joint seals.
- Using the wrong lube can result in burned trunnions
- Unless otherwise recommended, use a high quality E.P. (extreme pressure) grease to service most vehicular, industrial and auxiliary drive shaft applications.
End galling of the u-joint cross (The end of the trunnion
looks like material has been “gouged’ out.)
Usual cause: Operating angles are too large
U-joint fractures are usually by a shock load, but
can also be caused by an improper application
Calculate the torque transmitted by the engine/trans combination. Check
to make sure the drive shaft series is not too small for the application
Improper assembly procedures
Striking the bearing plate with a hammer can cock it on the bearing and may
cause the bearing to be pulled down crooked in the yoke. Cocking the
bearing in the bore of the yoke may put undue loads on the cross, its bearings
and the seals inside those bearings, which may cause premature failures
and make proper lubrication difficult.
Bent or deflected end fitting
Bent yokes will put abnormal loading on the U-joint bearings and lead to
premature failure. A yoke can be bent by a shock load or by over torquing
the yoke.
Mixing incompatible greases
All greases are a mixture of different additives and thickeners. Mixing
greases from different manufactures can lead to a mixture with a lower service
performance than either of the original grease products.
Thoroughly purging of all four bearing seals on each universal joint can
help alleviate the possibility of mixing incompatible greases.
Operating angles that are too large
Large U-joint operating angles can be caused by improper drive shaft installations,
(Vehicle owners sometimes make changes to the original vehicle that can
mess up operating angles) or a sagging suspension or even improperly adjusted
air bags
Keeping angles small and within recommendations will help to reduce wear
on U-joint components and help to lower the chances of having inertia and secondary
couple vibration problems.
Operating angles are NOT cancelled
Proper cancellation reduces the chance of having torsional vibrations.
Typical failures related to a broken part:
Shock loads are the usual cause of almost all
failures where a part breaks.
Some common causes of shock loads are:
- “Popping” the clutch
- Spinning tires that grab abruptly
- Backing into a loading dock, slamming under a parked trailer or trying to pull away with the trailer brakes locked.
- Improper operating angles that might allow “mating” yokes to make contact
with each other during suspension “jounce” and “rebound.”
Look in the drive shaft manufacturers catalog to check the angle capability of the mating yokes and change to a yoke, or yokes, with a larger angle or…
Check the u-joint operating angle and fix it if it does not meet recommendations. - Putting
too much torque thru the yoke.
Calculate the torque transmitted by the engine/trans combination. Change to a yoke having a higher “torque” rating if necessary.
Fractures specifically related to drive shaft yokes.
Tube yoke fractures:
- Improper welding when the drive shaft is assembled can cause
tube yoke failures.
- Most drive shaft manufacturers recommend that tube yoke circle welds be started in line with the tube yoke ear and 180 degrees from the tube weld seam. If you experience a failure of a tube yoke, contact Machine Service engineering for assistance.
Slip yoke fractures near the end of the slip spline.
- Slip yokes sometimes crack
near the outer end of the spline if the drive shaft is not properly installed.
- Machine Service usually recommends installing drive shafts so they operate at mid-slip. Contact Machine Service engineering if you suspect your drive shaft may have experienced this type of failure,. Your drive shaft tube may need to be lengthened.
End yoke fractures of the tang on yoke ear
- End yoke ear failures are sometimes caused by improper bearing strap bolt torque or by improper handling of the bearing strap and bolts.
- Bearing straps serve two purposes.
- They retain the U-joint in the yoke and they do not allow the U-joint bearing to rotate in the bearing “pocket” of the yoke ear. Rotation in the yoke ear will cause excess wear on the locating tangs in the yoke and will lead to misalignment of the drive shaft in the drive train. Misalignment causes vibrations.
- Proper torque on the bearing strap bolts actually “stretch” the strap around the bearing cap and stop it from rotating in the yoke ears. This “de-forms” the strap and makes it non-reusable a second time.
- Bearing straps that are re-used may allow the U-joint bearing to turn in the yoke and may cause excessive wear on the locating tangs in the yoke which may lead to premature failure of the yoke ears.
- Machine Service engineers ALWAYS recommend replacement of bearing straps when a U-joint kit or drive shaft is serviced
Failed tubing
Twisted tubing:
- If the failure is a twisted tube, the problem was probably a shock load.
- This type of failure is usually “driver’ related
- Popping the clutch or spinning tires that grab abruptly usually twist tubes.
Fractured tubing:
- Tubing failures sometimes start at drive shaft assembly and first appear as a failure of the tube-tube yoke weld seam (also called the “circle” weld.)
- Proper assembly requires alignment of the tube weld seam with one of the tube yoke “ears” and proper welding.
- NEVER start the circle weld on the tube weld seam. ALWAYS start it 180 degrees from the weld seam.
- This type of failure is usually a “fatigue” type failure where the tube cracks at the start of the weld and progresses around the tube.
- Failures of the tube weld seam can also be caused by high U-joint operating angles that cause large torsional vibrations. Large “torsionals” cause two twisting motions in the drive shaft every revolution and can cause a premature “fatigue” of the weld.
- Balance weight attachment can also cause premature failure of drive shaft tubing.
- Balance weights should NEVER be attached within 1” of circle weld.
- Balance weights that are welded too close to a circle weld can alter the metallurgical structure of the circle weld and can change the properties of the tube material. Either of these can cause premature fatigue failure of the tube
- Balanced
weights should never be placed directly over the seam weld of the tube.
Improper straightening can alter the metallurgical structure of the tube, changing the properties of the tube material and can lead to premature failure of tubing - An
improper installation can cause premature failure of tubing
Improper U-joint operating angles can cause excessively high secondary couple loads that cause a failure due to bending fatigue - Machine Service engineering recommends U-joint operating angles be no larger than three degrees.
- A drive shaft that is too long for its operating speed can cause a critical speed failure.
- A drive shaft tube that fails due to critical speed is always bent and usually fractures near the middle of the tube. The fracture has the appearance of a tube that has been bent back and forth until it fails.
- Machine Service engineering will calculate the critical speed of your drive shaft if you suspect you have experienced a critical speed failure.
- MOST IMPORTANT: IF YOU THINK YOU HAVE A FAILED DRIVESHAFT DUE TO CRITICAL SPEED, DO NOT REPAIR IT AND RE-INSTALL IT BECAUSE IT MAY FAIL AGAIN. CONTACT ENGINEERING FOR ASSISTANCE.
Premature Center bearing or pillow block failures
Center bearing and/or center bearing rubber failure
Large U-joint angles at the front end of a coupling shaft create secondary couple loads that try to “straighten” themselves out twice per revolution. This load causes a force that “travels” down the centerline of the drive shaft and creates a bending moment on the drive shaft
Since the driven end of the drive shaft is “bolted fast” and cannot move, the secondary couple load tries to “bend” the drive shaft at its connection points. (The attaching yoke at the front end of the drive shaft, or the bearing of the center bearing or pillow block.)
These components cannot bend so they load the bearing, or the surfaces that attach the bearing to the drive shaft
The first evidence of a secondary couple problem occurs in the rubber cushion of the center bearing.
It starts to fail and black dust appears around the bearing.
There is only ONE WAY to control secondary couple loads… ALWAYS make sure the operating angle at the drive end of your coupling shaft is less than ONE AND ONE-HALF degrees.
Do whatever is necessary to “adjust’ the operating angle at the front of the drive shaft. Add or remove shims from the bearing, if necessary, but if you add shims make sure you re-check all angles of any shafts connected to the coupling shaft to make sure they are still correct.
ALWAYS inspect your center bearing at normal service intervals. You should not see any black “dust” around the bearing or on the bearing bracket. If a black dust is present, you may have an operating angle problem and you should use a protractor to measure and calculate the size of the U-joint operating angle at the front end of the coupling shaft. It should be small, less than 1 1/2 degrees.
Center bearing failures related to weight
Extra heavy-duty drive shafts, those with heavy wall tubes, can easily weigh in excess of 100 lbs.
Engineers “tune” the center bearing in a shaft to the dynamics of the vehicle and the weight of the drive shaft
That is why there are center bearings with “slotted” rubber cushions and center bearings with “solid” rubber cushions.
Most center bearings with slotted cushions will not be “stiff” enough to support the weight of extra heavy duty shafts and the slots will collapse.
Center bearings should be inspected at regular service intervals. Check the slots in the cushion and if they are collapsed, replace the entire assembly with a center bearing that has a solid rubber
Keep this in mind, Most 1760 or 1810 series drive shafts have heavy wall tube and should probably have a solid rubber center bearing.