dshaft

After a year’s work and many thousands of Dinera, you’ve finally pulled your project out of the shop. Certainly the months you spent boxing the chassis, installing the suspension parts, and fabricating trick motor and transmission mounts are showing real promise. You’ve gone through the moves to get it right, putting the final loving touches on what should be a smooth ride. And it is - almost. Everything is fine except for that nagging vibration that courses through your finger tips on the wheel, shakes the seat of your pants, and insults your ears with a constant echoing howl. Although you’ve done your best to find the offending defective part, at some point you begin to suspect that your hands have made some critical error your brain won’t let you see. You’ve visited the alignment shop twice (maybe more than one shop) only to discover that the rims are straight, everything is in good balance, the axles are aligned, and no excess tire run-out exists. You replaced apparently flawless U-joints, and had the driveshaft checked for balance and straight-ness. You checked clearances and run-out in the differential, and maybe even swapped a set of bearings out. At some point you discover that you’ve either replaced or thoroughly inspected and passed on everything you can think of, and the vibration is still keeping you up nights. When you find that the U-joints are starting to show premature wear, you start looking for some fool with bad batteries in his hearing aid, a serious case of the shakes, and money burning holes in his pocket who’ll take this nightmare off your hands.

But wait!

Before you give up, there’s one last thing (actually it should have been one of the first things) that you should check. It’s a condition called driveline/universal joint cancellation, and has everything to do with what is often referred to as pinion angle. It’s another one of those often misunderstood and often neglected issues that brings out a lot of heated debate whenever it gets discussed. The reason is that there are a number of variables concerning both the equipment and the use of the vehicle that can change how you want to set up the driveline.To get to the bottom of this subject and provide the best and most reliable information available, lets take driveline geometry and troubleshooting down to what we feel are the most common problems you will encounter.

In practical terms, when you build your chassis and locate the driveline components, you need to create a situation where the angle between a line drawn though the center of the transmission output shaft A and a line drawn though the center of the driveshaft B is to equal but opposite the angle between a line drawn though the center of the pinion shaft C and the line drawn though the center of the driveshaft B.

If you visualize this correctly,you will see that the lines drawn though the pinion and
the transmission shafts (A,C) are parallel to each other but not in the same plane
(one is above the other).

The centerlines of the transmission output shaft, driveshaft, and pinion have a specific relationship that MUST be maintained to work correctly. Angle 1 and Angle 2 must be equal (but opposite) and centerline A and centerline C must be parallel but NEVER the same. Angles 1 and 2 must always be at least 1 degree but generally not more than 3 degrees. This means that the angles between the shop floor and the shafts should be identical if you are to achieve cancellation. This is most simply done with a tool called an inclinometer. Every driveshaft or chassis shop will have one, and you can get a perfectly serviceable tool from your local discount hardware for around $10 (a dial level). To set up or correct these angles, you can alter the transmission mounting, shim the transmission mount, slip angled shims (a parts-store, speed shop, or 4WD repair shop item) between the axle spring pad and the spring, or otherwise rotate the pinion into position depending on what type of rear suspension you are using. For example, if you have a four-bar setup, you can shorten the upper bars and lengthen the lower bars by the same amount to rotate the pinion upward without altering the wheelbase. Reverse this procedure to rotate the pinion downward.
Because the angles you are concerned with are measured solely in terms of their relationship between each of the three adjacent components of the driveline, the levelness of the floor or angle of the chassis to the pavement is not relevant. Just be sure to check all angles with the vehicle set up and loaded as it will be under most conditions.

First of all, the driveline is always set up with the driveshaft with at least one degree of angle between it and both the transmission output shaft and the differential pinion shaft so that the universal joints "flex" as they rotate.

The reason for this practice is to prevent running the roller bearings in the same position at all times which eventually pushes the grease away from the bearing surface, limiting lubrication, and ending in brinelling (those little ribbed marks that mean the death of a U-joint). By presetting a slight angle, the joint is put in a situation where each cup and bearing assembly rotates around the U-jointcross from front to rear twice per revolution. This assures an even coating of grease and prevents a wear pattern setting up, but creates a scenario which has a lot of potential negative results.

If the driveline is set up with a degree or more angle, the U-joint cross rotates with the transmission shaft in a circular motion and also moves from front to rear. If viewed in cross section, with rotation occurring in a counterclockwise motion, both arms of the U-joint cross fixed in the driveshaft yoke are closest to a simple circular path at 3 and 9 O’clock. From 3 to 12 O’clock, each arm (remember it is mechanically attached to the driveshaft) accelerates as it moves toward the rear of the vehicle. It decelerates from 12 to 9 o’clock as the arm of the cross returns to center, accelerates from 9 to 6 O’clockas it moves toward the front of the vehicle, and finally decelerates from 6 to 3 O’clock to complete one revolution.

Visually, as seen from the correct angle, the actual path the cross arms follow is an ellipse.
The speed of these arms when graphed, is seen as a regular sine wave reflecting aceleration - deceleration cycles. Finally, considering the U-joint cross arms alternately speed up and slow down twice per revolution, this also means the driveshaft - being mechanically linked to them must also speed up and slow down at the same rate.
You may not have thought about it, but there is a reason why U-joints are used in pairs.
You see, as one U-joint is going through its alternating cycle, so is the other - but because the opposite end of the shaft is installed at an opposite angle, the cycle occurs at exactly opposite timing.
If the joints are in phase, and the angle between the driveshaft and the equipment at both ends is the same, the acceleration/deceleration cycles tend to cancel out, resulting in smooth, silent operation.
This is called cancellation.

To get very technical, when the rotational inertia (the tendency for the mass of moving parts to continue moving along a given path) combines with the acceleration/deceleration cycles described above, what is called an excitation torque is produced. The more out
of cancellation, the higher the excitation torque becomes and the worse the vibration.

As seen from the end view, the path that the bearing cups takes is an ellipse, rotating
forward and backward as well as in a its path around the centerline of the shaft. From
the side view, the path appears as an angled line, again showing that the cup ends move
in more than one plane. Because of this front to back movement, the speed of the cups
accelerates and decelerates twice per revolution.

If cancellation is not achieved, you have a regular rotation on the transmission end translating through the U-joint into the acceleration/deceleration cycles in the driveshaft, but the translation back to a regular rotation through the U-joint at the differential is not achieved. Under these conditions, the alternating cycle of acceleration/deceleration is first taken up by any free-play in the driveline. Bearing surfaces begin to take a beating somewhat akin to what occurs if a piston wrist pin has excess play. The result is not only a noise, but a vibration which means reduced bearing life throughout the driveline and which tends to focus energy on the weakest point. Most often this is in the U-joints, but pinion bearings and transmission bearing wear as well. When free-play doesn’t allow enough slack, parts not designed to flex begin to do so in an effort to absorb the uneven energy transfer.
In some cases, the driveshaft literally begins to twist and untwist (this is called torsion) at a frequency which can be both heard and felt as vibration.

It doesn’t take too much imagination to understand why a driveshaft in torsion is not exactly a good idea. Many times, this transfer of energy extends beyond the driveline and into other components of the vehicle - called coupling.
Simply put, coupling is where torque excitation is transferred into the chassis and body from the driveline. It can often be "driven through" because the drivetrain has a critical frequency at which it begins to resonate and the noise and vibration increase dramatically.
It should be noted that the problem is still occurring at all speeds, but it is just more noticeable at this critical speed.

As I mentioned earlier, there are variables that can and do alter this static geometry. All vehicles, regardless of construction, will have some tendency for the differential to rotate upward under acceleration. If the vehicle is to pull heavy loads for long distances, or expected to undergo hard acceleration with lots of torque, you may want to rotate the pinion downward to pre-load the driveline so under normal operating conditions the pinion rotation will result in cancellation. Regardless of the application, additional pinion angle required beyond that resulting in perfect cancellation at rest depends on how stiff the suspension components, mounting bushings, springs, trailing arms, or other links with the chassis are. In addition, the total torque exerted on the differential and whether it is momentary or sustained must be considered. Most people believe that no more that three additional degrees of torque pre-load on the pinion to maintain cancellation is necessary and often only a degree or so is enough. Although not strictly a requirement of a fully functional suspension, a pinion snubber (a hard rubber or urethane bumper placed above the pinion housing) is a good idea. It can limit the rotation of the differential without sacrificing ride quality, limiting suspension travel, or causing damage to the housing.

The proof of whether you have achieved overall cancellation is found in the lack of vibration, singing, or ringing you have when you’ve finished the job. If under normal operation there is no vibration, you’ve hit it on the head. If you still have vibration that increases under acceleration, you need to add more downward pinion pre-load. If you find the opposite exists - the vibration tends to decrease or cease under acceleration - you need to reduce the downward pre-load. This should not be confused with a vibration that steadily increases with driveshaft speed (accelerating or decelerating). In most cases, this symptom is primarily the result of either imbalance or excess run-out in the driveshaft. Although a yoke run-out problem can sometimes be improved by rotating the driveshaft U-joint 180 degrees in the differential yoke, this is a common indication of a damaged or improperly built and balanced shaft.

The acceleration and deceleration of the joints occur at twice per revolution and must be
precisely in synchronization or the two ends will try to accelerate and decelerate in difference
phase. The result is flexing driveshaft, hammering in the splines and at the gear faces, and
vibration.The way to synchronize is to match the pinion to driveshaft angle with the output and
driveshaft angle so they work in unison. This is called cancellation.