A MISALIGNED driveshaft can create more damage in a shorter period of time, with fewer technicians able to properly diagnose the problem, than almost any other vehicle component, said Thomas Koedam, driveline development manager for Dana Corporation. Koedam presented the driveline session at the 38th annual NTEA Convention and Work Truck Show in Orlando.
“Many technicians and service managers haven't had training in driveshaft installation or more importantly in recognizing the problem signs that incorrect installation can produce,” Koedam said. “Improper driveshaft selection, installation, or U-joint operating angles lead to vibration, which will usually cause severe vehicle operating problems.”
How severe? A medium-sized fleet removed a transmission, took the transmission to a dealer to have it rebuilt, and within 150 miles of the fleet mechanic's reinstallation, the transmission locked up causing massive internal damage, Koedam said. The dealer again rebuilt the transmission, and upon installation into the customer's truck by the customer's mechanic, the transmission immediately failed.
“Everyone obviously wanted to resolve the problem,” said Koedam. “We were asked to look at the situation. Dana assisted the rebuilding of the transmission; however, we informed the customer that the driveline reinstallation procedure appeared to be the problem.
“After we went to the customer's location and explained the proper reinstallation procedures, the problems stopped. The customer didn't have an idea about the damage that an improperly installed or out-of-phase driveshaft can cause.”
Vibration is a key tell-tale sign of a driveshaft, U-joint, or slip yoke/tubing problem. However, Koedam warned that not all vibration problems are caused by the driveshaft. “When a power unit vibrates, many operators assume it's the driveshaft. With a new vehicle, that's usually not the case. The problems don't normally arise until there are length modifications or the slip yokes have been removed and reinstalled.”
Three types of vibration are associated with driveshaft problems: transverse, torsional, and critical speed.
Transverse vibrations are the easiest to repair and one of the most common, according to Koedam. The cause is an imbalanced driveshaft. “Anything could cause it from mud clumped to the shaft to a balancing weight that has broken off.”
The vibration is caused by each revolution of the driveshaft. “These are the easiest to repair,” said Koedam. “You simply get the driveshaft rebalanced.”
Torsional vibration
Torsional vibration occurs twice per revolution and is caused by U-joints that are required to operate at any angle. Unless the driveline is straight, the U-joint has to speed up and slow down twice for each revolution. The greater the operating angle of the U-joint, the more it is required to speed up and slow down with each rotation.
Torsional vibrations can be managed two ways:
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By installing the driveline so that the operating angle of the U-joints at one end is within one degree of the operating angle of the other end.
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By reinstalling the driveline in the exact position that it came from the factory.
Vehicle manufacturers install the drivelines “in phase”, which means that the forces generated at the front yoke are exactly offset by those at the rear yoke.
“Before taking a driveshaft apart, make certain that the slip yoke and tubing are marked so the driveshaft can go back in its original phase, especially if the driveshaft was manufactured with special phasing from the factory,” said Koedam.
However, Koedam said the majority of driveshaft torsional vibration problems are caused by improper U-joint operating angles. “U-joint operating angles are created at equipment installation time, whether you are installing a body on a shortened chassis, raising the drive component height, or any procedure where the factory set U-joint angles are changed. This can create unequal or out-of-phase U-joint angles that will cause problems.
“These driveshaft and U-joint principles are equally important to the auxiliary equipment mounting process,” Koedam said. “When installing a PTO or blower unit, vibration problems can start because of improper driveshaft alignment.
“U-joint operating angles are probably the most common causes of torsional driveline vibration in vehicles that have been reworked, or in vehicles that have had auxiliary equipment installed,” Koedam said.
Koedam listed three rules that Dana's Drivetrain Service Division recommends:
Rule 1. U-joint operating angles at each end of the shaft should always be at least one degree.
Rule 2. U-joint operating angles on each end of a driveshaft should always be equaled within one degree of each other (one-half degree for motor homes).
Rule 3. U-joint operating angles should not be larger than three degrees. If they are, make certain that they do not exceed the maximum recommended angles.
Calculate U-joint angles
Koedam said that calculating U-joint angles is an important part of the drive train equation.
“As calculating U-joint angles has become more important, Dana has developed the Spicer Anglemaster, a tool that helps installers when working on driveline installations.” It can be used to measure the simple one-plane angles as well as the compound angles.
The simple one-plane angle is the easiest to measure. “Using a protractor or Spicer Anglemaster, measure the angle of the two connected components and look at the direction of their slope. If the slope is in the same direction, subtract the smaller angle from the larger. If they slope in opposite directions, you add the smaller angle to the larger angle.”
Finding the compound U-joint operating angle is more complicated, Koedam said. “To determine the size of a compound angle, calculate the angle in the side profile view, then additionally calculate the offset angle while using the top profile view. Perform this measurement at each end of the shaft.”
The formula for this calculation is: C= x2 + a2 where C equals the true operating angle. The variables X and A represent the side profile angle and the top profile angle.
Perform the same operation at the other end of the driveshaft. The two calculated measurements should be equal or within three degrees of each other. Koedam said that if these measurements exceed three degrees, driveshaft vibration will increase.
“Vibration will occur under three degrees; however, these usually aren't large enough to worry about. Once the measurement gets past that figure, vibration starts to get very noticeable, very fast.”
Torsional vibrations can create some immediately noticeable problems. “There can be a failure of transmission synchronizers and free running gears, along with abnormal wear patterns on the clutch splines,” Koedam said. “For allied equipment, there are seizures of bearings in pumps, blowers, PTOs, and auxiliary transmissions.”
Loosening of the PTO mounting bolts is another sign of torsional vibration. “We've actually seen cases where the problem wasn't diagnosed in time, and the PTO fell off the transmission,” Koedam said. “In another case, the vibration from an incorrectly outfitted driveshaft caused the transmission case to rupture from PTO vibration.”
Critical speed
Critical speed is an issue that driveshaft manufacturers, OEMs, and driveshaft distributors face everyday, said Koedam.
Critical speed occurs when the driveshaft exceeds its maximum revolutions-per-minute (RPM) rate based upon the shaft's wall thickness, outside diameter, and overall length. Once this occurs, the driveshaft begins to bow off its centerline as it approaches critical speed. As it bows, it begins to vibrate violently.
If the speed continues to increase, the shaft bows further from the centerline. At the point of critical speed, the shaft has bowed to the degree that it disintegrates or pulls away from the yoke.
Koedam warned installers of auxiliary driveshafts that critical speed driveshaft failure is a catastrophic failure because of the potential for human injury.
“These are the failures that you've got to prevent because they can cause devastating consequences to equipment and bystanders,” Koedam said. “At the failure point, the shaft can break loose from the vehicle, escape from the undercarriage of the vehicle, and injure bystanders.
“Dana Spicer has developed our Spicer, driveshaft speed calculator that will help determine the safe operating speed and acceptable length of driveshafts,” Koedam said. “Contact us. We want you to have one of these.”
Koedam provided an example of how the calculator works:
A truck comes into your shop. It has a diesel engine governed at 3800 RPM and an SPL36 series, style “A” driveshaft that is 48 inches long with 3.5 inch tubing. The safe operating speed for that driveshaft material is 5500 RPM, well within the safe range of the engine's governed speed used in this application.
Extending the truck's frame by 20 inches would be catastrophic if the SPL36 series, 3.5 inch driveshaft is also extended by 20 inches.
The safe operating speed for this driveshaft material is reduced to about 2500 RPM, well below the governed RPM of the engine. When this driveshaft operates at its governed 3800 RPM, it would probably fail.
A distributor should suggest the use of a new 20" Style “C” driveshaft and the original 48" Style “A” driveshaft with center carrier-bearing. He should refuse to make a 68" shaft for this application.
“Remember, if you are the installer, it's your job to install the driveshaft so that it will safely operate at the required application speed. You may find it necessary to change your application to use multiple driveshafts even if it is more expensive.”