We’ve all spun a driveshaft by hand and observed the action of universal joints, and by the look of two yokes connected by a “spider”, machines don’t get much simpler. But what is the rotational speed of the input shaft compared to the output? Surprisingly, there is only one rarely encountered condition when they match. In fact, if you took a ratio of shaft speed from the transmission output shaft to the driveshaft, the relationship looks like this:
Fortunately, the math is less important than the message underneath: both the angle between the yokes and the amount the shaft rotates affects the speed of the driveshaft. And since the shafts spin a full 360 degrees per rotation, the speed ratios vary too. The result is a “pulsing” output shaft speed that’s responsible for power loss, wear and vibration. It’s one of the reasons why a slip joint is needed to keep the joint from binding.
So why worry about a little speed variation? The variation translates to vibration, or “NVH” (noise, vibration and harshness), a traditionally stubborn problem to solve without adding cost and complexity. One approach, used on rear-wheel drive Cadillacs among others, was to add a second yoke immediately after the first, canceling out the vibration-inducing wobble inherent in the single-spider design. This is the “constant velocity” universal joint, which trades off smooth operation for added complexity and replacement cost.
Transmission speed / Driveshaft speed = cos (shaft angle) / 1- sin2 (angle of rotation) x sin2 (shaft angle)
Front-wheel-drive adds another problem: angularity. Steering knuckles rotate at angles impossible for conventional joints to accommodate without binding or excessive vibration. The modern Rzeppa or “CV joint” solves the problem by allowing the elements that take the function of the spider, the balls, to slide axially within the outer pot’s splines for smooth power transmission. The same property gives them larger operating angles for tighter turning circles, all in a package that’s significantly smaller than a dual yoke constant velocity design. Its main disadvantage, of course, has made this segment a major earner in most shops; they’re not weatherproof, and torn boots quickly destroy the joints. “Inners” live in a less harsh environment, in terms of lower operating angles and better protection for the boots. Tri-pot designs and even rubber-isolated conventional Hooke joints can survive here.
Electronics reach the driveshaft
As purely mechanical devices, driveshafts and CV joints have escaped the trend toward computer control. That may be changing, however, as new technologies will soon be available that may conquer some of the last remaining NVH issues locked into driveshaft design. The main issue is how to create a vibration free shaft throughout its operating range. Any rotating shaft, including automotive driveshafts, vibrate. They’re like the strings in a guitar, and like a musical note, they vibrate at specific frequencies depending on the shape, length and stiffness of the shaft. They can be balanced, but design to reduce NVH is a compromise because the shaft can’t change its shape or stiffness to respond to changing road conditions. Two new technologies under study by Spicer parent Dana Inc. have the potential to eliminate the compromises and create smooth running driveshafts under all driving conditions.
The technologies are magneto-rheological fluid centre bearing brackets and piezo-electric dampers and actuators. The technology sounds exotic, but in principle, they’re both easy to understand.
The magneto-rheological fluid center bearing bracket is a semi-active mounting point on the driveline that uses magneto-rheological (MR) fluids to change the stiffness of its mount. In a magnetic field, MR fluids rapidly change their shear characteristics from liquid to solid or anywhere in between. This allows for adjustments to the stiffness of a bracket within the driveline suspension system while a vehicle is in motion. With this type of dynamic tuning, it is no longer necessary to sacrifice high-speed NVH for low-speed rigidity and strength. The fluid becomes more viscous when the intensity of the magnetic field is increased. To provide vibration isolation, a bracket with a very soft elastomer is used in conjunction with the fluid’s damping capability. MR fluid technology is also ideal for transfer case and shock absorber applications.
The other new technology uses a property called piezo-electricity to create a new class of dampers and actuators. Piezo-ceramics produce an electrical charge when deflected and conversely change shape with an applied voltage. A common use for peizo-materals is inside push button lighters used to ignite gas barbecues. Earphones for low-cost transistor radios are another application and in a future driveline, devices like the earphone could act as a damper by detecting driveline vibration, then sending a signal to a piezo-actuator which could apply a force on the driveshaft, changing its resonant frequencies like tightening or loosening a guitar string. To ensure the effectiveness of this system for acoustical vibrations, the dampers must dissipate the vibration energy as heat across a resistor bank.
Another potential application is to electronically force a small-diameter shaft to react in the drivetrain system as if it were larger in diameter. This could give Dana the ability to package rotating components in a confined space and achieve the same vibration characteristics as a larger shaft. Dana is working with an M.I.T.-linked company, ACX in Boston, Massachusetts, to develop piezo driveline technology.
Will future driveshaft service move from U-joints to scan tools? With the potential for greatly reduced NVH without complex mechanical CV solutions, the answer is likely, ‘yes’. In the meantime, keeping this part of the driveline running smoothly is still about tight boots, grease, and the least possible angularity.