A wheel loader was brought in with a severe, high-speed vibration that shook the entire cab violently, but only when driving faster than 15 mph. The shop had already replaced the front differential and the transmission, assuming internal gear damage. The vibration persisted perfectly.
We put the machine on stands and ran it in gear. The vibration was coming from the center of the machine, specifically the driveshaft connecting the transmission to the front axle. We pulled the driveshaft and laid it on the bench. The U-joints were brand new-the shop had replaced them during the previous repair. But looking closely at the slip yoke splines, we found the fatal flaw. The mechanic who installed the new U-joints had accidentally put the slip yoke back onto the shaft one spline off. The driveshaft was mis-phased.
Here is the physics of a U-joint: as it rotates at an angle, it does not turn at a constant speed. The output yoke speeds up and slows down twice per revolution. To cancel out this acceleration and deceleration, the U-joint at the front axle must be installed exactly in phase with the U-joint at the transmission-meaning the yokes on both ends of the shaft must be perfectly parallel. Because the slip yoke was one spline off, the yokes were at an angle. Instead of canceling each other out, the two U-joints were compounding their speed variations. At low speeds, it wasn't noticeable. At high RPMs, the driveshaft was violently accelerating and decelerating, trying to rip itself and the differentials apart. We slid the slip yoke off, aligned the splines so the yokes were perfectly parallel, and the loader drove as smooth as glass.
