A county road department reported that a Komatsu GD655-5 motor grader developed a violent front-end shimmy at 25 km/h when roading between job sites. When the operator turned the steering wheel to full lock at low speed, the wheel kicked back sharply against the operator's hands. The moldboard also chattered when finish-grading, preventing a smooth finish cut.
The technician first checked the steering hydraulic system, but pressures were within specification and the orbitrol was functioning normally. He then inspected the front axle articulation joint. The GD655's front axle is articulated-the front frame pivots relative to the rear frame through a horizontal pin and upper and lower tapered roller bearings. Using a pry bar between the front frame casting and the axle housing, he detected 8 millimeters of vertical play in the articulation joint.
He removed the articulation pin and extracted the upper and lower bearing cups. Both sets of tapered roller bearings were severely pitted, with the bearing cages disintegrated. The bearing race surfaces were deeply brinelled, indicating the joint had been operated dry-the grease zerk fitting had been sheared off by a rock strike years prior, and the joint had not received grease since. The excessive vertical play allowed the entire front axle to oscillate unpredictably. At 25 km/h, the oscillation amplified into a violent shimmy. The steer kickback occurred because the worn joint allowed the front axle to twist slightly under steering load; when the load released, the axle snapped back, kicking the steering wheel. The moldboard chatter was a secondary effect: the unstable front axle transmitted vertical oscillation into the moldboard's grade sensor mast, causing the blade to repeatedly hunt up and down.
The repair involved replacing the upper and lower articulation-joint tapered roller bearings and cups, machining the articulation pin to restore its factory surface finish, and installing a new grease zerk fitting in a protected location. The entire front axle housing was inspected for stress cracks. After reassembly and greasing, the shimmy at speed disappeared, the steering tracked smoothly through full lock with no kickback, and the moldboard maintained a chatter-free finish cut.
