We had a G150 aircraft arrive at our facility with an intermittent nose wheel shimmy squawk. The airframe technicians jacked the aircraft and inspected the installation of the NLG strut attach points, torque links, NLG Steering Unit and drag brace but did not finding any discrepancies. They proceeded to remove the NLG wheels to check for proper balance. The wheels were out of balance, so the techs rebalanced them to within specifications. They also cleaned, inspected and repacked the bearings.
After installing the wheels the aircraft departed.
After a few flights the pilots reported that the shimmy was still present. He arrived at another repair facility, and they replaced the bushings in the NLG scissors. The aircraft departed and after several flights still felt the shimmy.
Duncan Aviation technicians decided the next step was to replace the NLG wheel and tire assemblies. A technician was dispatched with a set of overhauled wheels that included new bearings and tires.
Several flights later, the pilots reported that the shimmy was no longer present.