Operators often ignore a "loose" swing bearing until the entire upper structure separates from the undercarriage. The swing bearing is a massive double-row ball bearing. As it wears, axial clearance increases. Measuring this clearance requires a dial indicator with a magnetic base, and it must be done under a specific load condition.
We mount the base to the track frame and the stylus on the outer race of the bearing. With the stick fully extended and the bucket empty, we boom down slightly to lift the front of the tracks off the ground (removing the eccentric load). The indicator shows 1.1mm of axial clearance. The OEM discard limit is 0.25mm for a 20-ton machine.
We then measure the gear mesh backlash using a feeler gauge between the swing pinion and the bearing ring gear. It measures 2.5mm (limit is 0.8mm). Why is 1.1mm of clearance fatal? The bearing bolts (M24, 10.9 grade) are tensioned to 350 kN. With 1.1mm of play, the heavy upper structure "heels and toes" on the raceway during every swing start/stop. This cyclic shock load stretches the bolts past their 0.2% yield offset, relaxing the clamp force to under 150 kN. The joint begins to micro-slide, fretting the machined mounting surfaces. One hard counter-swing against a bank, and the bolts shear, dropping the 20-ton house. If axial clearance exceeds 0.5mm, the bearing must be replaced immediately; do not wait for the 0.25mm limit.
