A municipal highway department reported that a John Deere 770G motor grader's circle - the ring-gear assembly that supports the moldboard and allows it to rotate about a vertical axis during operation - had developed progressive degradation. The operator reported that the circle hydraulic motor struggled to rotate the blade, and hydraulic pressure would climb to relief-valve setting when the blade was turned to steep slope angles. The circle would stall mid-rotation when attempting to adjust blade angle under load, and the operator had to rock the blade back and forth to "break free" the stall before rotation could continue. A distinctive scraping sound was audible from beneath the machine when the circle rotated.
The technician first inspected the circle. The circle is a large ring gear bolted to the blade assembly, engaging a small drive pinion driven by a hydraulic motor through a reduction gearbox. The entire assembly rides on a set of "Circle Shoes" - replaceable wear blocks mounted to the lower surface of the upper frame, which support the circle assembly's weight and locate the circle assembly vertically at the correct height for pinion engagement. When the shoes wear or the circle rack gear loses lubrication, the circle sags or distorts, breaking pinion engagement and forcing the circle hydraulic motor to overwork to complete a rotation.
He visually inspected the circle rack gear's teeth. The teeth were coated in a dry, powdery lubricant residue, and in several sections of the circle, the teeth showed no fresh lubricant at all. Several teeth showed "excessive wear profiles," with pronounced flank wear and minor pitting - consistent with metal-to-metal contact as the lubricant film failed. He then inspected the circle shoes - visible from beneath the upper frame through an access hole beneath the circle assembly. Three of the six shoes were worn heavily, their UHMW surfaces worn to 60 percent of remaining thickness, and one shoe had broken completely, leaving only the metal backing plate exposed beneath the circle assembly.
The root cause was traced to lubrication neglect and accelerated shoe wear. Motor grader circles require regular lubrication - the rack gear teeth and shoes require greasing at every grease interval, and in abrasive environments (which motor graders routinely work in) typically require more frequent greasing. The machine's service history showed circle lubrication was performed only at annual major services, not at the machine's standard 50-hour grease schedule. Years of dry running had caused the rack gear wear, and shoe wear - accelerated by the increased friction and by the circle assembly's weight pressing on unlubricated dry surfaces - had caused the circle to sag, misaligning the pinion and stalling the circle hydraulic motor.
The repair involved replacing all six circle shoes with new units, replacing the backing plates beneath the circle assembly, and replacing the circle drive pinion (whose worn tooth profile would have quickly destroyed a new rack gear). The rack gear was thoroughly lubricated with fresh, spec-compliant grease. The circle reduction gearbox's gear oil was checked (correct), and its breather was replaced (the old breather was clogged with dust). The circle assembly was reassembled and centered on the new shoes, with pinion engagement adjusted to JCB specification for correct gear depth. Circle lubrication - rack gear and all shoes - was re-added to the machine's 50-hour grease schedule. After the repair, the circle rotated through its full range without stalling or hydraulic pressure surges, the scraping sound was eliminated, and blade rotation was smooth and even during operation.
