A building contractor reported that a SANY SY215C excavator had developed a travel fault over the past two weeks. When the operator commanded both tracks forward at full travel, the machine would track noticeably to the left - the right track moving at approximately 65 percent of the left track's speed. When attempting to climb a soil pile or travel up a slight grade, the right track would lose power first and slow further, requiring the operator to command full travel on the left track and partial travel on the right track to maintain a straight path. The machine had approximately 4,800 operating hours with no prior travel-system maintenance history.
The technician first understood the SY215C's travel architecture. Each track is driven by a "Travel Motor" - a bent-axis piston hydraulic motor coupled to a planetary reduction gearbox that provides the high torque reduction required for track drive. The travel motor receives high-pressure oil from the main pump via the "Swing/Travel Joint" (a rotary manifold mounted on the machine's center pivot that transfers hydraulic oil between the rotating upper structure and the stationary undercarriage), and a separate "Case-Drain Circuit" returns internal leakage from the motor back to the reservoir through a fine filter.
He installed pressure gauges at the travel-motor inlet ports on both sides and ran the machine with full travel commanded. The left travel motor's inlet pressure read 28 MPa (within specification). The right travel motor's inlet pressure read 34 MPa - significantly higher than the left, indicating the right motor was demanding excessive pressure to deliver reduced output. This pattern suggested either internal leakage within the right travel motor (the motor leaking oil internally past its pistons rather than delivering it to the output shaft) or restricted supply oil to the right motor.
He checked the travel-motor case-drain filter, which is designed to remove wear debris from the case-drain oil that returns from both travel motors and the swing motor. The filter element was found heavily contaminated with a dark metallic deposit, and the filter housing showed evidence of the bypass valve having operated recently - indicating the element had become restricted enough that case-drain oil was bypassing unfiltered. He then checked the rotary manifold - the joint that transfers oil between the upper and lower structure - by installing pressure gauges on both sides of the joint and comparing pressures. On the right-side travel circuit, the manifold showed a 4 MPa pressure drop across the joint (spec maximum is roughly 1 MPa), indicating internal leakage between adjacent ports within the rotary manifold. On the left side, the drop was within specification.
The root cause was traced to a combination of rotary-manifold seal wear and a clogged case-drain filter. Rotary-manifold seals wear naturally over extended service as the manifold rotates continuously during slewing; the seal wear allowed oil from the right travel circuit to leak internally into an adjacent low-pressure circuit, reducing the pressure available to the right travel motor. The clogged case-drain filter had raised back-pressure within the travel motors' case-drain circuits - which, on piston-type hydraulic motors, increases the risk of shaft-seal extrusion and accelerates internal wear. The combination had degraded the right travel motor's efficiency to the point of noticeable power loss.
The repair involved replacing the rotary manifold with a new unit (the seal wear had progressed beyond field-repairable), replacing the case-drain filter with a new element, and replacing the right travel motor's rotary-shaft seal as a precaution. The hydraulic oil and return filter were replaced with fresh specified oil, and the system was flushed with a portable flushing rig before fresh oil was introduced. After reassembly and bleeding, the right track's speed matched the left during forward and reverse travel, and the excavator tracked straight on a level test surface. Travel pressure on both sides read within specification across the operating range. Rotary-manifold pressure-drop checks and case-drain filter maintenance were added to the machine's 500-hour checklist.
