Evaluation of Lubricant Selection and Lubrication Intervals for Pin–Bushing Bearings Operating Under High-Temperature Conditions in Heavy-Duty Construction Machinery

Pin–bushing bearings in heavy-duty construction machinery operating in severe industrial environments are susceptible to accelerated wear, grease degradation, and lubrication failure, yet application-specific guidance for lubricant selection and re-greasing intervals under such conditions remains limited. This study evaluates the combined effects of bushing material (hardened steel, cast bronze, and Cu–Sn alloy), grease type (three commercially used greases with viscosities of 120, 460, and 150 mm2/s at 40 °C), and lubrication interval (8, 12, and 24 h) on grease-condition indicators in a field-operating wheel loader used in slag handling, where surrounding slag temperatures may reach 700–800 °C. A Taguchi L9 orthogonal array was used to define nine experimental configurations, each applied for approximately one week under real operating conditions. Grease samples were characterised using the SKF grease analysis kit based on NLGI consistency grade, base oil release rate, and contamination particle count. All greases showed an increase in NLGI grade from 2 to 3–4 during service, indicating thickening and a possible risk of lubrication channel blockage. Oil release rates decreased by up to 60% in some configurations, indicating reduced base oil mobility during service. When the three grease-condition indicators were evaluated together by Grey Relational Analysis, the combination of steel bushing, type B grease (ISO VG 460, lithium complex with MoS2), and a 12 h lubrication interval showed the most balanced overall response. These findings provide field-based guidance for grease selection and maintenance scheduling in pin–bushing systems operating under demanding service conditions.