The use of tractors for agricultural works showed an important role to mechanized agricultural sector. A repairable\nmechanical system (as agricultural tractor) is subject to deterioration or repeated failure. In this study, the regression model\nwas used to predict the failure rate of MF399 tractor. The machine failure pattern was carefully studied and key factors\naffecting the failure rate were identified in five regions of Khouzestan province. The tractors grouped in four sub groups\naccording to annual use hours and maintenance policies (corrective maintenance and preventive maintenance). Results\nshowed that the majority of recorded failures were observed in the electrical system (20.3%) for each machine sub-group.\nMaximum and minimum annual repair and maintenance costs were related to other (tire, ring, ball bearing and operator seat) and\nbrake system respectively. According to the results of the research, different annual use hours and maintenance policies\naffected failure rate. The tractors included in 300-1,000PM subgroup were mostly in the useful life period while the machines\nwere towards the wearing out period of machine life for 300-1,000CM subgroup. The tractors maintained under 1,200-2,000PM\nsubgroup showed an obvious indication of wear-out period entrance, while the tractors were mostly in the wear-out period for\nthe 1,200-2,000CM sub group. The tractors included in 300-1,000 annual use hours were commonly in a randomized\nbreakdown period during their useful life but these tractors tend to enter the wear out period in the 1,200-2,000 annual use hours.\nSo, annual use hours were more effective on failure rate in comparison with maintenance policies.
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