M. Poozesh et al./ Elixir Project Mgmt. 47 (2012) 9074-9078 9074 Introduction Today, tractor is one of the most important power sources in agriculture (Singh, 2006). Farm tractors must be maintained and kept in good repair condition if they would render efficient service (Beppler and Hummeida, 1985). Its versatility and high efficiency have made it suitable for most field and barnyard operations. However, the need for high management skills and susceptibility of tractors to breakdown has made its maintenance very imperative. Timeliness in farm operations is a crucial factor for successful agricultural operations. Farm tractors failure, especially, during the engaged part of the season, causes delays which result in losses and inefficient labour utilisation. As more and more capital in the form of machinery replaces manual labour on the farm, the reliability of this equipment assumes greater importance. Indeed, deeper insight into failures and their prevention is to be gained by comparing and contrasting the reliability characteristics of systems that make up the tractor (Amjad and Chaudhary, 1988). Reliability is defined as the probability that the equipment or system will complete a specific task under specified conditions for a stated period of time (Ebeling, 1997). Hence, reliability is a mathematical expression of the likelihood of satisfactory operation. A failure may be referred to as any condition which prevents operation of a machine or which causes or results in a level of performance below expectation. The failure rate of a population of items for a period of time t 1 to t 2 is the number of items which fail per unit time in that period expressed as a fraction of the number of non- failed items at time t 1 . Hence, in reliability, the reciprocal of failure rate is the mean time to failure [MTTF] (Wingate-Hill, 1981). Amjad and Chaudhary (1988) reported that machine failures can be categorised into: early life failures, random failures and wear-out. Likewise, Lewis (1987) asserted that reliability considerations appear throughout the entire life cycle of a system. He claimed that the data collection on field failures are particularly invaluable because they are likely to provide the only estimate of reliability that incorporates the loading, environmental effects and imperfect maintenance found in practice. At both component and system levels, such a database is invaluable for predicting the reliability of future design and for improving design. Owing to the importance of timeliness of operations in obtaining high yields, machinery breakdown especially at busy period such as sowing or harvest can lead to large losses of revenue quite apart from the cost of repairing the equipment. If estimates could be made of when equipment is likely to fail, this would assist in planning machinery purchases and spare parts inventories and reduce costs. The results of numerous individual experiments confirm that the crop yield changes in a predictable way related to the timing of various filed operations (witeny, 1985). Unless the enterprise is on a very small scale or the machine used is very large, it is unlikely that the operation can be completed at the scheduling time. This inability to complete an operation within a short period incurs a penalty which increases on a daily basis as the duration of the period is extended (Fig. 1). The evaluation of this penalty cost requires the selection of a unique yield/time response for a multiplicity of crop yield experiments (witeny, 1985). Time from optimum day of establishment (day) Fig 1. Percentage yield loses from untimely crop establishment Tele: E-mail addresses: Asakereh@ut.ac.ir © 2012 Elixir All rights reserved Determining the reliability function of farm tractors M. Poozesh, S.S. Mohtasebi, H. Ahmadi and A. Asakereh Agricultural Machinery Engineering Department, University of Tehran, Karaj, Iran. ABSTRACT There is an optimum time for implementing field operation of various crops in each region. If the operation is accomplished sooner or later, it will cause a reduction in yield quantity and quality. The cost of this reduction is named “timeliness cost”. Annual timeliness cost is estimated by an equation reported in ASAE standards. MF285 tractors have low reliability in Iran. Generally, low tractor reliability has an uncertainty in implementation of farm operation which it is not considered in ASAE equation for calculating of timelines cost. This uncertainty causes a cost being a part of timeliness cost and is produced due to tractor failure within farm operation. The purpose of this study was to determine the reliability function of MF285 tractors operated in Debal Khazaei Agro-Industry Co. in Khuzestan province of Iran. Thus, in this study a function-fit model for reliability of farm tractors was determined. Afterwards, tractors working times to a failure (hours) in tenth year of tractor life were fitted with normal, exponential, lognormal, poisson and weibull age distribution functions using moment estimation. Depending on Chi square test, the best reliability function for this given situation was exponential distribution function with  = 0.025. © 2012 Elixir All rights reserved. ARTICLE INFO Article history: Received: 12 April 2012; Received in revised form: 15 June 2012; Accepted: 20 June 2012; Keywords Timeliness, Mean time to failure, Age distribution function, Farm management. Elixir Project Mgmt. 47 (2012) 9074-9078 Project Management Available online at www.elixirpublishers.com (Elixir International Journal)