ORIGINAL ARTICLE A tabu search algorithm for two-sided assembly line balancing Uğur Özcan & Bilal Toklu Received: 9 October 2007 / Accepted: 8 September 2008 / Published online: 23 September 2008 # Springer-Verlag London Limited 2008 Abstract Two-sided assembly lines are often used at the assembly of large-sized high-volume products, such as cars, trucks, and buses. In this type of a production line, the left side and the right side of the line are used in parallel. This paper presents a tabu search algorithm for two-sided assembly line balancing. The line efficiency and the smoothness index are considered as the performance criteria. The proposed algorithm is illustrated with two numerical example problems, and its performance is tested on a set of test problems taken from literature. The performance of the proposed algorithm is compared to the existing approaches. The computational results show that the proposed algorithm performs well. Keywords Assembly line balancing . Two-sided assembly lines . Tabu search 1 Introduction Assembly lines have been widely used in various production systems to produce high volume standardized products. An assembly line includes a series of stations arranged along a material handling system. The components are processed depending on a set of tasks for a given cycle time. The tasks are assigned to an ordered sequence of stations according to a given precedence relationship among tasks to aim the optimization of some specific objectives, such as the minimization of the number of stations for a given cycle time, the minimization of the cycle time for a given number of stations, or the maximization of the efficiency of assembly line [1]. The problem of assigning tasks to stations in such a way that some specific objectives are optimized subject to the precedence relationships among tasks is called the assembly line balancing (ALB) problem. Main constraints of ALB problem are (a) each task must be assigned to exactly one station (the assignment constraint), (b) all precedence relation- ships among tasks must be satisfied (the precedence con- straint), and (c) the total task times of all the tasks assigned to a station cannot exceed the cycle time (the cycle time constraint). In the literature, generally, assembly lines are classified as straight (traditional) assembly lines and U-lines (U-shaped assembly lines) by means of the line layout, and it is also classified as means of the number of product models produced on the line: single-model and mixed/multimodel lines. For more details on classification of assembly lines, refer to Boysen et al. [2]. Many studies on assembly lines including exact solution methods, heuristics approaches, and metaheuristic approaches have been reported in the literature. The detailed reviews of such studies are given by Baybars [3], Ghosh and Gagnon [4], Erel and Sarin [5], and more recently by Scholl and Becker [6] and Becker and Scholl [7]. Assembly lines can also be categorized in two groups as one-sided assembly lines and two-sided assembly lines [8]. The difference between them is associated with the design of the assembly line, i.e., the left side and the right side of the line are used in parallel in two-sided assembly lines, whereas only one side of the line is used in one-sided assembly lines. Two-sided assembly lines are typically Int J Adv Manuf Technol (2009) 43:822–829 DOI 10.1007/s00170-008-1753-5 U. Özcan (*) Department of Industrial Engineering, Selçuk University, 42075 Selçuklu, Konya, Turkey e-mail: uozcan@selcuk.edu.tr B. Toklu Department of Industrial Engineering, Gazi University, 06570 Maltepe, Ankara, Turkey