Assembly Systems

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Assembly Systems & Line Balance : 

Assembly Systems & Line Balance SUDHIR REDDY S.V.R (09311D0412)

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ASSEMBLY SYSTEMS: There are various methods used in industry to accomplish the assembly processes. Major methods can be classified: 1. Manual single-station assembly. 2. Manual assembly lines. 3. Automated assembly system.

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1. Manual Single-Station Assembly: *Consist of a single workplace to accomplish the product or some major subassembly of the product. *Generally used on a product that is complex and produced in small quantities, one or more workers depending on the size of the product and the production rate. *Such as machine tools, industrial equipment, aircraft, ships and complex consumer products (appliances, car,...).

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Manual Single-Station Assembly:

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2. Manual Assembly Lines: *Consist of multiple workstations in which the assembly work is accomplished as the product (subassembly) is passed from station to station along the line. *At each workstation one or more human workers perform a portion of the total assembly work on the product, by adding one or more components to the existing subassembly.

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Manual Assembly Lines:

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3. Automated Assembly System: *Use of automated methods at the workstations rather than human beings.

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Automated Assembly System:

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REASONS FOR MANUAL ASSEMBLY: Demands for the product is high or medium. The products made on the line are identical or similar. The total work required to assemble the product can be divided into small work elements. It is technologically impossible or economically infeasible to automate the assembly operations.

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THE LINE BALANCING PROBLEM: *The line balancing problem is to arrange the individual processing and assembly tasks at the workstations so that the total time required at each workstation is approximately the same. *If the work elements can be grouped so that all the station times are exactly equal, we have perfect balance on the line and we can expect the production to flow smoothly. *In most practical situations it is very difficult to achieve perfect balance. When workstation times are unequal, the slowest station determines the overall production rate of the line.

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Method of Line Balancing: Largest-Candidate Rule(LCR). Kilbridge and Wester's Method (KWM). Ranked Positional Weights Method (RPW).

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Largest-Candidate Rule(LCR): Procedure: Step 1:-List all elements in descending order of Te value, largest Te at the top of the list. Step 2:-To assign elements to the first workstation, start at the top of the list and work done, selecting the first feasible element for placement at the station. A feasible element is one that satisfies the precedence requirements and does not cause the sum of the Tej value at station to exceed the cycle time Tc. Step 3:- Repeat step 2.

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Kilbridge & Wester's Method (KWM): *It is a heuristic procedure which selects work elements for assignment to stations according to their position in the precedence diagram. *This overcomes one of the difficulties with the largest candidate rule (LCR), with which elements at the end of the precedence diagram might be the first candidates to be considered, simply because their values are large.

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Procedure: Step 1:- Construct the precedence diagram so those nodes representing work elements of identical precedence are arranged vertically in columns. Step 2:-List the elements in order of their columns, column I at the top of the list. If an element can be located in more than one column, list all columns by the element to show the transferability of the element. Step 3:-To assign elements to workstations, start with the column I elements. Continue the assignment procedure in order of column number until the cycle time is reached (Tc).

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Ranked Positional Weights Method (RPW): *Introduced by Helgeson and Birnie in 1961. *Combined the LCR and K-W methods. *The RPW takes account of both the Te value of the element and its position in the precedence diagram. Then, the elements are assigned to workstations in the general order of their RPW values.

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Procedure: Step 1:-Calculate the RPW for each element by summing the elements Te together with the Te values for all the elements that follow it in the arrow chain of the precedence diagram. Step 2:-List the elements in the order of their RPW, largest RPW at the top of the list. For convenience, include the Te value and immediate predecessors for each element. Step 3:-Assign elements to stations according to RPW, avoiding precedence constraint and time cycle violations.

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Compare LCR, K-W, and RPW: *The RPW solution represents a more efficient assignment of work elements to station than either of the two preceding solutions. *However, this result is accordingly by the acceptance of cycle time Tc = 1 and make those methods different. *If the problem were reworked with Tc = 0.92 minute, it might be possible to duplicate the efficiency.

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Ways to Improve the Line Balance: Dividing work elements:-A minimum rational work element was defined as the smallest practical indivisible task. Advantages- eliminates the bottleneck and increases the tool life. Changing work lead speeds at automatic stations:-Through a process increasing the speed/feed combinations at the stations with long process time, and reducing the speed/feed combinations at stations with idle time.

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Method analysis:-The study of human work activity may result in better workplace layout, redesigned tooling and fixturing or improved hand and body motions. Pre-assembly of components:-Reduce the total amount of work done on the regular assembly line by another assembly cell or by purchasing. a. Required process may be difficult to implement on the regular assembly lines b. Variations in process times (adjustments or fitting) Inventory buffers between stations. Parallel stations.

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