Unit 8 (2017)Apunte Inglés
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Unit 8: Design of production process by product: lines balancing
In a line layout, workstations are placed one after the other according to the operations
sequence. The material goes through the different steps. If a workstation requires more time
than the rest, then it acts as a brake. Idle time appears at the rest of workstations.
Line balancing intends that workstations have a similar workload so that their inactivity time (or idle time) is minimised.
Definitions - - Work element: Task that cannot be divided between two or more workers without creating an interference between them.
Workstation: Area close to the production line, where is performed an operation.
Usually it is assigned to one operator although it can be assigned to more than one.
Operation: Set of work elements assigned to a workstation.
Line balancing: Assignment of work elements to workstations.
Cycle time: Maximum time than a product stays in each workstation. Also, it can be defined as the time from the end of a workstation until the entrance to the next.
Consequently, it’s the mean time used to manufacture a product.
Balancing delay: Total idle time existing in the production line that arises from an irregular division of workstations.
Tasks, capacity and cycle time Firstly, workload is divided in smaller units that can be performed independently.
Minimum cycle time is defined as the longest workstation time.
Maximum cycle time is the time required to execute the whole production process; in other words, the sum of all workstations times. If the desired production by time units is known (Q), the cycle time is: 𝑡𝑐 (𝑚𝑖𝑛𝑢𝑡𝑒𝑠 𝑝𝑒𝑟 𝑢𝑛𝑖𝑡) = 60 𝑚𝑖𝑛𝑢𝑡𝑒𝑠 𝑝𝑒𝑟 ℎ𝑜𝑢𝑟 𝑄 𝑢𝑛𝑖𝑡𝑠 𝑝𝑒𝑟 ℎ𝑜𝑢𝑟 Line capacity and cycle time Consider a product that requires several components for its production. The mounting activities have been divided in 10 basic elements (a….j). Activity times are shown below: Minimum cycle time is 10 minutes. Elements b) and g) require those minutes for its execution.
Maximum cycle time is the sum of all workstation times = 58 minutes.
Minimum number of necessary workstations The formula to calculate the minimum number of workstations that are necessary is: 𝑁= ∑𝑛𝑖=1 𝑡𝑖 𝑡𝑐 When the result is NOT an integer, then we adjust that number by excess. The result is the minimum number of workstations but, in practice, it can be used more workstations if it’s necessary.
Idle time or inactivity time Idle time is the difference between the total time really used and the theoretical time needed to obtain one product unit.
𝑛 𝑡𝑜 = 𝑁 ∗ 𝑡𝑐 − ∑ 𝑡𝑖 𝑖=1 Free grouping of 10 elements Theoretical minimum number of workstations Calculation of annual production Consider just one working shift Adequate cycle time Calculating the most adequate cycle time should take into account the maximum annual production and also the present market demand.
Task precedence Task precedences are activities that should be carried out before the considered task can start.
The sequence is collected in the Precedences diagram.
Task assignment Conditions to assign a task to a determined workstation: a. The considered task should NOT be assigned to any previous workstation.
b. The preceding tasks should be assigned to a previous or the same workstation.
c. The sum of execution times of all the tasks assigned to the same workstation should NOT overpass the cycle time Tc.
Rules for task selection: 1. Rule of maximum successive task. Selection of tasks with longer execution and leaving the rest for precise adjustment.
2. Hegelson and Birnie methodology. For each considered task, execution time of this task is added to execution times of all subsequent tasks. The one that shows a greater result is selected.
Efficiency calculation Ordering technique of weighted positions: H&B methodology 1. Position weigh is determined for each element by adding the time of this element and all the rest that should follow (according to the Precedence diagram) until the process end. In the example: 2. Work elements are placed in decreasing order according to position weights. In the example: 3. Cycle time should be specified. In this case, tc=10 min.
4. Work elements are grouped in workstations, taking into account that none workstation can exceed Tc and preceding relations between elements must be respected. In each workstation, the elements are grouped according to the decreasing order of position weights (Wi). The process is repeated until all elements have been assigned to a workstation.
5. Max. Efficiency.
Where ti = time of the work element i Tc = cycle time N = Minimum integer number of workstations.
6. Efficiency of the balance.
where M is the number of workstations used. In the example: 7. If e.real<e.max, new groupings can be tried by focusing in workstations with higher idle timing that lead to a better balance ( respecting preceding relations).
Production flow and necessary resources Operations planning should take into account the total production time and NOT just the effective productive time.