Journal of Manufacturing Technology Management
Emerald Article: Improving factory layout under a mixed floor and overhead material handling condition
By Manocher Djassemi
Purpose – In spite of having a number of general-purpose algorithms for solving plant layout problems, facility planners may still face a challenging task to adjust these algorithms to handle special, but not uncommon, layout problems. The purpose of this study is to propose a new method for addressing the impact of overhead space utilization on a plant layout solution.
Methodology– A new method for adjusting material flow under a mixed floor and overhead material handling condition is incorporated in an existing plant layout procedure. A case study involving the layout improvement in a lawn mower engine assembly facility is presented to illustrate the effectiveness of the proposed method.
Findings – The analysis of solutions for the case study shows that the layout generated by the proposed modified material flow approach is a more economical solution. The case also shows, when it is important to optimize the use of space, the overhead space should be considered as part of any methodology for designing a good layout.
Research limitations/implications – The proposed modified material flow approach can be applied to any facility where the use of overhead space for material handling is justifiable by limited floor space and/or by high cost of land. The proposed method can be applied to small to medium size problems with minimal computational effort. However, as the size of facility grows, the manual calculation becomes more time consuming and potentially erratic.
Originality/value – This paper should be useful to both researchers and practitioners who deal with overhead space utilization in designing facility layouts.
Findings – The analysis of solutions for the case study shows that the layout generated by the proposed modified material flow approach is a more economical solution. The case also shows, when it is important to optimize the use of space, the overhead space should be considered as part of any methodology for designing a good layout.
Research limitations/implications – The proposed modified material flow approach can be applied to any facility where the use of overhead space for material handling is justifiable by limited floor space and/or by high cost of land. The proposed method can be applied to small to medium size problems with minimal computational effort. However, as the size of facility grows, the manual calculation becomes more time consuming and potentially erratic.
Originality/value – This paper should be useful to both researchers and practitioners who deal with overhead space utilization in designing facility layouts.
Introduction :
The efficiency of a manufacturing facility depends on a number of factors, including the layout of machinery and departments. Typical plant layout procedures determine how to arrange the various machines and departments to achieve minimization of overall production time, maximization of turnover of work-in-process, and maximization of factory output.
A number of procedures have been developed to aid facilities planners in designing new layouts or improving existing layouts. Meller and Gau (1996) listed more than 90 published layout models and algorithms. These layout algorithms are based on two types of objective functions: distance-based and adjacency-based. The common parameters in these objective functions are interdepartmental flow, fij; unit-cost values, cij; distance between departments, dij; and department closeness rating, xij. A distance-based objective function aimed at minimizing distance between departments is expressed mathematically as:
The efficiency of a manufacturing facility depends on a number of factors, including the layout of machinery and departments. Typical plant layout procedures determine how to arrange the various machines and departments to achieve minimization of overall production time, maximization of turnover of work-in-process, and maximization of factory output.
A number of procedures have been developed to aid facilities planners in designing new layouts or improving existing layouts. Meller and Gau (1996) listed more than 90 published layout models and algorithms. These layout algorithms are based on two types of objective functions: distance-based and adjacency-based. The common parameters in these objective functions are interdepartmental flow, fij; unit-cost values, cij; distance between departments, dij; and department closeness rating, xij. A distance-based objective function aimed at minimizing distance between departments is expressed mathematically as:
Modified material flow procedure
In general, there is strong relationship between the amount of material flow and the proximity of departments when one of the facility planning procedures is deployed. That is, the larger the amount of flow between two departments, the closer they are positioned to each other. In addition to material flow data, other factors such as environmental factors, building constraints, and/or the MH system configuration may play a critical role in attaining a practical solution.
As mentioned earlier, the issue of overhead space utilization and its trade-off with interdepartmental distances has not been addressed in the literature. Most existing procedures aim at placing departments as close as possible to each other due to significant material flow and regardless of whether the material is transported by floor equipment or by overhead equipment. However, when the overhead space utilization is considered, the material flow should not have the same impact on determining the closeness of the departments. This is due to the fact that using overhead space can be beneficial in that, first, it plays the role of a buffer zone for holding parts while the materials are in transit, and second, the overhead space can balance the workload between departments. Therefore, to reflect the impact of overhead space utilization in the final layout, it is necessary to adjust the material flow data such that it would not cause the departments on the two ends of a particular travel path to be placed so close to each other that the benefit of overhead space utilization is undermined.
In general, there is strong relationship between the amount of material flow and the proximity of departments when one of the facility planning procedures is deployed. That is, the larger the amount of flow between two departments, the closer they are positioned to each other. In addition to material flow data, other factors such as environmental factors, building constraints, and/or the MH system configuration may play a critical role in attaining a practical solution.
As mentioned earlier, the issue of overhead space utilization and its trade-off with interdepartmental distances has not been addressed in the literature. Most existing procedures aim at placing departments as close as possible to each other due to significant material flow and regardless of whether the material is transported by floor equipment or by overhead equipment. However, when the overhead space utilization is considered, the material flow should not have the same impact on determining the closeness of the departments. This is due to the fact that using overhead space can be beneficial in that, first, it plays the role of a buffer zone for holding parts while the materials are in transit, and second, the overhead space can balance the workload between departments. Therefore, to reflect the impact of overhead space utilization in the final layout, it is necessary to adjust the material flow data such that it would not cause the departments on the two ends of a particular travel path to be placed so close to each other that the benefit of overhead space utilization is undermined.
In view of above discussion, a cost-based modified material flow (MMF) measure is defined as
equation 2 as follows
Where Uoverhead(ij) represents the overhead material flow volume in unit loads between departments i and j. Rij is a weighting factor that represents the ratio of the operational cost of moving materials between departments i and j by overhead equipment to the operational cost of moving the same volume of materials by floor equipment in a hypothetical scenario :
equation (3)
where, Lij actual length of overhead MH equipment between departments i and j; COH operating cost of overhead MH equipment per foot per part; DFL(ij) distance between departments i and j if material were moved by a floor material handler; CFL operating cost of moving material by a floor material handler per foot per part.
By using Rij factor in equation (3), the actual material flow, Uoverhead, is intentionally increased or deceased. For example, if the operational cost of using of fork lift truck is less than the cost of using an overhead conveyor, then Rij . 1 and as a result, the modified material flow, MMF, would be higher than the actual material flow between the two involved departments. This intentional increase in material flow volume may result in positioning the two departments closer to each other, which translates to less utilization of overhead space. In this example, it is obvious that the use of a fork lift truck is more economical than using an overhead conveyor.
The algorithmic form of a procedure for applying modified material flow, MMF, to an existing plant layout problem is as follows:
(1) Determine the number of unit loads (pallets, baskets, or boxes) for floor material flow
(2) For material flow moved by an overhead MH device, determine the equivalent unit loads in number of pallets or baskets (Uoverhead(ij))
(3) Determine the adjusted material flow using equation (3)
(4) Set up a From-To chart that includes unit loads for floor and overhead moves as well as modified unit loads for overhead moves
(5) Use the From-To chart generated in step 4 as input to one of the available plant layout methods to generate two layout alternatives,with and without material flow data modification
(6) Compare the performance of the alternative layouts
(1) Determine the number of unit loads (pallets, baskets, or boxes) for floor material flow
(2) For material flow moved by an overhead MH device, determine the equivalent unit loads in number of pallets or baskets (Uoverhead(ij))
(3) Determine the adjusted material flow using equation (3)
(4) Set up a From-To chart that includes unit loads for floor and overhead moves as well as modified unit loads for overhead moves
(5) Use the From-To chart generated in step 4 as input to one of the available plant layout methods to generate two layout alternatives,with and without material flow data modification
(6) Compare the performance of the alternative layouts
This paper described a procedure for incorporating overhead space utilization in existing plant layout algorithms using a modified material flow approach. The SLP method was used as a means for to develop a solution methodology. The combined SLP and MMF procedure generates two solutions for improving the plant layout. The first solution is based on actual material flow data. The second solution is based on actual material flow data adjusted by a material flow factor, to account for overhead space utilization and associated MH costs.
