In line layout, equipment is dedicated to parts and products, and if the same process is needed elsewhere in the system, it is duplicated almost regardless of utilization.
Production line concepts have found their greatest held of application in assembly rather than in fabrication. This is true because machine tools commonly have fixed machine cycles, making it difficult to achieve balance between successive operations. The result can be poor equipment utilization and relatively high costs when production line concepts are applied to fabrication operations. In assembly operations, where the work is more likely to be manual, balance is much easier to obtain because the total job can be divided into smaller elements. If station 10 is too short whereas 16 is too long, part of the work of station 16 is too long, part of the work of station 16 probably can be transferred to station 10 (perhaps the tightening of a single bolt). Because very little equipment is involved in assembly, its utilization may not be of great importance.
Decision to Organize Facilities by product:
The managerial decision to organize facilities with a product focus and a line layout involves important requirements, and there are some consequences affecting the work force that should be weighted carefully.
The following conditions should be met if facilities are to be organized as a product focused system,
1. Adequate volume for reasonable equipment utilization
2. Reasonably stable product demand
3. Product standardization
4. Part interchangeability
5. Continuous supply of materials
When the conditions for product focused systems are met, significant economic advantages can result. The production cycle is speeded up because materials approach continuous movement. Since very little manual handling is required, the cost of material handling is low. In process inventories are lower compared with those of batch processing because of the relatively fast manufacturing cycle. Because aisles are not used for material movement and in process storage space is minimized, less total floor space is commonly required than for an equivalent functional system, even though more individual pieces of equipment may be required. Finally, the control of the flow of work (production control) is greatly simplified for product-focused systems because routes are directed and mechanical. No detailed scheduling of work to individual work places and machines is required because each operation is an integral part of the line. Scheduling the line as a whole automatically schedules the component operations.
Given the decision for designing a line layout, a major problem is subdividing the work so that smooth flow can result. The subdivision process is called line balancing.
Line Balancing Concepts:
Let us take the case of a wooden toy car, the parts of which are named and numbered. We will use the assembly of the car to illustrate the concepts of precedence requirements, cycle time, and capacity specification and the meaning of optimum solutions.
By examining the toy car, we can see the sequence restrictions that must be observed in its assembly. For example, the hubcaps must be installed on the wheels prior to subsequent assembly steps to ensure that the wood axle is not broken as a result of impact. Finally, the wheels cannot be assembled until the axle has been inserted in the car body.
These sequences must be observed because the toy car cannot be assembled correctly in any other way. On the other hand, it makes no difference whether the headlights are assembled before or after the wheels are assembled. Similarly, whether the front or rear wheels are assembled first is irrelevant.
Cycle time and capacity:
Now we can proceed with the groupings of tasks to obtain balance. But balance at what rate of output? What is to be the capacity of the line? This is an important point and one that makes the line balancing problem difficult. If there were no capacity restrictions, the problem would be simple; one could take the lowest common multiple approach. For example, if we had three operations that required 3.2, 2.0, and 4.0 minutes, respectively, we could provide eight work places for the first, five for the second, and ten for third. The capacity of the line would be 150 units per hour at each of the operations, and the cycle time would be 0.4 minutes. But capacity would then be specified by balance rather than by market considerations.