Mixed Assembly Line Balancing

Assembly lines are meant to be a cost-efficient way to manufacture an item through standardization. Balancing the assembly line allows for low-volume, made-to-order production up to high-volume, mass-produced items. Essentially, balancing the assembly consists of allocating or reallocating tasks to a workstation to minimize downtime or constraints.

Read this article. The article proposes balancing production lines to attenuate capacity restrictions and increase balancing efficiency. Pay particular attention to section 2.2 on assembly line balancing.

Results and discussion

The method was applied in a AL belonging to a manufacturer of agricultural machines of the type drawn with unit manufacturing lot. The AL is organized into five workstations; in each station there is an operator. Moreover, among the workstations there is a buffering unit that has the function of absorbing excess processing time for some models about the cycle time. Figure 9 illustrates the AL current flow, with the number of workstations within the marked blocks and buffers marked with "x" in the center. All computational procedures were performed in spreadsheet.

Figure 9 AL flowchart with current balance. Source: The authors.



The AL in study manufactures seven models of products, called CL, O, MX, CB, ATI, ATU and CH, which are assembled through up to 29 different tasks depending on the model. Table 1 shows the product models, the tasks of each workstation and the precedence relationship.


Table 1 Performed tasks in the AL for each modelthe current AL workload is shown in Figure 10, with the times (in minutes) per workstation and model.

Work station Taks Description Precedence CL O MX CB ATI ATU CH
1 1 Introduce pre-assembly - X X X X X X X
1 2 Preassemble servostat 1 X X X X X X
1 3 Mount Steering Column 1 X X X X X X
1 4 Preassemble reservoir 1 X X X X X X
2 5 Preassemble accelerator pedal 1 X X X X X X
2 6 Place preassembly device at post 2;3;4;5 X X X X X X X
2 7 Preassemble the console structure 6 X X X X X X X
2 8 Preassemble servostat at column 7 X X X X X X
2 9 Pre-mount clutch cable 7 X X X X X X
2 10 Pre-mount brake pedals support 7 X X X X X
2 11 Mount Valve Brake POWERFILL - 40Km 7 X
2 12 Mount brake valve and reservoir 10 X X X X X
2 13 Pre-mount brake sensor 12 X X X X X
2 14 Mount switch differential lock 7 X X X X X X X
2 15 Mount clutch pedal 9 X X X X X X
3 16 Mount accelerator pedal 7 X X X X X X X
3 17 Pre-mount plugs from the brake lines 11;12 X X X X X X X
3 18 Mount Steering Column on the console 8 X X X X X X
3 19 Assemble Hydraulic Hoses Direction 18 X X X X X X X
4 20 Assemble hoses servostat 8 X X X X X X X
4 21 Assemble hose and return pipe 20;18 X X X X X X
4 22 Mount servostat hoses clamp 21 X X X X X X X
4 23 Assemble steering wheel on the console 18 X X X X X X X
4 24 Plase devide in horizontal position 21 X X X X X X X
5 25 Mount firewall plate 24 X X X X X X X
5 26 Mount firewall nuts clips 25 X X X X X X X
5 27 Seal outside on the console 26 X X X X X
5 28 Plase devide upright 27 X X X X X X X
5 29 Final test Todas X X X X X X X

Figure 10 Actual AL balancing. Source: The authors.



The current planned demand of AL is 37 products per shift, which are thrown randomly into production (no structured sequencing). Each turn consists of 480 minutes of production time, excluding stops for meeting, lunch and breaks.

As can be seen in Figure 10, only the stations 3 and 5 have ability to 37 product/shift for any product, since the time all product models is less than the cycle time. Currently, AL capacity at bottleneck situation is 28 products per shift.

Table 2 presents the mix of products using the company's production plan and the proportion of each model calculated from Equation 8.

Table 2 Definition of the production.

PRODUCT MODEL Τ
CL O MX CB ATI ATU CH
MIX
(quantify for model) 		0.8 1.2 0.4 12.5 1.7 20.0 0.4 37.0
Proportion of model 2% 3% 1% 34% 4% 54% 1% 100%

Then it was drafted precedence matrix equivalent to all models, as shown in Figure 11.

Figure 11 Equivalent precedence diagram. Source: The authors.



Based on the total demand of 37 products per turn, we calculated the cycle time (T_c) using Equation 2, which resulted in 12.97 min.

Table 3 shows the time weighted average processing for each task and the RPW for each task calculated using Equation 9.

Table 3 Weighted average processing time and tasks RPW.

    PRODUCT MODEL 
   
     CL O 
 MX CB
ATI
ATU
 CH    
Taks
    2% 3% 
1% 
34% 
4% 
54% 
1% 
\overline t_{k}
RPW
1 Introduce pre-assembly 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37
2 Preassemble servostat 4.49 4.49 0.00 4.49 4.49 4.49 4.49 4.44 4.81
3 Mount Steering Column 0.00 5.59 5.59 5.59 3.40 8.77 5.59 7.08 7.44
4 Preassemble reservoir 3.50 3.50 0.00 3.50 3.50 3.50 3.50 3.46 3.82
5 Preassemble accelerator pedal 3.99 3.99 3.99 3.99 3.99 3.99 0.00 3.94 4.31
6 Place preassembly device at post 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 20.28
7 Preassemble the console structure 0.65 0.65 1.25 1.25 1.25 1.25 1.25 1.22 21.50
8 Preassemble servostat at column 3.42 1.47 0.00 2.29 2.29 2.29 2.29 2.26 23.77
9 Pre-mount clutch cable 0.56 0.56 0.56 0.56 0.56 0.56 0.00 0.55 22.05
10 Pre-mount brake pedals support 0.96 0.96 0.00 0.96 0.96 0.96 0.00 0.94 22.44
11 Mount Valve Brake POWERFILL - 40Km 0.00 0.00 0.00 0.00 0.00 0.00 2.08 0.02 21.53
12 Mount brake valve and reservoir 2.08 2.08 0.00 2.08 2.08 2.08 0.00 2.03 24.47
13 Pre-mount brake sensor 0.47 0.47 0.00 0.47 0.47 0.47 0.00 0.46 24.93
14 Mount switch differential lock 0.52 0.52 0.52 0.52 0.52 0.52 0.52 0.52 22.03
15 Mount clutch pedal 0.97 0.97 0.97 0.97 0.97 0.97 0.00 0.96 23.01
16 Mount accelerator pedal 0.93 0.93 0.93 0.93 0.93 0.93 0.47 0.93 22.43
17 Pre-mount plugs from the brake lines 0.80 0.80 0.16 0.80 0.80 0.80 1.58 0.80 25.30
18 Mount Steering Column on the console 0.00 1.89 1.35 1.35 1.52 5.68 1.52 3.68 27.45
19 Assemble Hydraulic Hoses Direction 5.44 5.44 5.44 5.44 5.44 5.44 5.44 5.44 32.89
20 Assemble hoses servostat 3.56 3.56 6.07 10.88 6.07 6.07 6.07 7.55 31.31
21 Assemble hose and return pipe 3.03 3.03 0.00 3.03 4.55 4.55 4.55 3.90 38.89
22 Mount servostat hoses clamp 0.99 0.99 0.82 0.99 0.99 0.99 0.99 0.99 39.88
23 Assemble steering wheel on the console 0.84 0.84 0.84 0.84 0.84 0.84 0.84 0.84 28.29
24 Plase devide in horizontal position 0.68 0.68 0.68 0.68 0.68 0.68 0.68 0.68 39.58
25 Mount firewall plate 2.14 2.18 3.10 3.10 3.10 3.10 3.10 3.05 41.94
26 Mount firewall nuts clips 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 42.49
27 Seal outside on the console 0.00 0.00 1.39 1.39 1.39 1.39 1.39 1.31 43.80
28 Plase devide upright 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 44.76
29 Final test 3.10 3.10 3.10 3.10 3.10 3.10 3.10 3.10 63.03

After increasingly ordering the tasks in accordance with the value of the RPW, it was determined the number of the last workstation (W). To do this, we calculated the minimum number of workstations for each model (Equation 12), and then it was defined as a worst case model with higher MinW (in this case, the ATU model, according to Table 4).
Table 4 W calculation.

PRODUCT MODEL
CL O MX CB ATI ATU CH
CTTm 46.0 51.6 39.6 62.1 56.8 66.3 52.3
TC 12.97 12.97 12.97 12.97 12.97 12.97 12.97
MinW 3.55 3.98 3.06 4.79 4.38 5.11 4.03
6
W=j 6

Further, steps 9 were performed by 14 of the method presented in section 3. The values of MVM_{j,m} are shown in Table 5; stands out in bold the value of MVM_{j,m} according to j allocation of tasks to workstations.

Table 5 Results of MVM_{j,m}.

PRODUCT MODEL
CL O MX CB ATI ATU CH
MinW 6 6 6 6 6 6 6
CTTm 46.0 51.6 39.6 62.1 56.8 66.3 52.3
j = 6 CTAj+1 0 0 0 0 0 0 0
AVMm 7.7 8.6 6.6 10.3 9.5 11.0 8.7
j = 5 CTAj+1 8.4 8.5 10.6 10.8 10.8 10.8 10.8
AVMm 7.5 8.6 5.8 10.3 9.2 11.1 8.3
j = 4 CTAj+1 18.2 18.2 16.2 20.5 22.0 22.0 22.3
AVMm 7.0 8.3 5.9 10.4 8.7 11.1 7.5
j = 3 CTAj+1 23.5 23.6 24.1 33.2 29.9 29.9 31.3
AVMm 7.5 9.3 5.2 9.6 9.0 12.1 7.0
j = 2 CTAj+1 31.5 31.5 26.9 41.4 38.2 42.4 35.6
AVMm 7.3 10.1 6.4 10.4 9.3 11.9 8.4
j = 1 CTAj+1 33.6 39.2 35.3 49.7 44.4 53.9 44.0
AVMm 12.3 12.3 4.4 12.3 12.3 12.3 8.4

The new balance of AL is shown in Figure 12, with the times per workstation and per model.

Figure 12 New balance of AL. Source: The authors.



It can be seen in Figure 12, all workstations have less processing time than the cycle time. Thus, the new balancing provided the AL able to produce 37 products/turn, regardless of the proportion or type of product manufacture. A summary of the tasks allocation to the workstations is shown in Chart 1. As can be seen, there was an increase of 1 workstation from the current balance; however, all workstations have processing time less than the cycle time, which ensures a production capacity of 37 products/shift for any model.

Chart 1 Tasks allocation to workstations for balancing the AL.

Workstation Task
1 1; 2; 4; 5
2 3; 6; 7; 14
3 8; 9; 10; 12; 16; 18
4 11; 15; 20; 23
5 13; 17; 19; 21
6 22; 24; 25; 26; 27; 28; 29

Another observed improvement refers to buffers which become dispensable for no longer exists over processing time in relation to the cycle time for any model. Thus, the amount of product in the new AL will not exceed the number of workstations (6); this situation differs from the current balance, which allows up to 9 products in the AL. So you can see a 33% reduction of inventory in process. Table 6 presents the variables and indicators used to assess the current state of the AL compared to new balance.

Table 6 Variables and Indicators (Current × RPW-MVM).

Current balancing New balancing
VARIABLE Time available in the period p (min) 480 480
Demand (parts) 37 37
Cycle time (min) 12.97 12.97
Number of buffers 4 0
Model bottleneck ATU CB
Tg (min) 17.12 12.69
CTTg (min) 66.3 62.1
INDICATOR Amount of AL workstations 5 6
Capg (parts) 28.0 37.8
TCestimg (min) 154.08 76.14
Line efficiency bottleneck situation (LEg) 63% 83%
Balancing efficiency (BE) 85% 94%

The results presented in Table 6 indicate significant improvements from the point of view of the company's experts. The ability of 37 products/shift exceeded the capacity index of the current state by 35% for the AL bottleneck model. This increase is due to the new distribution of tasks. Furthermore, the new balancing allows the release of the products in AL without particular sequence, which gives great flexibility to the system.

The crossing time estimated in the bottleneck situation was reduced by 50%, which represents that a product can reach the customer 78 minutes faster than today. Finally, the line Efficiency bottleneck situation showed an improvement of 32% under the influence of reduction in processing time bottleneck station (due to rebalance). The balancing efficiency improved 11% (from 85% to 94%), resulting in a better balanced distribution of tasks between the workstations about to the current balance.

Finally, a comparison was performed between the resulting RPW-MVM balancing against traditional RPW applied to mixed-AL, according to Table 7. The flexibility of precondition for a demand for 37 products per shift was not met for the case of balancing proposed by traditional RPW, as capacity in the bottleneck situation was 31.4 products/ shift. Moreover, a reduction in line efficiency at bottleneck situation was observed, despite the reduction of a workstation having occurred, along with the increase in 4% balancing efficiency.

Table 7 Variables and Indicators (RPW-MVM × RPW).

New balancing
RPW-MVM 		New balancing RPW
VARIABLE Time available in the period p (min) 480 480
Demand (parts) 37 37
Cycle time (min) 12.97 12.97
Number of buffers 0 0
Model bottleneck CB CH
Tg (min) 12.69 15.28
CTTg (min) 62.1 52.32
INDICATOR Amount of AL workstations 6 5
Capg (parts) 37.8 31.4
TCestimg (min) 76.14 76.38
Line efficiency bottleneck situation (LEg) 83% 82%
Balancing efficiency (BE) 94% 98%