## 4 Results  
### Selection and Description of Critical Tasks  
Numerous manual assembly tasks are performed on the assembly line, but for the purpose of the case study presented in this paper, three basic tasks were selected as critical tasks. These tasks are related to assembly parts that represent a substantial proportion of the company's quality issues. Results showed that, during the 36-month period prior to the study, 67% of all wrongly installed parts were associated with brackets (27%), loop clamps (25%) and bolts (15%). Similarly, 65% of all missing parts were associated with cap covers (40%), brackets (13%) and loop clamps (12%). Following this, the three basic critical tasks selected were:  
1. Install three brackets at specific locations according to the work procedure.
2. Secure a data cable to the main structure with three cushioned loop clamps.
3. Install four cap covers to protect access points to the structure.  
Figure 3: Workstation’s layout (not to scale) and general workflow: (1) read work instruction on the PC, (2) pick the parts from bins, (3) pick tools from the tool cabinet, (4) execute assembly operations and (5) close work instructions on the PC.  
To provide a comparative idea of the different cycle times, these were estimated using MTM-1 (method-time measurement), a predetermined motion time system (PMTS) [83]. The estimated cycle times for the tasks analyzed were 155 s, 51 s and 28 s, respectively. Basic cycle times, calculated with PMTS, are often used as part of different models developed for complexity assessment in manual assembly operations [84; 85]. However, the cycle times calculated here are not part of any formal complexity assessment of the assembly tasks under study. Therefore, these values must be interpreted with this element in mind even when, to some extent, they might express differences in complexity between the tasks analyzed. Brackets are secured with nuts and bolts, while cushioned loop clamps are attached to the main structure with tie wraps. Torque is applied with a wrench according to specifications provided in the assembly instructions. Parts used in the assembly tasks are presented in Figure 4. A check for missing parts is carried out by the worker at the end of each task cycle. The selected tasks can be considered representative, to some extent, of the manual assembly process under study.  
An example of a 2D drawing provided to the assembler in the work instruction of task No. 1 (install three brackets) is shown in Figure 5. Work instructions are displayed on a 19-inch stationary PC monitor. The computer's input devices are a mouse and a keyboard, both connected by cables to the PC. Figure 6 shows bracket A installed and secured with two bolts to the assembly structure. Similarly, Figure 7 shows the data cable and the cap covers installed on the assembled object. The data cable was secured to the structure using cushioned loop clamps and tie wraps. Cushioned loop clamps were used to avoid damage to the data cable in an operational context. Cap covers were installed to protect different inlets.  
Figure 4: Parts used in the assembly tasks: (1) bracket A, (2) bracket B, (3) bracket C, (4) nut and bolt, (5) cushioned loop clamp, (6) tie-wrap and (7) cap covers.  
Figure 5: 2D drawing showing the exact location where brackets A, B and C should be installed.  
Figure 6: Bracket A secured to the structure in the final assembly location.  
Figure 7: Installation of data cable and cap covers in their respective assembly locations.  
Figure 8 shows the breakdown of the critical basic task No.1 (install brackets). This HTA diagram is presented as an extract of the diagrams developed for the breakdown of each of the critical tasks under analysis. Subtasks 1.1-1.4 repeat for each of the brackets A, B and C, representing 12 subtasks. For the sake of diagram simplification, repetitions are not included.