## 3 Case Study
### Ergonomic Assessment
#### 3.3.1 Indexes Evaluation  
In this section, the indexes evaluation is presented. The indexes have been evaluated by means of tools implemented in Tecnomatix Process Simulate, except for the OCRA checklist which has been filled-in by using data provided by the simulation. Figure 7 shows the results concerning the OWAS index evaluation. The blue dotted line represents the average value of the OWAS index (\(I_{\mu}\)), while red lines represent the standard deviation \(\pm\sigma\) (\(I_{\sigma}\)) values. Table 3 resumes the statistical values and their comparison with medium risk threshold value (\(I_{t}\)). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Index** & \multicolumn{3}{c}{**Range Values**} \\ \hline  & **Low Risk** & **Medium Risk** & **High Risk** \\ \hline OWAS & \(<\)200 & 201–300 & 301–400 \\ NIOSH & \(\leq\)0.85 & 0.86–0.99 & \(\geq\)1 \\ OCRA checklist & \(\leq\)11 & 11.1–22.5 & \(>\)22.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Risk areas for the selected indexes. Figure 7: OWAS index scores for each working cycle and their statistical values. By applying the Equation (1), it is possible to deduce that, as well as demonstrated by the results in Figure 6, it is widely verified:  
\[I_{\sigma}=8.3<I_{t}-I_{\mu}=53.1 \tag{4}\]  
Thus, no further investigations are needed about working postures. Regarding exerted forces, there are 4 operations that require the application of force. They are related to screwing operations (OP40 in Figure 3): the worker applies a counter-reaction force at the tightening end. The value of the force is the same for each screwing and in each working cycle, so there is no variability. It depends on the tightening torque (\(T\)), which is 30 Nm, and the lever arm (\(a\)), which for the used gun screwdriver (Figure 8b) is 100 mm. So the exerted force (\(F_{EX}\)) is given by the following equation:  
\[F_{EX}=T/a=300\ \mathrm{N} \tag{5}\]  
The Force Solver tool in Tecnomatix Process Simulate enables to evaluate the maximum applicable force, based on the posture and on the direction of application. Since it is not possible to predict the direction of the tightening end force, the worst-case scenario has been considered, which corresponds to the direction, in the transverse plan, along which the minimum value of the maximum applicable force is depicted (Figure 8a). The maximum applicable force is equal to 44 N.  
Hence, the exerted force (\(F_{EX}\)) is widely higher than the maximum applicable force, so an improving solution is necessary. Concerning MMH, NIOSH lifting index (\(LI\)) has been evaluated by considering two lifting operations that are performed for each task:  
1. The first lifting regards the component 1, which has a weight of 6 kg. The heights of initial and final positions are 1400 mm and 900 mm respectively and the vertical displacement represents the main contribute to _RWL_. This component is handled with a frequency of 1 per cycle, thus during the work shift, a total of 900 components are lifted;
2. The second lifting concerns the assembly, which has a weight of 8 kg. The heights of initial and final positions are 900 mm 500 mm respectively. In this case, the main contribution to _RWL_ is given by the maximum distance between the object and the body, which is equal to 400 mm. The frequency of handling is 1 per cycle too. The NIOSH Lifting Index (\(LI\)) is constant throughout the working cycles and it is equal to:  
\[LI=0.96\]  
Figure 8: (**a**) Maximum applicable force. (**b**) Screwdriver lever arm. Concerning OCRA checklist, the scores varies along the cycles. Figure 9 shows the results concerning the right limb, which is the most stressed one. The blue dotted line represents the average value of the OCRA score (\(I_{\mu}\)), while red lines represent the standard deviation (\(I_{o}\)) values. Table 4 resumes the statistical values and their comparison with medium risk threshold value (\(I_{t}\)).