# Closing Force Evaluation of a Sample Return Capsule for a Phobos Sample Return Mission
## 5 Closing Force Measurement
### Piston-Toroidal Shape and Comparison Evaluation of Their Impact over the Mechanism Design  
To be able to perform a comparative analysis between the two piston geometries, a test was firstly conducted for the toroidal piston at environmental temperature and the results are presented in Figure 21. Following the toroidal piston test, a first comparison regarding the necessary force for the closing operation was made using the results obtained for the two pistons (conical and toroidal shape) at ambient temperature and presented in Figure 22.  
Figure 19: Closing force vs. temperature dependency for Type I and Type II seals–piston with a conical shape.  
Figure 20: Type I and Type II seals closing forces for the piston with a conical shape.  
The results show that the closing force is lower using the piston with a toroidal shape for both types of sealing elements. Thus, a decision was made to analyse in detail the closing force values for the piston with toroidal shape along with their influence over the design of the sealing and closing system.  
For the piston with a toroidal shape (TO), the same testing sequence was followed with regard to the temperature, types of sealing elements and testing procedure.  
Based on the partial results, it was concluded that the worst-case scenario (the highest values for the closing force) corresponds to the tests performed at \(-60\)\({}^{\circ}\)C. Thus, taking into account the temperature influences, several closings and closing force evaluation tests were performed at a temperature of \(-60\)\({}^{\circ}\)C, but also at a positive temperature (+70 \({}^{\circ}\)C). In Figure 23 the maximum force/displacement curves are presented for both sealing elements (Type I and Type II), at different operating temperatures and in Table 4 are summarized the results from the previous tests using the piston with a toroidal shape.  
Figure 21: Closing force curves obtained at environmental temperature–piston with toroidal shape.  
Figure 22: Type I seals vs. Type II seals closing force for conical shape vs. toroidal shape pistons.  
The previous tests performed with the piston having a toroidal shape at \(-60\)\({}^{\circ}\)C lead to the decision of increasing the test number for Type II seals to have a better understanding of their behaviour. Maximum obtained forces for each Type II seal test at \(-60\)\({}^{\circ}\)C are marked in Figure 24.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Type of Test** & **Type I Seal [N]** & **Type II Seal [N]** \\ \hline +20 \({}^{\circ}\)C & 471 & 317 \\ \(-60\)\({}^{\circ}\)C & 1932 & 897 \\ \(-60\)\({}^{\circ}\)C & 1915 & 997 \\ \(-60\)\({}^{\circ}\)C & 2265 & 896 \\ +70 \({}^{\circ}\)C & 384 & 232 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Closing force evaluation results were obtained for the piston with a toroidal shape.  
Figure 23: Type I and Type II seals closing forces for the piston with toroidal shape–displacement vs. load.  
Figure 24: Closing force curves were obtained for Type II seals at \(-60\)\({}^{\circ}\)C with toroidal shaped lid.  
Based on all the tests performed with the piston with toroidal shape, the closing force as a function of temperature dependence is plotted in Figure 25 for each scenario. It can be observed that the negative temperature has a significant influence over the closing force and that the force value on the cylindrical area remains approximately constant for both types of seals. As in the previous case, the highest values of the closing force for each type of sealing element were considered and interpolated.  
Summarizing all the data previously presented, it was determined that the toroidal shape piston along with Type II seals ensure the lowest closing force, leading to the decision of considering these aspects later in the mechanical press design.