## 5 A Quantitative Evaluation of Sequences and Time  
Having reached the end of the search for the disassembly sequence, the three sequences found have been checked in order to decide upon the optimal sequence in terms of savings of time. The calculation of times, referring to the disassembling of the gearbox, has been made using TMUs due to the unavailability of the gearbox that was only a CAD model. The environment with tools has been hypothesized too, compatibly with a credible production environment. These assumptions have been made according to References [50; 51] that allow for an estimate of the disassembly time through a process of assigning points to then be converted into seconds.  
[PERSON] [50] considers DfD to optimize the workings and the interaction of the operator with the process, being the aim of the project activity. The resulting life cycle of the product is already influenced during the first design phases, defining structure, and components. In this phase, if a careful choice of the elements and a homogenization of the parts are adopted, 10-20% of gain on the following EOL cycle of the product or on the maintenance phase can be obtained. After the design concerns a \"disassembly judgment,\" which can be defined as the degree of difficulty related to disassembly. In this context, the following aspects have to be highlighted:  
* _Use of force:_ to quantify the effort of the operator and the relative conditions.
* _Disassembly mechanism:_ a lean and fast path minimizes time.
* _Use of tools:_ ideally, try to disassemble without using tools, but in practice, try to reduce the tools used in terms of cost and versatility of the workstations.
* _Repetition of the parts:_ a limited repetition of the components allows the operator to identify the elements, reducing possible errors.
* _Recognition of the joints:_ an easy access of the connections allows the operator to work in an optimal position without fatigue.
* _Product structure:_ a simplified structure, or at least one without redundant elements, makes the process easier.
* _Accessibility:_ the elements that must be disassembled must not be positioned in such a way as to make it difficult to remove them through unfavourable disassembly directions.
* _Positioning:_ tools and connections are preferable when they do not require high precision in the positioning and disassembly approach.
* _Basic time:_ an attempt is made to take care of and to quantify the time required for the operator to perform elementary movements, which repeated in a given sequence, define the whole operation.  
Following the aforementioned elements, eleven parameters have been defined, which allow one to best describe the dismantling operation of an element. By performing the sum of these values, an evaluation of the operation under examination is obtained, expressed in TMUs. This has to be combined with a classification of the \"EOL option,\" i.e., how the component will be used/disposed of once it has been disassembled from the assembly: or how to allocate the recycling of elements such as precious metals, plastic materials, and elements that cannot be used; or how to recondition processes and reintegrate into the production cycle; or how to best perform maintenance. As performed in the publications, the total value of the TMUs has been converted using the formula:  
\[\text{Time (s)}=(10\times\text{TMU})\times 0.036 \tag{1}\]  
where 0.036 is the standard value of conversion from a TMU task to seconds.  
For the calculation, an Excel sheet was prepared in order to easily calculate the various amounts of TMUs. It was possible to associate the correct tool for each operation, and the aim was to evaluate the number of tool changes in each sequence, a further evaluation parameter.