Mobile and permanent fasteners
The union of parts made by the fasteners can be of two types: mobile or permanent.
In the mobile type of union, the fasteners can be placed or removed from the set without causing any damage to parts that were attached. This is the case, for example, unions made with bolts, nuts and washers. The type of permanent union, the fasteners, when installed, can not be removed without becoming unusable. This is the case, for example, for unions made with rivets and welds. Both mobile and permanent bonding should be used with great skill and care because they are usually the weakest components of the machine. Thus, to design a mechanical assembly, it is necessary to choose the fixture for the type of parts that will be attached or fixed. If, for example, if you join robust parts with weak and poorly planned fasteners, the assembly will present failures and may become unusable, occasioning waste of time, material, and financial resources.
It is still important to plan properly and choose the correct fasteners to be used to avoid stress concentration in the parts set. These strains can cause disruption due to fatigue of the material, ie, the decrease of strength or weakness of the material due to stress and constant effort.