In the assembly stage, as we have seen in the previous paragraph, one runs the risk of damaging the external area of the O-Ring making it slide on abrasive surfaces or against incorrectly joined edges. The installation of the O-Ring on many seal elements, to avoid superficial damaging, requires that the job of lubricating the stretching process of the ring to widen the inside diameter and exceed in this way the superficial roughness. In this phase, the O-Ring undergoes a highly increased lengthening. The lengthening value must be correctly evaluated and in any case less than the limit of the elastic lengthening of the compound, to avoid permanent deformation that compromise the seal capacity of the element. This risk is particularly noticeable in the O-Rings with a small cross section diameter. When the deformations imposed in the assembly phase are increased, even if the elastic deformation limit was not attained, the ring may require a certain amount of time before reacquiring its initial dimensions; it is advised to be aware of this elastic inertia in the automated assembly systems.
Another factor of physical stress that can present itself is the torsion section of the O-Ring, when during the assembly the ring meets resistance in the sliding. This problem presents itself the most noticeable way for the O-Rings that exercise a strong pressure onto the seal walls, and for O-Rings with increased correlation between internal and the cross section diameter, with consideration to the base value of resistance to the torsion. If the ring is unable to recuperate elastically the torsion stress, the seal system may be compromised. In dynamic seals, one can reach the destruction of the ring as a result of the internal tension. We have indicated the position of the ideal line of symmetry on the level of the O-Ring in an assembly carried out correctly (figure 5.5a) and in an assembly which has provoked the torsion of the ring. (figure 5.5b)