A Frank Read source sends dislocations through a random set of cubic unshearable precipitates. They are by-passed according to the Orowan's model. One can notice loop pile-up around each precipitates crossing the glide plane containing the source. The shape of the Orowan loops is either triangular or hexagonal corresponding to the intersection between a (100) cube and a (111) plane. When many loops are present, passages between precipitates are narrower. Consequently the passage of dislocations is more difficult and the rythm of the source is slowed down.