Buckle for restraint systems for car safety seats for children, provided with a device for indicating the correct tensioning

The buckle comprises: a case (18) connected to a lower belt of the restraint system, a coupling mechanism arranged to releasably engage a pair of tongue-like coupling elements each connected to a respective upper belt of the restraint system, and an indicator device (32,36,38) for indicating to the user the correct tensioning of the belts. The indicator device (32,36,38) comprises an indicator element (32) having a visual symbol which can be seen by the user through one or more openings provided in the case (18) and which is received in the case (18) so as to be movable between a first end-of-travel position, corresponding to the condition of loose belts, in which the visual symbol is not visible by the user, and a second end-of-travel position, corresponding to the condition of correctly tensioned belts, in which the visual symbol is visible by the user through the opening(s). The indicator device (32,36,38) further comprises first springs tending to urge the indicator element (32) towards the first end-of-travel position, a locking element (36) received in the case (18) and directly or indirectly connected to an end of the lower belt, and second springs (38) interposed between the case (18) and the locking element (36) so as to be resiliently deformed as a result of the application of a tension force on the lower belt, thereby bringing about a corresponding movement of the locking element (36). The locking element (36) is configured to cooperate with the indicator element (32) in such a manner that when the indicator element (32) is in the second end-of-travel position once the correct tension in the lower belt has been reached, it is locked in this position by the locking element (36).