A device for crimping synthetic bundles or slivers of yarns, the device has a stuffing box provided with an inlet for a yarn bundle or sliver and including a movable press plate and a stationary counter plate opposite to the press plate, two axially parallel rollers disposed at the inlet for the yarn bundle or sliver and forming a gap through which the yarn bundle or sliver enters the stuffing box, a pivot arm pivotally mounted on a stationary pivot bearing, one of the two axially parallel rollers being rotatably mounted at a fixed position in a machine frame, another of the two axially parallel rollers being seated on the pivot arm so as to be rotatable about a rotation axis thereof and the press plate being pivotally connected to the pivot arm by means of a rocker so as to be pivotable about the rotation axis of the roller seated on the pivot arm or another rotation axis parallel with the rotation axis of the roller seated on the pivot arm, whereby the gap between the rollers is varied, a contact pressure device for applying a contact-pressure force directed toward the counter plate to the press plate at an engagement point movable in a longitudinal direction of the stuffing box, an adjusting device for moving the engagement point of the contact-pressure forced on the press plate along the press plate, and a closed-loop control circuit unit for adjusting a reaction force acting between the rocker and the pivot arm to a predetermined fixed value by actuating the adjusting device.
