A movable strap-engaging member (20) is provided for being connected at a first pivot axis to a vibrator arm (30) for accommodating pivot movement of the vibrator arm (30) about the pivot axis while the movable strap-engaging member (20) is pressing a first length (U) of flexible thermoplastic strap against a second length (L) such strap in an elongate strap-receiving path. The movable strap-engaging member (20) is reciprocated generally transversely of the path to effect a friction-fusion weld of the strap lengths (U, L) . The movable strap-engaging member (20) has a configuration in which first and second centers of mass (M1, M2) are spaced apart on a line (75) that passes through the pivot axis and that is oriented generally perpendicular to the path. The product of the square of the distance (r1) along the line (75) between the pivot axis and the first center of mass (M1) is substantially equal to the product of the square of the distance (r2) along the line (75) between the pivot axis and the second center of mass (M2).