Abstract: The invention relates to an improved apparatus for tensioning and fusing together strips of plastics material, having a pair of clamping members mounted on a support base and between which members overlapping strip portions are inserted tensioned, and fused together by frictional heat generated upon movement of one clamping member relataive to the other; the improvement resides in that the clamping members comprise clamping jaws of which at least one is adaptd to be reciprocable at a frequency corresponding to the speed of a drive shaft and that the drive is effected via a gearing which presses said driven reciprocable clamping jaw against the strip abutting it during its movement in the tensioning direction.