Abstract: A device for butt welding pipes of thermoplastic plastic includes two pipe clamping units, aligned flush relative to each other, for clamping pipe ends therein. A first of the two units can be moved in the alignment direction by an electric motor and via mechanical drive elements. The reaction force corresponding to the infeed force is supported on the bottom via a support structure and a pipe end held in the second of the two units which is fixed in relation to the bottom. The mobile pipe end, held in the first pipe clamping unit, can be pressed against the fixed pipe end which is clamped in the second pipe clamping unit with a force measurable by a measurement device.