the pipe abut with sufficient force to permit effective fusion of the two ends the pipes for which the machine of this invention is intended to be used are large and heavy for example 24 34 in diameter and four pounds per linear foot and the section being moved is generally on the order of 40 feet long so that the momentum of the pipe is substantial the provision of the accumulator 8 serves automatically to absorb the kinetic energy of the fluid and minimize the sensitivity required when setting the pressure controls this has been found to produce a more uniformly satisfactory fusion of pipe of various sizes numerous variations in the construction of the machine of this invention within the scope of the appended claims will occur to those skilled in the art in the light of the foregoing disclosure merely by way of example the hydraulic system by which the movable clamp is moved can be given a differential speed and force by other arrangements of the hydraulic cylinder to provide an effective capacity in one cavity greater than in the other in the embodiment described the double track surfaces 36 and 37 were provided by setting the square box beam on edge however angles or sloped plates.