Abstract: A machine for gas-free sealing/welding of overlapping edges of thermoplastic material is provided and is of the type which includes a framework carrying an ultrasonic transducer horn assembly having a hammer at a lower end thereof. An anvil is positioned below the hammer. A mounting mechanism for mounting the framework for side-to-side movement, and a drive mechanism for moving the framework on the mounting mechanism are also provided. The improvement resides in the mounting mechanism comprising a beam which extends between spaced apart support members and which has at least one upper rail edge and one lower rail edge and wheels rotatably mounted to the framework and including at least one upper wheel and at least one lower wheel which engage, respectively, the upper and lower edges.