A spin welding apparatus is employed for hermetically sealing seamless overpacks. The seamless overpacks are utilized, for example, for isolating corroding 55 gallon drums which contain hazardous materials. The overpacks include a receiver which contains the 55 gallon drum and a cover which is spin welded onto the receiver. The receiver and cover have circular welding surfaces which oppose one cover which is spin welded onto the receiver. The receiver and cover have circular welding surfaces which oppose one another across a horizontal plane. The welding surfaces have a composition of heat fusible plastic. During the spin welding process, the cover is rotationally driven by the spin welding apparatus while simultaneously being forced downward onto the receiver, which is held stationary. Frictional heat melts the opposing welding surfaces. After the rotational motion stops, the opposing welding surfaces cool and solidify to form a seamless weld joint. During the spin welding process, the covers are rendered susceptible to centrifugal imbalance due their large size. The resultant centrifugal imbalance can couple to other components of the system. Several features serve to ameliorate this centrifugal imbalance: vibrational coupling between the receiver from the cover is minimized by the horizontal configuration of the welding surfaces; rotational imbalance transmitted to the drive shaft is damped by pillow blocks; the damping efficiency of the pillow blocks is enhanced by mounting the pillow blocks and the torque driver onto a linear positioning table for effecting the axial movements of the drive shaft.