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 .