maintaining constant tension over {HEAD ~ the length} of {TAIL ~ each web} as the webs travel over the support surface 
maintaining constant tension over { HEAD ~ the length } of { TAIL ~ each web } as the webs travel over the support surface
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH