1. A roll system for tensioning an elongated flexible workpiece, comprising: a pair of roll stations spaced apart in a direction of advance of said workpiece; a respective roll set, each including at least two rolls with peripheries engageable with said workpiece, journaled at each of said stations and disposed mirror-symmetrically with respect to the rolls of the other set; drive means for advancing said workpiece in said direction and thereby rotating said rolls; a respective differential transmission rotatably interconnecting the mirror-symmetrical rolls of said sets; a respective controlled-torque hydraulic drive motor operatively connected to each of said transmissions; and a variable-pressure source of hydraulic fluid including a variable-output hydraulic pump connected in parallel to said motors, each of said transmissions including: a respective shaft connected to a roll of each set, the shafts of each transmission being coaxial; a sun gear connected to one of said shafts; a planetary-gear carrier connected to the other of said shafts; planetary gears journaled on said carrier and meshing with said sun gear; and a ring gear meshing with said planetary gears and connected with the respective hydraulic motor, each of said stations being provided with a roll support, the rolls of said sets being journaled at said supports, said sets including mutually distal upper rolls and mutually proximal lower rolls in mirror-symmetrical relationship, said workpiece passing initially over the lower roll of one of said sets and thereafter around the upper roll of said one of said sets, the upper roll of the other of said sets and the lower roll of said other set.

3. A roll system for tensioning an elongated flexible workpiece, comprising: a pair of roll stations spaced apart in a direction of advance of said workpiece; a respective roll set, each including at least two rolls with peripheries engageable with said workpiece, journaled at each of said stations and disposed mirror-symmetrically with respect to the rolls of the other set; drive means for advancing said workpiece in said direction and thereby rotating said rolls; a respective differential transmission rotatably interconnecting the mirror-symmetrical rolls of said sets; a respective controlled-torque hydraulic drive motor operatively connected to each of said transmisSions; and a variable-pressure source of hydraulic fluid including a variable-output hydraulic pump connected in parallel to said motors, each of said transmissions including: a respective shaft connected to a roll of each set, the shafts of each transmission being coaxial; a sun gear connected to one of said shafts; a planetary-gear carrier connected to the other of said shafts; planetary gears journaled on said carrier and meshing with said sun gear; and a ring gear meshing with said planetary gears and connected with the respective hydraulic motor, said stations each being provided with a roll support, said rolls being journaled on said roll supports and including a set of mutually proximal upper rolls and a set of mutually distal lower rolls, each set being provided with a respective one of said transmissions, said workpiece passing initially over the upper roll of one of said sets and thereafter over the lower roll of said one of said sets, the lower roll of the other set and the upper roll of said other set.

5. A tensioning device for continuous strip comprising a pair of spaced apart roll stands each having at least a pair of rolls engaging the strip for tensioning said strip between said stands, said strip being slung around said rolls, the rolls of each stand being in mirror-symmetrical arrangement with respect to the rolls of the other stand, and respective independent variable drive transmissions interconnecting the mirror-symmetrical rolls of said stands and independent of any means for advancing the strip.