A robotic surgical instrument comprising a shaft, an articulation attached to the distal end of the shaft, and a driving mechanism at the proximal end of the shaft. The articulation comprises a plurality of joints for articulating an end effector, the plurality of joints driveable by at least first and second pairs of driving elements. The driving mechanism comprises a pulley arrangement and a pulley drive. The pulley arrangement comprises first and second pulleys attached together such that their separation is fixed, wherein the first pair of driving elements is constrained to move around the first pulley, and the second pair of driving elements is constrained to move around the second pulley. The pulley drive is configured to linearly displace the pulley arrangement, such that a linear displacement in one direction away from the distal end of the shaft causes the first pair of driving elements to be tensioned and the second pair of driving elements to be de-tensioned, and a linear displacement in an opposing direction towards the distal end of the shaft causes the second pair of driving elements to be tensioned and the first pair of driving elements to be de-tensioned.
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