teeth 22 and 23 respectively which engage as shown in fig2 and 4 such that when a driving arm is rotated anti clockwise the associated driven arm rotates clockwise and vice versa thus a locked hub may be unlocked by a limited clockwise rotation of the rotor sleeve relative to the hub body and the springs 20 will assist the unlocking once they have passed the over centre position conversely an unlocked hub may be locked to a tape reel by a limited anti clockwise rotation of the rotor sleeve relative to the body and the springs will assist the locking when past the over centre position the friction pads are of a resilient material such as a synthetic elastomer and friction pads of a fluoroelastomer obtainable from du pont under the trade name 34 viton 34 having a thickness of 1 6 mm have been found to be particularly effective the gear teeth engagements 18 21 and 22 23 are each designed to have a controlled backlash of substantially 2 5 u00b0 this backlash permits a small independent rotation of each arm relative to the remaining arms and to the rotor sleeve as the friction pads approach the inner cylindrical surface of the tape reel during a locking operating and the resilience of the springs and of the friction pads ensures the smooth engagement of all six pads if there were no backlash the six arms would be tightly linked to one another and one or more of the friction pads may not fully.