a variation of the going barrel called the motor barrel or safety barrel. Mainsprings usually broke at their attachment to the arbor, where bending stresses are greatest. When the mainspring broke, the outer part recoiled and the momentum spun the barrel in the reverse direction. This applied great force to the delicate wheel train and escapement, often breaking pivots and jewels. In the motor barrel, the functions of the arbor and barrel were reversed from the going barrel. The mainspring was wound by the barrel, and turned the arbor to drive the wheel train. Thus if the mainspring broke, the destructive recoil of the barrel would be applied not to the wheel train but to the winding mechanism, which was robust enough to take it. Safety pinion A safety pinion was an alternate means of protection, used with the going barrel. In this, the center wheel pinion, which the barrel gear engages, was attached to its shaft with a reverse screw thread. If the spring broke, the reverse recoil of the barrel, instead of being passed on to the gear train, would simply unscrew the pinion. The myth of 'overwinding' Watches and clocks are often found stopped with the mainspring fully wound, which led to a myth that winding a spring-driven timepiece all the way up damages it. Several problems can cause this type of breakdown, but it is never due to "overwinding", as timepieces are designed to handle being wound up all the way. One cause of “overwinding” is dirt. Watch movements require regular cleaning and lubrication, and the normal result of neglecting to get a watch cleaned is a watch stopped at full wind. As the watch movement collects dirt and the oil dries up, friction increases, so that the mainspring doesn't have the force to turn the watch at the end of its normal running period, and it stops prematurely. If the owner continues to wind and use the watch without servicing, eventually the friction force reaches the 'flat' part of the torque curve, and quickly a point is reached where the mainspring doesn't have the force to run the watch even at full wind, so the watch stops with the mainspring fully wound. The watch needs service, but the problem is caused by a dirty movement or other defect, not "overwinding". Another common cause of a watch stopped at full wind is that if a watch