the barrel provides approximately constant torque over the watch's designed running period; the torque doesn't decline until the mainspring has almost run down. The built-in tension of the spring in the going barrel makes it hazardous to disassemble even when not wound up. Broken mainsprings Because they are subjected to constant stress cycles, up until the 1960s mainsprings generally broke from metal fatigue long before other parts of the timepiece. They were considered expendable items. This often happened at the end of the winding process, when the spring is wound as tightly as possible around the arbor, with no space between the coils. When manually winding, it is easy to reach this point unexpectedly and put excessive pressure on the spring. Another cause was temperature changes. If a watch was fully wound in the evening and the temperature dropped at night, without any slack between the coils the thermal contraction of the long spring could break it loose from its attachments at one end. In earlier times, watch repairers noted that changes in the weather brought in a rash of watches with broken mainsprings. Broken mainsprings were the largest cause of watch repairs until the 1960s. Since then, the improvements in spring metallurgy mentioned above have made broken mainsprings rare. 'Knocking' or 'banking' Even if the mainsprings were not prone to breakage, too much force during winding caused another problem in early watches, called 'knocking' or 'banking'. If very little slack was left in the spring after winding ('overwinding"), the pressure of the last turn of the winding knob put the end of the spring under excessive tension, which was locked in by the last click of the ratchet. So the watch ran with excessive drive force for several hours, until the extra tension in the end of the spring was relieved. This made the balance wheel rotate too far in each direction, causing the impulse pin on the wheel to knock against the back of the fork horns. This caused the watch to gain time, and could break the impulse pin. In older watches this was prevented with 'stopwork'. In modern watches this is prevented by designing the 'click' with some 'recoil' (backlash), to allow the arbor to rotate backward after winding by about two ratchet teeth, enough to remove excess tension. Motor or safety barrel Around 1900, when broken watchsprings were more of a problem, some pocketwatches used