Quote:
Originally Posted by Clifford Pope
It will stop, but not instantaneously (remember Newton's Laws of motion?  )
The momentum of the spinning shaft will keep it turning until the effort of compressing the valve springs or hitting some of the valves overcomes it.
It would depend on the speed of rotation at the moment the belt broke. Belts normally break at low speed, because the belt is then stressed more by the slow hammering of the cam lobes. At high speed the varying stress happens so quickly it evens out, so a fast spinning shaft takes less power to turn than a slow one.
Obviously in practice any damage is largely a matter of chance. I've read of engines completely trashed through a breaking belt, but also of miraculous escapes where even an interference engine has escaped all damage just through the lucky position of the cam shaft. |
Don't underestimate the power of valve springs to stop a camshaft turning! Remember they are designed to return the valves to the close position at high speed! Therefore Newtons Laws of Motion do come into play but the opposite way to how you're suggesting!
The main reason belts break at low speed is because of having to overcome the inertia of changing speed from zero (it it's during starting) to idling or from idling to 1500-2000rpm to pull away.
It can also happen at the end of a long, fast run where the change from being on throttle to being off throttle causes many dynamic changes within the engine.