From an old Nick Lappos post in Rotorheads:
"Arm,
You ask a good question. Let me try and explain it this way:
Imagine that each minute at TO power extracts one bean from a big jar of beans. One minute, one bean. If you stay at low cruise power, say 65% power, then five hours takes one bean out of the jar.
There is nothing unsafe about running at TO power, it is not "beyond limits". But if there are 10,000 beans in the jar, then you only get so much life from your engine, transmission and drive train.
We manufacturers set a 5 minute limit per takeoff as a way to remind you of that. Inside the factory walls, we imagine that you make about 2 1/2 takeoffs per hour (don't try a 1/2 takeoff, it should only be done by professionals!). We calculate that if one does 2 1/2 TO's per hour, and the rest in cruise then the power systems will last 3707 hours, and you will have performed 9200 takeoffs.
If you make more TO's, plan to overhaul sooner, if you make less, your engine will last a lot longer.
The numbers above are made up, but the idea that a takeoff is several times more damaging than a cruise hour is entirely true.
No thundering limit, where 6 minutes at TO power will end your life. Just a faster draining of those beans from the jar."
http://www.pprune.org/rotorheads/178...ml#post1942676