I prefix this with I stand to be corrected by engineers and design bureaus from OEMs, but this is my understanding and have seen the visual evidence of such.....
As it was explained to me once upon a time, (and I hold this explanation true for many, if not all helicopter transmissions), the primary reason for the 85%, (other than the mast bending moments with ->IAS), is due to increased potential for damage to sun/planet/ring gears within the Tx.
As we all know, the Tx in most helos consist of a mix of sun/planet/ring gears conducting a massive rpm reduction and power transfer function. As >85% tq is delivered to the Tx to transfer to the Rotor system, the sun/planet gears rotate furiously to absorb, reduce, and transfer this power to the mast/rotor drive.
In doing this, the gears spin at a higher rpm and increased Tq. This creates physical movement (small, but significant) of the gear teeth intermeshing contact areas. Normal gearing and load transfer is calculated on a certain percentage of teeth contact area under normal operating power ranges. As RPM increases, this percentage meshing or contact area can significantly reduce. This results in an increasingly smaller contact area having to transfer a much greater load to the next reduction gear via the teeth.
If this goes on for any extended time (5min in this case), the teeth will become very hot. Particularly at the teeth tips or reduced contact area. The evidence of this can be seen by visual inspection and a localised burnishing will be seen. You may get away with this for one, two maybe 3 or 5 times....who knows...before the metal changes its characteristics, becomes more brittle (due excessive localised heating) and the tips of the teeth or the whole tooth/teeth could develop fatigue cracking and ....hey presto, chips if you are lucky, Tx failure if you are not.
I believe the REASON behind limits should be taught to every pilot so that they can better understand ....and abide by the intent of the limit rather than simply deciding to blow it off by making up an arbitrary fairy tale about their own private ratioanle why they thin the limit exits. Or as is often the case, thinking "got away with it once before without any oil leaks", without understanding the potential problem they could be passing on to the next poor shmuck who has fly his abused machinery.
It is another reason I am STRONGLY in favour of retro fitting monitoring systems such as ALtair/Shaddin etc to record EXACTLY what that poor little abused Tx has seen. If manufacturers will accept the reports from these exceedance monitors and give "maintenance credits" back to the operators with the balls to invest in such equipment especially if they can call the OEM and say, "hey we just have an exceedance of 5min 10 sec recorded on electronic monitoring with no other history of exceedances....if the OEM were able to say....check chip plugs, drain and change oil...maybe send sample for full wear debris analysis and if all OK...return to service...maybe with another chip check in 25 hrs.....saves the operator money and validates his investment in electronic monitoring,....makes us all a whole lot safer and affordable
I'll standby for incoming on this one.....