If you keep the search to engine rather than drive chain and transmission, from previous research only three documented cases I can find, worldwide.
*SA CAA ref 7819 13 June 2004 "The engine failure was as the result of the number 6 piston pin plug failing."
*German Federal Bureau of accident investigatins. Sept 04 2001 ref 3X241-0/01 engine damage evident on spark plugs.
*Canadian CAA ref A04P0422. Engine vibration causing V belt to fail.
More reading here;
http://www.kreindler.com/kreindler_n...rrent/2006-05-
Legal_Victory_Robinson.htm
Numerous cases of "total loss of power" where no mechanical cause could be found. In many cases post fire damage making conlusive investigation impossible.
A search for any failed mechanical elements to do with the search term "failure to maintain lift" would be a good comparitor across helicopter types?
Mickjoebill