Invrtd(Clark),
I can't speak for other helicopters but for the E6/7, setup your helical pinion height correctly and you will not have an issue. I designed it to cancel out helical forces.
On the E6/7 the bottom of the pinion should be in contact with the lower shaft support bearing. If it is not set up this way the helical forces will pull down on the shaft.
If this is happening on a Trex700E you might get better answers concerning this topic in a Trex700E forum.
Thanks,
Matt