SFX; Rotating around a unit?

I've created a sfx: several rocks that rotate around the center point (created a shape and made it into a bone). When I use it ingame, the effect acknowledges the units facing, turning, and by doing so, it turns, and changes rotation. I don't want this. I want it to simply rotate normally (and by this I mean, to keep the z constant, while the x and y changes). oh, and btw, I'm talking about 3ds max 5.

Is there some option that I've overlooked while making it?