Generally in industrial applications the lead screw will be turned down at each end so that it can sit on a thrust bearing package. In my plant, we then drive them with a timing belt.
An easy way to do that is to get a shaft collar that fits over the threads and pin it in place. Then you don't have to machine down the end.
http://img.directindustry.com/images...84-4327425.jpg
http://gallery.nibou.com/albums/LINE...w_Supports.jpg
Your system is putting a huge amount of axial load into the motor shaft directly. They aren't made to support that type of load.