What material coupler did you use? Aluminum helical couplers are total crap, and wear out rather quickly if used out of spec. The ones I have on the way are stainless, and rated far beyond what I'll need for torque. I worked in a manufacturing plant where we used these for a number of different operations - some with good results, and others with horrid results. The stainless couplers are indeed not cheap, but it's a lot easier to spend the money once than to worry about grinding your bearings to bits over misalignment that you can't even effectively measure. They're plenty stiff, and enjoy a fatigue limit that aluminum can't achieve.