Oh No! Again

Broke a coupling again, one of the new ones. So the new coupling isn’t any stronger than my original design. This time it was in the X axis where there has never been a problem. I replaced a perfectly good original style coupling with the new one and the trouble began.

This kind of “random error” coupling failure has been driving me crazy, especially now in a place where it has never been an issue. It is a good example of how powerful the stepper motors are at low speeds. They can break things well above their holding torque ratings.

What I think happened this time is I may have changed the preload on the X drive screw end bearings when I replaced the coupling.  Those bearings are straight radial bearings not designed to take end thrust loads. After I replaced the new failed coupling, I could hear a slight binding (groaning noise) near the middle of the axis nut travel. I spent hours fine tuning the X axis and double checking every alignment. The problem was I could not tell what component was generating the sound. The steppers themselves make a buzz type noise when rotated, even when not powered.

Finally I backed off on the axis bearing cap to as loose as I could make it without end play. I would say as near zero end load as can be set. The groan went away and the X axis started running extremely smooth with no complaints of a binding noise.

Now I am thinking my oversize screws may drag on the case or outside rim of the bearings when under any type of end loading. The original design was to use 1/2 inch screws. I am using 5/8 inch on X and Y. There could be some length growth in the screw with use that contributes to the drag. I would think the end load shouldn’t be too critical.

I am going to look for some 1/4in x 3/8 thin flat washers to place on the ends of the screws just inside the bearings and see if that will keep the screws from impinging on the bearing case. One other option is I may have damaged the bearings causing them to rumble under thrust or pre-load. I may need to just replace them.

Update 3/23/10  Precision washers (center hole) are hard to find and buy in small quantities. I don’t want them flopping around loose on the shaft ends. I should be able to make a pair on my lathe (two washers) and try them on one axis to see if this really solves the problem. Then make four more if it works! New bearings (not expensive) are on order.