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Here are the pictures of both pair of Y axis bearings. The drag I suspected wasn’t on the outside case edge but right in the middle of the shields. I most likely started this problem when I was experimenting with end thrust. I have replaced these but not tested the new bearings yet. I’ll do that this weekend.

I found a source for some 1/16″  thin solid thrust washers if I have to use them. They are available in several types of material and the center hole is very accurate.

One shield shown here is actually dented. I don’t know what caused that but it had to be a very hard stop or tapping on the bearing. This was also on the Y axis screw shaft that was factory supplied turned slightly undersized (loose rather than slip bearing fit.)

I also note the scraping is not 360 degrees on the bearings. That indicates some shaft/bearing misalignment. That is a big clue.

The lesson learned is to keep the preload as close to zero as possible. Mine were and are now set where I can turn the holding screws at their heads and the nylon lock nuts will turn if they are not held. I have had them a bit tighter when I first set up the machine, which I soon discovered is incorrect. A thin thrust washer may have prevented the digging in scrape. I’ll also make sure the end cover (pressure) plate holding screws are adjusted equally side to side (a part of bearing alignment).

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… Continue reading →

I received the new couplings today for the HB2 from PDJ, Inc.. The new one is the bright and shiny one in the picture. The black one is the original style that has been giving me problems with breakage. (6208K6 @ McMaster-Carr)

The new one is rated for 495.6 oz-in. The old one is rated for 42 in-pounds (peak). That is 672 oz-in! So the old one should be the strongest, right? The new one LOOKS stronger and is a different style. They are both considered Helical Beam Couplings. If you want to see what a broken one looks like check HERE.

So what gives? Well, the old couplings certainly do. Maybe the key word is the “peak” rating rather than a continuous rating. I can’t tell. The Mark One eyeball says the new one is stronger. That’s old Navy jargon for eyeball engineering.

I think I will soften the acceleration on the steppers a bit more to lessen the start stop shock loads.PDJ has been using these shiny couplings without failure so I hope my problems are over, at least with the couplings.