Irregular Verbiage
from the desk of Colin Nicholls

The Daily Grind, part 2

January 29, 2012 13:05 by colin

The big red coffee grinder failed again. This time, the motor just raced while the burr refused to rotate. Clearly the rotating burr had come loose from the shaft of the motor. 

Last time I disassembled the mechanism, I didn't take it far enough to see how the shear pin locks the burr to the shaft, but now I had no choice. 

Yup, the shear pin was snapped into three pieces, two of them knocking about in the base of the grinder, the third piece solidly jammed in the hole in the shaft.

Shear pins are supposed to fail before the motor does, when something jams the shaft (like, say, a stone in the beans makes its way into the grinder). If they don't fail, the motor can burn out.

After I'd repaired the grinder the first time, I went on ebay and purchased a black unit that was advertised as "non-functional parts only", which was fine with me. I verified that the motor had indeed burnt out, and then ended up using parts of it to replace bits in our grinder.

So I had high hopes that the shear pin in the ebay-sourced unit would be intact and that I would be able to use it to replace the broken one in ours. No such luck:

Here you can see the motor, shaft, and bottom burr holder from the black unit, along with the broken shear pin from our original red unit, and the replacement pins I obtained via another ebay purchase.

It's tricky to see in the image above, but the shaft of the black unit is damaged from the failed pin:

I suspect that the pin fragment jammed the motor and that's why it burned out.

After obtaining replacement pins from a seller on ebay, I knocked out the remaining fragment from the shaft with a nail, and then tried inserting the new pin. Man - those things are tight! I tried using a hammer, but with limited success. This thing really needs to be pressed into place in a vise or similar device.

The problem is, you need to apply symmetric pressure on both sides ofthe hole in the shaft, or you can very easily bend the shaft - at which point, you've got an unusable component.

What's really needed is a jig of some kind. Something like this would have been perfect:

Unfortunately, I was impatient and I bent the shaft slightly before I knew what was happening. Dang! I managed to straighten it up most of the way, and tests show that it does not seem to impare rotation - that was lucky.

I ended up using the vise to force the pin into place, using a socket from my wrench set to support the shaft on the opposite side. 

Re-assembling the unit was not difficult, and now it is back grinding coffee for us. Hooray!


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