A Lesson in Thermal Expansion

The “Fixed Spindle Reel” is my first design with a bushing that runs the length of the spool. I make the bushing of either Delrin or acetal copolymer, and it rides on a spindle of stainless steel.

The fit of the bushing length to spindle length has to allow a little end play. The drawings that I produce for the reel do not have tolerances on the dimensions, but I like for the assembly to show an end play of no more than .005 inch. Some that I produced were less than .005, but they turned freely when on my work bench this past winter (ambient temperature 68 F).

On the first warm day this spring, a customer (who is also a neighbor) noticed that his reel spool was dragging more than the ratchet-pawl would cause. I shortened the bushing by about .003 inch and the reel has been OK on the warm days since.

The problem is thermal expansion. Delrin has a thermal coefficient of expansion of .000120 per degree C, while stainless steel is only .000017 per degree C. For the 1.2 inch long bushing and a temperature rise of 20 deg C, the bushing grows by .0025 inch relative to the spindle.

So far I have repaired 3 reels that had room temperature end play of .002 inch or less. It is easy to measure the end play by pushing a feeler gage under the head of the front screw.
IMG_4063
If any other reel owners have this problem, please contact me for repair.

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This entry was posted in My Reels, Repair, Spindle. Bookmark the permalink.

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