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 gauge under the head of the front screw.

If any other reel owners have this problem, please contact me for repair.

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

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