Pawl Wear Test, Redone

Refer to earlier posts titled “Results of Pawl Wear Test” (Aug 2010) and “The gearmotor that wasn’t a gearmotor” (Sept 2010).

After discovering a possible misoperation of my first pawl test fixture, I built a new tester using a real gearmotor. This gearmotor has a shaded pole ac motor and a 3 stage geared reducer; one stage helical and two stages straight tooth.

This is a view of the new tester:

The new tester runs at 20 rpm (vs. 25 rpm for the first one), so I extended the test time to 7 days (vs. 6) to partly compensate.

The wear results are not dramatically different from those obtained in the first test, suggesting that the faux gearmotor did operate continuously while not being watched. Measured wear is, if anything, less in the second test. Perhaps the intermittant, ratcheting motion of the first tester was a factor.

The following table summarizes both tests.

Before trying plastic pawls, I tried pawls of aluminum, brass, and stainless (303). All of these produced rapid wear (of the ratchet, in the case of stainless). Any of the plastics is better than any of these metals. My bronze ratchet is alloy 642. It doesn’t appear to have worn as a result of testing plastic pawls.

I believe that low coefficient of friction is desireable in ratchet-pawl contact. Turcite X did less well than Delrin, but some other property must have come into play.

Update 8 Dec 2010:  When I previously tested a 303 stainless pawl, it was with a 360 brass ratchet, and this caused wear of the brass ratchet in a short time.  I just finished running a 303 pawl on the 642 bronze ratchet, 1 week, and there was no observable wear.  I think that the difference is Rockwell B hardness of 80 for 360, vs. 90 for 642.  For the present reel design, I will settle on a Delrin pawl, because the metal pawl is too loud.

Update 13 Oct 2011: It has just come to my attention that the material that I have called “Delrin” is actually acetal copolymer. There is a minor difference, read here.
The “Delrin AF” is this material from McMaster.

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