Some background for this post is in a discussion on Rod and Reel Maker’s Forum.
The discussion made me think of using plastic for the pawl. (In my reel design, replacing a worn pawl takes only a screwdriver, for backplate removal.) Also, about the same time, I had a meeting with Michael Hackney wherein he suggested that pawls may benefit more from low coefficient of friction than from hardness. I decided to try a harder ratchet and to conduct tests to compare different pawl materials under controlled conditions.
Two things to decide are how long to run the test and how to measure pawl wear. I needed a few preliminary tests to help decide these issues.
I made the first test pawl from 6061 aluminum. After running only a few hours, the pawl was no longer effective. It was notched on its flanks (from striking ratchet tooth tips) but was also shorter than its starting length, preventing proper engagement with the ratchet. This suggests using pawl length as a measure of wear. The flank notches are obvious, but I could not determine a way to measure them.
My second test pawl was plain, natural Delrin. I ran it for 2 weeks and it was still going strong, with no obvious length change. This was encouraging, but it did not help me to decide a practical duration for the test.
The third pawl was Ultem 1000 (a harder plastic). I would have tried PEEK or Torlon, except for cost. At six days, the sound emitted from the test fixture had obviously changed. I stopped the test and measured pawl length (this time I had remembered to measure at the start of the test). It had lost 0.045 inch in six days, and so was near wear-out (whole depth of the ratchet teeth is 0.060 inch). My test duration would henceforth be six days.
I ended up testing 6 plastics, including a re-do of natural Delrin at 6 days. Here is a chart that shows the materials and the measured wear, in terms of pawl length change. I have also given some data on mechanical properties, but these are from a variety of sources and may not be consistent.
(Sept. 6, 2010 : A fault has been discovered with the gearmotor used for these tests. Read further on this matter in my post of today. The wear data in the table below cannot be trusted.)
(Nov. 16, 2010 : I have now completed a second set of pawl wear tests, using a real gearmotor. The results are not greatly different. See the new post of this date.)
So why did natural Delrin do better than black? What is the most important property: strength, hardness, or coefficient of friction? It is difficult to draw a general conclusion.
If anyone would like to compare another material, please provide me with a material sample (a disc 0.125 thick and 0.75 diameter would work) and I will make the pawl and run under the same conditions.
I would like to use the fixture to study plastic ratchets with metal pawls, but don’t know how wear could be measured.