Re: Out with Clutch Dogs: In with Ball Bearings in the OMC 6 HP

[Don]

quote Fisherman6:

Don,
You are correct in your thinking about the difference in friction coefficient between the gear and the thrust washer vs. the gearcase and the thrust washer. I’m assuming the thrust washer is phosphor bronze. I realize the gearcase is in an oil bath and the numbers I’m going to give are dry, but since the lubricant is so subjective, that seemsounds like the best for the sake of comparison. The friction coefficient between aluminum and steel is 0.61. The friction coefficient between phosphor bronze and steel is 0.35. I couldn’t readily find any references for phosphor bronze and aluminum, but being that the aluminum is a much softer and gummier material, it will certainly be higher. Theoretically the thrust washer would not rotate in this application, but… we all know theories don’t always work in the real world. If one could find a bronze thrust washer with a diamond knurled on one face and place the knurled face against the aluminum gearcase, it would certainly bite into the gearcase casting and prevent further rotation with a firm shift into reverse.

Bearing materials are a bit outside of my everyday engineering expertise. I am an industrial machine and automation design engineer so I do certainly have some familiarity with them and their applications.
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As far as how they could have thought this gearcase would survive. I also understand how things get designed to do a certain job in a certain way. Inevitably the end user will find a way to use the product in the real world in a way that it was not necessarily designed to be used. I could be off base in my thinking here, but it seems likely to me the this gearcase was designed for a fishing motor that spends nearly 100% of its time in forward gear. Reverse would be used only momentarily mostly when leaving or returning to the dock. For that purpose, it seems OK if not optimal. I’m guessing the heavy use of reverse in sailboat use and possibility of back trolling wasn’t given enough consideration in the design phase. I bet they realized like they had a problem in a lot less time than it took them to fix it. There is a lot of politics and accounting involved in basically scrapping a new design and replacing it with the previous one.

Whew! I may have gotten a little windy on that one myself. Everything below the line is just my theory based on what I know about engineering for industry.
-Ben

Ben-
Yeah, OMC was just like any other company, cost was the only thing that trumped politics. I’m thinking you are correct, the engineers felt the design was "good enough" for most uses, but then they had no business marketing "sailmaster models" with high thrust props and extra exhaust reliefs. I could forgive this if after seeing higher than acceptable failure rates, they added some sort of bronze type thrust washer that could be retro-fitted to older models. In this case the politics probably kicked in thinking these small engines were not worth the engineering effort diverted from larger models along with the cost. This is a shame because these engines run very nicely and develop quite a bit more power than the conventional 6hp models they replaced, but I’m sure many customers went running to the competition after being socked with gearcase repair bills and driveshaft spline problems.