Busting/mushrooming Hubs

[richale]

Ive got a 28 ft twin 302 sterndrive i/o, 78 vintage fiberform. My problem has been that the starboard outdrive keeps mushrooming the hub. The drive is electric shift. The drive is pin drive. This has happened at low rpms(2000) usually after about 35 miles of travel even though it was at 8 knots. Now I did reduce the rpms at idle ( 650) instead of 850. Ive been very carefull to not shift to quick and have got to the san juans and back, 120miles. But I feel maybe my lucks running out. Heres the ? Has anyone had this problem and am I on the right track or just lucky till next time? Thanks What I mean by mushrooming is that the pin that is through the 2 ears at the end of the hob are being busted letting the pin spin and hust mushroom the end of the hub if that helps.

 

[Texasmark]

Re: Busting/mushrooming Hubs

Not the expert in your arena. Just applying physics to the problem.

The lower rpm probably helps rather than hurts your situation.

What kind of hub do you have on your props? Are they solid metal, or do you have a rubber "slip" hub. The rubber slip hub helps to cushion the transition from idle to engagement.

I assume the pins on the two outdrives are identical in material and size.

The pin (known around here as a shear pin....assume that's what you are talking about) is designed to lock the prop to the hub if you don't have a splined system with a locking nut. You can go to the old_omc.de site and look at some OMC (Johnson-Evinrude outboard) advertisements back in the 50's-60's timeframe where the rubber cusioned hub was introduced and read the why they did it. Course in present day engines, the spline has become the method of choice for reasons such as you are (I think you are) experiencing.

Other thing is that the pin is in shear. Metals have a tremendous tolerance for pressure in shear, but you can't cut them (score them) or you loose a lot of that ability.

Sharp edges on the propshaft or prop can score the pin and cause it to yield prematurely. You don't want them round, but you don't want sharp edges protruding either.

Last thing is that there should be a very slight slop in the interface, not much, but just enough to be able to determine that nothing is binding.

Best I can do from here.

Mark