Busted Merc D Quickie Gear

[sam]

What do you think caused this failure?

[sam]

The prop shaft gear only has one chipped tooth.

Anyone else seen one explode like this?

How often?

[kws]

is the wheel banged up?

[kws]

if i had to make a bet and the prop isnt ruined i would say metal fatigue

[sam]

Nope, I was in deep water the whole time, no debris. The prop still looks like new, the shear pin looked normal (resuable, but a little bow from being tightened, it dropped out with out being pushed when I loosened the prop nut).


Metal fatigue is my impression; that the gear just got hard from being old.

It broke right when I squeezed the throttle wide open the first time in milling.

[sam]

The bolt and end of drive shaft look fine.

If it had been used a lot, I think the teeth would have been worn down more. Except for the busted teeth, the shape of the teeth on both gears look pretty good. A wear pattern, but by no means worn out.

[Ron Hill]

The "USED TOO MUCH WHEN IT WAS"...

That quote is like a family joke, which means.....we have no idea why it broke, it just quit running...


Another family quote, "She was running when I SHUT HER OFF." Which is translated to mean, "YOU BUSTED IT, NOT ME."

Are you running that "D" Quickie on a 75-H???? If you are, the 75 may just have too much torque for the gears to handle.

When I ran Harry Bartolomie's 75-H with a Konig 15:16 unit, Harry would always remind me to get on plane easy and to get of the throttle easy out of the corners.. The Konig gearcase would take HIGH RPM but would not take torque...as Harry had broken several, I never broke one...

Same thing is true today: Guys take a Bravo drive that is designed for maybe 500 HP, they add a "HUFFER" (Blower) and now their engine has a thousand horsepower..... they idle out of the five mile an hour zone, and throw the hammer down, two blocks later, they have a BLOWN BRAVO DRIVE....I could run a Bravo drive with a blower all summer with no problem, but I would bring the power on slowly...

When you've runined as much stuff as I have, you start asking yourself why??? When I raced for Evinrude, I always told them there was nothing they made that I couldn't break, if I wanted to!!!!

Percentage wise, I finished more marathons than most people (Fishished second three years in a row at Parker--27 race hours)...sometimes your equipment needs to be treated like a LADY.....gently!!!!

ADD: I would jump on the throttle faster with Harry's 75-H on the hydro than the runabout...two in the boat, big mother 13' 6" DeSilva.....had to get on it easy... if for no other reason, I might go on my ASS!!!

[trader]

Sam, It's difficuilt to tell from the pic's. But the break on the bottom left, was probably last, because all I see is crystal. If any of the areas of the break's are shiney, and peened, it may have had a hairline crack due to some previous stress. What happens is they rub or beat against each other, (much like bending a wire back and forth)(opening and closing) until they break. The rubbing or beating against each side is what peens or flattens the grain structure, and identifies an old stress crack, and where it began. A texured clean crystal surface determines it was a sudden fracture/break. Sometimes a stress crack can be stable for million's of cycles, and suddenly it has a San Andreas moment...trader...

[sam]

I looked for that right away. All surfaces show crystalized metal grain, no peening evident. Wango, bango all at once, I guess. The boat was not hopping or jumping and I was at about 55mph (1/3 or 1/4 throttle) when I squeezed the throttle to go thru the bridge and back around (I think there was about 40 seconds on the clock).

[nbass]

When we began designing gears for the Bass Machines lower unit we discovered a number of potential failure modes. All of the above comments have merrit, but I will add my suspecions. It looks like, and I believe that these gears are, "thru-hardened" rather than ones that have a ductile core with hardened surfaces. Thru-hardened gears will take a good load with little wear, but will sudenly fail if there is an overload, material flaw, or stress crack. Additionally, if a "chunk" breaks of one of the teeth and lodges in the root of one of the gears, the splitting forces will break the gear simular to what you show. The subject is interesting and we can all learn from these failures, unfortunately at your expense.
Good Luck
Neil Bass