Corrosion in a Driven Gear

[EngDesServices]

Having the ability to understand and perform hands-on work is something I think helps make engineers better at their job. I re-purposed the old driven gear from my outboard engine as a talking piece about this point and keep it on my desk to remind me that even small design decisions can lead to big problems.

My theory is the failure of a $5 seal, when combined with a not easily inspected location, was what resulted in a total engine failure while 15 miles offshore.

When the seal failed, saltwater was able to make its way into the splines mating the driveshaft of the lower unit to the driven gear. The splines are very shallow and the corrosion of them allowed the shaft to slip, no longer transferring power to the driveshaft, prop-shaft and therefore the propeller.

Replacement of this component required removing the entire powerhead and 8ish hours of labor.

What design ideas do you have for what could have been done to prevent a failure like this?

Michael Hall, PE (TX) PMP - President
Engineering Design Services LLC

http://www.engdess.com

 

[hydtools]

What did the manufacturer reccomend for routine inspection and maintenance? Was that recommendation followed before experiencing the failure?

Ted

 

[MJCronin]

Was this the best seal choice for offshore service ???

Would some kind of double seal be better ??/

Was the entire unit certified for saltwater splashed service ????

MJCronin
Sr. Process Engineer

 

[Tmoose]

What make and model of Outboard.

From the diagram my first guess is all the components shown are well above the water line.

 

[IceStationZebra]

My first thought is pack it full of grease to keep the water out, but where would it go next? Would have to better understand the system and how it goes together. And reliance on inspection is not a good solution because most people won't do it.

I'll put on my reliability hat and say that this is what failure modes and effects analysis (FMEAs) are for. Designers moan about having to do them, but we have found something significant on every one I've done. Completed one right before Christmas and the designer was surprised at the number of failure modes and effects he hadn't considered for a "simple field update kit" that only involved a circuit breaker and three wires.

ISZ

 

[EngDesServices]

Sorry for the late replies!

"What did the manufacturer reccomend for routine inspection and maintenance? Was that recommendation followed before experiencing the failure?"

There is no routine inspection and maintenance on this component as it is located within the oil pan on the bottom of the powerhead.

"Was this the best seal choice for offshore service ???

Would some kind of double seal be better ??/

Was the entire unit certified for saltwater splashed service ????"

Yes, this is a component from a Suzuki DF300 which is designed for salt water service.

"What make and model of Outboard.

From the diagram my first guess is all the components shown are well above the water line."

2010 Suzuki DF300. All components are above the waterline.

"My first thought is pack it full of grease to keep the water out, but where would it go next? Would have to better understand the system and how it goes together. And reliance on inspection is not a good solution because most people won't do it."

This is actually what Suzuki did to remedy the problem. They added a zerk fitting in 10-39 in the figure and added a maintenance step to grease the connection.



What I found most surprising is that the driveshaft splines were almost completely unaffected while the driven gear splines were totally destroyed. I find it much easier to inspect the driveshaft splines when removing the lower unit as opposed to inspecting the splines of something located in the oil pan. Perhaps future models will use a different material for the driven gear so that the driveshaft becomes the first-to-fail component.



Michael Hall, PE (TX) PMP - President
Engineering Design Services LLC

http://www.engdess.com

 

[BananaMana]

Hi Michael,

I'm not to surprised the gear failed before the driveshaft. Seems like the closer to the water (lower) the more important corrosion risk is. So when saltwater gets up much higher than designed, corrosion would start at the top most component, the lowest, most prone parts are best protected (via the anode etc).