What causes lower unit failure?

Hi there,
I'm trying to figure out what generally causes lower unit failure with the goal of running a lower unit at higher than rated RPM. I get the feeling that the internal bearings are probably rated for much higher than the operational RPM, and the issue is exceeding torque limits (not really an issue due to the low drag of the vessel) or shock loading (definitely possible). I'm curious if anyone can comment on common causes of failure and how likely they are and how to mitigate them.

Chronic causes:

• Gear wear due to age/use
• Bearing failure due to age/use

Acute causes:

• Overspeed (probably not really an issue because powerheads typically cut spark over max RPM)
• Shock loading due to ventilation/breaching the surface
• Oil foaming? (How exactly does lubrication on a lower unit work? When you fill the oil until it comes out of the top hole, is the cavity 100% filled? 90%? Is using a fresh marine rated oil with anti-foaming properties enough to prevent all lubrication related failures?)

 

How are you going to overspeed the rated RPM?

The short answer to your question is: fatigue.

 

Fatigue, defects, wear and tear , and skipping required maintenance, would be my guesses. Not sure anything will completely mitigate, but use of higher rated quality units than needed, may postpone it.

 

I see a lot of lower unit failures due to water intrusion. It’s not weakness of gears and bearings, but lack of lubrication. Fishing line, even seagrass can open the seals to water intrusion.

 

Most catastrophic failure from impact of some kind, progressive failure from a myriad of seal failures. (Electrolysis, seal west, foreign object).

What are you overspeeding that can't be better serves with proper gear and prop pitch selection.

 

I'm converting a small gas outboard to high speed electric. Yes, a proper gear ratio would do it, but the outboards in the size range I need usually have pretty dumpy gear ratios (2 or 2.5 to 1) since they are usually designed around pushing a dinghy or sailboat. I picked up two of these 5hp johnson/evinrudes that have a 2.44 to 1 ratio lower for basically nothing, and I'm not opposed to blowing up one for the sake of testing.

Propeller selection is quite limited -- my options are 7 or 9 inch -- and I'd need something around 22 inches in pitch. I am working on designing a custom prop, which will be my first choice, but I was curious about the overspeed possibility in case that avenue fails.

 

22 inches of pitch, that must be some spectacular input torque.

 

You could rebuild the lower unit with the least internal wear, with new seals, and good quality bearings, (not from the land of instant land fill), and use high speed synthetic oil or mineral oil. High speeds cause shear in cheap oils, allowing more metal to metal contact. Or could you run a small pulley on the motor, and a larger pulley on the leg, a cheap simple ratio change, if belt / pulley side thrust loading is accounted for.

 

More info trickles in...
So, the most common reason you're going to get a gear failure now is from over torque and over RPM.
But, if you have such low hull resistance ( that's still a mystery ) you may not encounter over torque if you go easy on the throttle.
Over speeding RPM may not be such an issue if it's low torque by the time you get to high RPM.

 

Yep, I'd start with gear tooth design. Gears are designed with a specific torque to rpm profile. Get too far off that and you will have all sorts of issues.

 

A large percentage of the failures are from oil seals damage or deterioration.

 

Some guys use ATF for high rpm .
ATF fluid in lower unit ? https://www.screamandfly.com/showthread.php?135618-ATF-fluid-in-lower-unit

 

Since you have two units, try atf in the worst condition one first, and see how long it lasts, then you still have the option of using better oil in the second one. See what types are used in heavy transmissions, drag cars, and what the militarys use in their equipment, then choose. Since legs have cooling by default, I think a full synthetic oil would perform best, but unsure how they go after some salt water ingress messing with the chemistry.

 

Racing applications don't translate directly if you want something to have a long life in usual use. Drag cars only run seconds at a time; so do drag boats.

 

What exactly does "high speed" mean? These outboards run 5000rpm engines, so unless you plan to run the electric motor at a constant 10,000 rpm the gearbox should be fine. If you do plan on such high rpm the transmission ratio is to high, it will give you 4098rpm at the prop.