if you dont mind me asking, who did your imco sc lowers?

Thanks for the info.

I skipped a few replies so I hope I'm not repeating ground already covered.

Let's talk about straight-cut gears. They are easier to visualize. The principles are the same as on helical gears and bevel gears (or helical bevel gears..) but let's stick to something we can see better in our mind.
Gears transmit power through a contact line to another gear. That "line" is theoretically 0.001" wide and is as long as the width of the gear. Depending on the pitch diameter of the gear, you can have several teeth in contact at the same time, multiplying the number of "lines" you have. The smaller the pitch diameter, the lesser number of teeth that can be in contact at the same time.

If the gears are machined absolutely perfectly (which is impossible in the real world), then the contact lines mesh perfectly as the gear turns. This also assumes there is ZERO change in the shaft centerlines as well (which does change slightly with increased loading). This contact line scrapes across the gear tooth faces and wears away the gears as they are used. The more load, the more wear. Add to that a small amount of case distortion under load which translates into slightly greater shaft centerline distances, and you lose the "help" of the other gear teeth, focusing a greater portion of the load onto the ONE primary contact line. This accelerates the gear wear as well.

So why don't all gears die a horrible death in 3 minutes?
Lubrication.

The lubrication is designed to "live" between the gear faces and buffer the contact line. The oil film increases the "effective width" of the contact line from 0.001" to probably .005" which is helpful, but as long as the thickness of the oil film is "greater than zero" it completely ELIMINATES gear wear.

Operating within a set of parameters that does not cause the film strength of the oil film to break down and allow gears to actually TOUCH results in those gears lasting virtually forever.

So optimally, here is a gear's job.
1) Provide as high of a combined sum of contact line area as possible to allow the oil to work.
2) Provide enough tooth strength to prevent permanent tooth deformation under load.
3) Provide enough flexibility to allow the teeth to temporarily deform under load to buffer tooth impact and load cycling.

A gear made of something insane like tungsten carbide will indeed be strong enough, but when it's lube strength is exceeded, two carbide gears working against each other will chew each other up just like two cast iron gears will. Those ultra strong (unflexible) gears will also transmit harmonics and vibration into the shafts and bearings, shortening bearing life dramatically.

The BEST way to get gear life is to give the oil more tooth area to work with.
How do you do that?

1) Increase the width of the gears.
2) Increase the diameter of the gears.
3) Increase the number of teeth on a given diameter (higher diametral pitch).
4) Increase the number of gears sharing the same load.

Number 3 is a double edged sword. Smaller gear teeth means that you have to control the shaft centerlines better. The amount of case flex and bearing distortion that is acceptable with a lower diametral pitch (larger gear teeth) is no longer acceptable with higher diametral pitch (smaller gear teeth). Therefore, shaft sizes must be larger, bearings must be upgraded, and case material must be thickened or changed to something less flexible. Obviously the aluminum castings we use in outdrives have a limiting factor on how well a smaller gear tooth will work - bottom line is that we're pretty much at the correct tooth size already..

Look at a BMAX.
Larger upper gears - huge massive gears. The same torque that kills an XR will live happily on those larger gears. The lowers, however, use pretty standard gears and set the limit for the torque.

Look at a Six.
Huge upper gears.
DOUBLE vertical shafts.
This shares the load between two complete transmission devices and allows a small gearcase bullet while doubling the torque capacity.

Lets talk seriously now.
Magic gears can indeed give you "redline" protection. In other words, when you DO choose to operate the outdrive in excess of what it's lubrication can protect, then magic gears can allow you to run in that zone a bit longer - but you are still operating in a zone where the gears are not protected properly.

Premium lubes, proper lash setup on the gears, good metallurgy on the gears, and upgraded bearings and case stiffening devices (good billet caps, etc) will do the MOST for letting your drives extend the zone in which you are operating where the lube still works.

Operating above the lube's effectiveness, however, is a temporary proposition.
Space age hardcoatings can help, ensuring good metallurgy (cryo treatments, etc) can help.

But the bottom line is that if you are running beyond the film strength of your lube, you're on borrowed time. Period.

MC

Nice post. Thanks for the engineering explanation.

+1

my choice now would be BMAX had them on for 5-6 seasons and only lost a propshaft and that's with 850s on a 98 42 fountain, but if I have to do this again im going to look real hard at arnesons. RIK sounds like a nice guy.

There is a local guy that does the setup on those lowers for me.
It takes a few assy and dissassy to get the exact shimming but its worth the extra hour or so spent on eachgetting them perfect.

Let's dig a little deeper into exactly what we are asking our gears to do in our outdrives. Here's what comes out of an industrial gear calculator used in sizing gears for industrial applications. This spreadsheet will calculate the max hp you should run through a specific gear.

First is the loading environment the gear will see. The choices are uniform, light shock, med, and heavy shock. Let's consider this in industrial terms and settle on "light shock". Okay.

The next choice is duty cycle. The choices are: Intermittent Duty up to 3 hrs a day. Medium Duty 3-8 hrs a day. And constant duty 24hrs/day. Let's go with Intermittent. Okay.

The next choice is helical or spur. Non XR's use helical bevel, XR's use straight bevel, but a helical choice will translate into a higher hp load so let's go with that. Ok.

The next is the tooth form. I say we choose a DP of 4. That's pretty close to the big beefy teeth of an upper gear. Ok.

Pressure angle - not sure what those drive gears use, but let's say 14.5 deg. Ok.

Number of Teeth: 16. Bravos run 16, 17, 19, 21 tooth upper gears. We will go with 16. Ok.

Face width of the gear. Ah, let's say the upper gears have a 1.1" long tooth form. Ok.

RPM: 5000 (the pinion will run at crank speed).

Helix Angle: 45. I would say the helix angle is actually closer to 25 degrees, but we will go with 45 since it gives us a higher result.

Material: The meanest material on the chart is 8620 case hardened, so let's go with that.

Guess what the chart says is the max hp you should attempt to transmit through that gear?
Cmon guess.

Yep, that's right - 97 horsepower.

Wait, that calculator uses a 3:1 load factor. That means that the rating is based on 1/3 of the yield strength of the gear material. Let's change that to 100% of the yield strength......Manually change the safe stress factor from 57000psi to 170000psi. Now what do we have?

Now we have 290 hp.

But how do we run boats with 600hp through gears that are rated to fail at 290 hp?
Hang on - they still aren't rated to FAIL at 290hp. They are rated to have no safety factor at 290hp.

THAT's where a garden variety industrial duty gear lube in a splash environment would start giving problems with this gear. What if we run the gear submerged in lube?

The number jumps to 380 hp. But that's still for garden variety gear lube.

So, it's safe to say that any Bravo that is being fed more than 380 hp is relying on the magic properties of its lube to protect the gears from eating themselves.

If the best lube is 30% better than the baseline (in EP scuff resistance and film strength properties) then you MAY be looking at an effective limit of 510hp.

Even with the best lubes, and magic gears, running above that is causing incremental damage to your gears. Hopefully a little bit at a time, but still is damaging them.

Larger gears is the only good answer.

MC

Great gear/engineering analysis! This is why i love this site!

I have that set up on my OL