Driveshaft won’t engage powerhead

[walleye1]

I have seen the drive shaft splines twisted from impact or the end of the shaft broken, most of the time the pinion gear will split.

[zenoahphobic]

I do believe the drive shaft splines are "case hardened" meaning just the end, and the outer surface of the metal is hardened, the crank however might be a very different story, it is cast, machined then hardened, and I think (I have never seen a crank made) only the bearing surface areas are hardened? has anyone ever seen a mass produced crank made? I don't know, but maybe the spline end is "soft" or perhaps the entire crank is hardened as a unit?

Not many people would witness them being made, as it would be in a large dangerous factory involving hot molten metal. But the process would be the same, selection of the steel, forging, hardening with carbon and nitride(?, Xraying etc.); the aerophiles would be very familiar with the rigorous manufacturing and testing involved with aeroplane engine crankshafts.

I think these outboard cranks don't have a lot of treatment and essentially rely on the composition of the steel.

I think most driveshafts are stainless steel and don't have any additional hardening so the strength is the same from the spline to the centre of the shaft.

On the other hand, if the cranksplines are hardened this hardening can only go to a certain depth. What the difference will be the shear strength at the base of the spline, the crankshaft being weaker there than the corresponding driveshaft spline.

[TownsendsFJR1300]

Many moons ago, I worked in a machine shop. Making custom, narrowed, racing automotive racing axles from scratch was one thing we did,

I know the boss would machine the splines in his mill (and other parts of the shaft) BEFORE HARDENING.

After he got done with that, I would deliver the axles to a place that would THEN harden them. I don't know the process and as I re-call (I was 17 at the time),
it went to another place for further hardening with his SPECIFIC spec's on hardening. These were for high end dragsters/ funny cars as well as street cars..

These axles were mostly for 9" Ford diff's and Chrysler 8&3/4 rear ends..

We also narrowed rear ends, made roll bars, shortened driveshafts, sold Chevy Vega V8 kits (which I made built my own after my Chevelle got stolen)


Just to add re Yam drive shafts.
Anyone that's had to remove a stuck key (for the WP impeller) knows how EASY it is to nick/ damage the lower drive shaft if the shaft is accidently tapped with a screw driver/hammer (pretty darn soft)… I have to gather the entire shaft isn't hardened. Who knows, the hardening processed may have been done incorrectly (or not at all) on the splines (perhaps not hardened at all)

[zenoahphobic]

What we also don't know is what was the fit between the two parts when new. Was the taper of the splines exactly fitting? Theoretically they are not the same angle, as there is space to slot (not compression fit) the shaft in. I.e. were they touching exactly flat when torque is applied? I bet not.

[ausnoelm]

The driveshaft to crank spline fit is really quite "sloppy" by engineering standards, it is certainly not a tight fit, (it doesn't need to be) brand new shafts are blue tinged at the spline, so, some sort of hardening takes place, in a strange kind of way, wouldn't you think they would make the drive shaft the weaker part, it's easier to replace a drive shaft (or complete gearcase) than it is to replace a crankshaft! or maybe even a replaceable adapter, but then, stripped splines are not all that common.

[zenoahphobic]

The driveshaft to crank spline fit is really quite "sloppy" by engineering standards, it is certainly not a tight fit, (it doesn't need to be) brand new shafts are blue tinged at the spline, so, some sort of hardening takes place, in a strange kind of way, wouldn't you think they would make the drive shaft the weaker part, it's easier to replace a drive shaft (or complete gearcase) than it is to replace a crankshaft! or maybe even a replaceable adapter, but then, stripped splines are not all that common.

I think that is the point, the splines NEED to be an exact fit so that all the energy passes through to the shaft and not have it absorbed at this junction where it can cause damage. But because of the difficult access design of these outboards, the slightest corrosion will make it impossible to separate them later. So they are made sloppy.
An adaptor should be an obvious design advantage whereby it can be more exactly hardened separately from the crank requirements for the purpose and replaceable. It's connection to the crank would not be a spline but some other tight fit......but then more cost to make.
Sometimes the weakest part is not what one might first expect. Townsend spoke of driveshaft manufacture in his youth, he mentioned shafts fitting Ford 9 inch differentials. I remember an axle in my F100 failing nearly leading me to my death. Those shafts were extremely thick but had a propensity to snap cleanly straight through, like a carrot, just ouside the wheel bearing, letting loose the entire wheel. Although the shaft was huge by other makes, the hardening, just like a weld, has a "boundary of stress" where it is vulnerable if it is not shifted away where it can dissipate. Hardening stiffens and reduces flexing and distortion, but the downside is fracture and catastrophic failure when stressed too far.

[boscoe99]

The driveshaft to crank spline fit is really quite "sloppy" by engineering standards, it is certainly not a tight fit, (it doesn't need to be) brand new shafts are blue tinged at the spline, so, some sort of hardening takes place, in a strange kind of way, wouldn't you think they would make the drive shaft the weaker part, it's easier to replace a drive shaft (or complete gearcase) than it is to replace a crankshaft! or maybe even a replaceable adapter, but then, stripped splines are not all that common.

There are all sorts of engineered "fits". Loose tolerances, tight tolerances, interference fits, etc., with all sorts of fits in between. Just depends upon the application. As you note, the drive shaft to crank shaft fit has to be such that the drive shaft can be readily removed. So there will naturally be a bit of slop between the two parts.

Crank shaft steels are typically very brittle. Thus the propensity for a crank shaft to break and not bend. Whereas a drive shaft is engineered such that it can bend to some degree.

The drive shaft is generally made of stainless steel. Very tough. And as you note, heat treated for additional toughness. Crank shafts are more prone to corrosion than are the drive shafts. I can easily see a lack of lubrication between the splines leading up to corrosion within the crank shaft splines. The tough stainless then uses the corrosion particles to wear away the crank shaft splines over time. Nil lubrication so the splines are hammering each other with each pulse of a piston. Finally the crank shaft splines fail.

While not unheard of, crank shaft spline failures do happen. Just one of many many failure modes of a motor.

[boscoe99]

I think that is the point, the splines NEED to be an exact fit so that all the energy passes through to the shaft and not have it absorbed at this junction where it can cause damage. But because of the difficult access design of these outboards, the slightest corrosion will make it impossible to separate them later. So they are made sloppy.
An adaptor should be an obvious design advantage whereby it can be more exactly hardened separately from the crank requirements for the purpose and replaceable. It's connection to the crank would not be a spline but some other tight fit......but then more cost to make.
Sometimes the weakest part is not what one might first expect. Townsend spoke of driveshaft manufacture in his youth, he mentioned shafts fitting Ford 9 inch differentials. I remember an axle in my F100 failing nearly leading me to my death. Those shafts were extremely thick but had a propensity to snap cleanly straight through, like a carrot, just ouside the wheel bearing, letting loose the entire wheel. Although the shaft was huge by other makes, the hardening, just like a weld, has a "boundary of stress" where it is vulnerable if it is not shifted away where it can dissipate. Hardening stiffens and reduces flexing and distortion, but the downside is fracture and catastrophic failure when stressed too far.

Millions and millions of outboard motors are being used daily with crank shaft spline failures being extremely low. Those few failures more than likely could have been mitigated with better maintenance. Given the low failure rate, is there really a need to change the design?

Achieving perfection is a worthy goal but at some point the added expense is not worth whatever additional improvements could be realized.

[panasonic]

I think most crank shafts these days are forged, machined, induction hardened and then final machining and grinding.

Different parts of the crank are induction hardened to various depths and hardness. The whole crank is not hardened the same. Look up crank hardening on YouTube to see how it is done.

The spline socket on the end of the crank would be hardened more than the splines on the drive shaft so the drive shaft will wear before the crank end will.

[panasonic]

The drive shaft would be some type of stainless that can be heat treated, such as 17-4PH stainless. I would think it is only hardened on the ends where the splines are.

Boscoe or Rodnut do you know what type of stainless is typically used for the drive shafts?

[boscoe99]

I think most crank shafts these days are forged, machined, induction hardened and then final machining and grinding.

Different parts of the crank are induction hardened to various depths and hardness. The whole crank is not hardened the same. Look up crank hardening on YouTube to see how it is done.

The spline socket on the end of the crank would be hardened more than the splines on the drive shaft so that hopefully the drive shaft will wear before the crank end will.

Suggestion.

[boscoe99]

The drive shaft would be some type of stainless that can be heat treated, such as 17-4PH stainless. I would think it is only hardened on the ends where the splines are.

Boscoe or Rodnut do you know what type of stainless is typically used for the drive shafts?

Yamaha does not say that I know of. Perhaps there are multiple types of SS used depending upon the model. I will look and see what I can find.

[panasonic]

Yamaha does not say that I know of. Perhaps there are multiple types of SS used depending upon the model. I will look and see what I can find.

Thanks I would be interested to find out what it is..

For those who are interested here is an example of induction hardened steel. It is a piece of 1045 chrome plated induction hardened hydraulic rod I had in the lathe today. It is 1.500" diameter with .0005" chrome plating.

In the picture you can see the depth of the hardened material as the dark ring around the outside, probably about 0.100" thick, I did not measure it. The lighter color core is the original material not hardened.

Don't mine the poor surface finish, it was first pass, insert needed changing and it was time to go home.

[ausnoelm]

Are all drive shafts stainless?

[DennisG01]

Are all drive shafts stainless?

Personally, I've never seen one that isn't. Not that I've seen everything, of course, but have probably seen easily over a hundred of various manufacturers' driveshafts.