Myles, The "guarantee" is the splendid bit of engineering you do that calculated the shear values. The fellow wanted to avoid welding and machining, which is why I came up with this solution.
When I said "buy a coupler" I meant one of the many types that allow movement and misalignment like a Lovejoy connector. Prop shafts should never be hard connected to motors, especially in boats because the boat itself can flex, causing excessive force on the motor bearings.
Regarding dropping parts and tools, well what can I say? It happens. It could even happen when installing your solution, too, right? If you are concerned, then tether your tools and safety wire your small parts.
Dan
--- In electricboats@
>
> Yes, I agree that it would work---and you're right, that's what a full size
> keyway does. But where's the guarantee that this coupler doesn't become a
> weaker link than the shear pin at the prop? I can replace the prop sheer
> pin easily enough---not so the motor coupler. At 5/8" ID, the keyway is a
> 3/16" square key with nom. 3/32" protruding from the coupler and available
> to drive the 14-pt spline. With but 3/16" width available in the key,
> there's not enough angle to make 2 spline teeth and so you're only going to
> engage 1 tooth with nom. 1/16" (less than 5/64") of that 3/16" keyway
> remaining for tooth engagement. While 2 of these might suffice as you
> suggest, with but 1/16" tangentially, 3/32" radially (or less) and 1" length
> to drive the spline, you might need to go with a stronger material for the
> key, especially if using a torky motor and expecting to deliver over 3HP.
> Further, if you need a machine shop to broach a second keyway, the cost may
> end up exceeding the cost to purchase, grind and adapt a pinion gear which
> engages all teeth, not just 2.
>
> Sample coupler dimensions: http://www.ruland.
>
>
>
> Other important considerations:
>
> 1) Using a setscrew to force keyway to engage tightly with spline makes
> the driveshaft-spline-
> asking for problems as the motor axis should be allowed to be skew from the
> driveshaft--
> resonances, etc.
>
> 2) There's the added risk of dropping the key(s) or its setscrew into
> the lower unit on installation, or worse, while the unit is operational if
> the driveshaft fights the adapter/motor for shaft alignment.
>
>
>
> You should consider bonding the key in place to avoid the above problems
> and allow the motor shaft to be misaligned with the driveshaft. As long as
> the bond is good and the key(s) don't sheer, you're good. Downside is that
> if the key shreds itself, you might be back to the machine shop to rebroach
> that keyway or end up doing what I did---use the full 14-point spline
> interface.
>
>
>
> -Myles Twete, Portland, Or.
>
>
>
> Comment on shear: ON my boat, my prop shear pin is nom. 0.2"dia, resulting
> in approx. 0.3 sqin of shear area at the prop. With nom. 2:1 lower unit
> gear ratio, the torque required at the prop to shear this pin is reduced by
> approx. 2 at the power head. This means that for the same material, I
> should expect that a coupler at the motor should have at least 0.15sqin of
> shear tooth engagement with the spline or it might shear before the prop
> shear pin. At 1" engagement length, this comes out to 0.15" of tangential
> coupler cross section at the spline, minimum. Coincidentally, this comes
> out to just about the cross section shear area of that key ends up being
> when machined down to slip into one of the 14-point spline tooth slots. The
> upshot: One key used as a drive for a 14-point spline has nearly the same
> likelihood to shear as the shear pin on an older motor like mine. Two keys
> or stronger material would give you some overhead or extra life, but you
> might want to analyze this a bit----my motor will shear prop shear pins made
> from drill bit stock if I am not careful with direction changes!
>
>
>
> The Reach Of Tide electric barge cruiser: www.evalbum.
>
>
>
>
>
>
>
> From: electricboats@
> On Behalf Of danbollinger
> Sent: Tuesday, December 29, 2009 8:13 AM
> To: electricboats@
> Subject: [Electric Boats] Re: Source for coupler (14 spline, 5/8ths shaft)
> to motor or pully
>
>
>
>
>
> Yes, you got it exactly right. Granted, you are only using one spline, but
> then that's what a keyway typically does. Also, you aren't tranmitting as
> much torgue as the gasoline engine, and the electric motor is smooth, not
> pulsating torque. I think it would be fine. Worst case, you have a machine
> shop broach a second keyway.
>
> --- In electricboats@
> <mailto:electricboa
> wrote:
> >
> > I'm racking my brain to understand what you're suggesting, but I think I
> > understand now.
> >
> > It sounds to me like you are describing a 5/8" ID coupler that slips over
> > the spline and to make it mesh with the spline, you'd need to machine down
> > the key and either (1) narrow the key width at the OD to the spline gap
> > width or (2) cut a slot in the key along its length to make the key grab 2
> > teeth instead of one.
> >
> > You're asking for the coupler to transmit a lot of torque thru a very
> narrow
> > 1 or 2 teeth instead of 14.
> >
> > It would work though.
> >
> >
> >
> > -mt
> >
> >
> >
> > From: electricboats@
> <mailto:electricboa
> [mailto:electricboats@
> <mailto:electricboa
> > On Behalf Of danbollinger
> > Sent: Monday, December 28, 2009 6:12 PM
> > To: electricboats@
>
> > Subject: [Electric Boats] Re: Source for coupler (14 spline, 5/8ths shaft)
> > to motor or pully
> >
> >
> >
> >
> >
> > Myles' idea is a good one. Here's another. Buy a regular 5/8" drive
> coupler
> > (there are dozens of different types) that uses a square key to transfer
> > power. Then, grind or file the key to fit one of the spline grooves.
> >
> > --- In electricboats@
> <mailto:electricboa
> > <mailto:electricboa
> > <williamdickenson55
> > >
> > > I am about to get seriously into converting an old outboard to electric.
> > I've got all the parts laid out except one. Is there a commercial source
> for
> > a coupler that would slide onto that 14 spline 5/8ths inch outboard that
> > would be suitable for either direct drive to an outboard, or into a
> pulley.
> > I'm trying to avoid the cutting and welding end of this if I can.
> > >
> > > I've seen quite a few comments, but not one that specifically mentions a
> > source.
> > >
> > > Thanks
> > >
> >
>
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