[Public-List] stern tube question

[americo silva]

hello
All this talk about alighnment and thousands of an inch
just wondering how the factory did it
I don't think they would spend a lot of time on this not the way Whitby was
cheap on everything


On Sun, Feb 3, 2019 at 9:21 AM David Fay via Public-List <
public-list at lists.alberg30.org> wrote:

> [Clay, I hope you don't mind my including your email to me (below) on the
> Public-List. I'm sure it will be useful to others.]
>
> You raise a couple of issues that have been bothering me also. See my
> comments below.
>
> David
>
> > On Feb 2, 2019, at 7:38 PM, Clay Pass <clay.q.pass at gmail.com> wrote:
> >
> > I'm going to throw this out there for discussion.  When I got #449 I had
> to replace the engine.  I rang my hands quite a bit when I read the
> alignment procedure calling for .004" (don't remember exactly what the spec
> was but it was in the thousandths) or tolerances on the engine shaft flange
> mounting surfaces.  I just didn't see how that was going to be possible to
> measure.  When the engine was removed the shaft dropped loosely to the
> bilge with movement in the order of inches (because of wear in the cutlass
> bearing).  So there is no way to hold that shaft by hand or assume it is in
> the correct position to consider positioning the engine such that a .004"
> tolerance is achieved at the flange.
>
> I think that might be why aligning an engine is so tedious and
> time-consuming. You have to align it initially very roughly by eye, then
> bolt the two flanges together, then measure the gap between them all
> around, then shift the engine on its mounts to make the gap more uniform,
> then measure again, and so on. It could take hours to get it to the 0.004"
> or whatever tolerance.
>
> > In manufacturing it is about impossible to repeatably measure .004" on
> anything other than a slab (rigid) table and carefully calibrated
> instruments.  Considering that the shaft is held in place with a rubber
> bushing on one end then there is no way to align the other end to achieve
> flange to flange tolerances that tight.  Also consider the engine itself is
> mounted on rubber bushings.
>
> Yet people claim to be able to do it.
>
> By the way, not only is the cutless bearing lined with rubber and the
> engine mounts riding on rubber, but the stern tube is also floating in a
> flexible caulk (thiokol). Nothing is fixed to thousandths of an inch.
>
> > I think it would be better to recommend that any time you remove the
> engine, that you also remove the cutlass bearing and get a true alignment
> at the business end of the shaft to exit hole interface.
>
> > Theoretically, your flex coupling is supposed to compensate for small
> amounts of misalignment, so you might expect to see some small amount of
> wobble as you rotate the shaft in neutral.  However, I would still try to
> minimize this since you obviously have some resulting damage to your
> cutlass bearing from too much side pressure.
>
> You might be right about expecting a "small amount" of wobble. But how
> much is small? I wonder how much I can ignore.
>
> Yes, I think I did have some damage to the cutless bearing from the
> sideways pressure (that plus it was old and worn anyway). That's why I
> could move the prop shaft back and forth by 1/4" or so. And it's why I
> replaced the cutless bearing. When I dry fit the new cutless bearing and
> housing onto the shaft, I couldn't wiggle the shaft at all. But that
> doesn't mean it isn't loading one side of the bearing more than the other.
> To eliminate that, I need to align the engine perfectly (assuming the shaft
> is not bent or the flexible coupling damaged).
>
> > Ideally, the engine with the shaft mounted can be considered one rigid
> unit.  What you really want is that the shaft exit the stern tube and be
> perpendicular to the mounting interface of the cutlass bearing.  Such that
> when you slide the cutlass bearing onto the shaft there is no side pressure
> on the bearing surface and it perfectly aligns with the mounting interface
> and bolt holes so that when you tighten it down you are not introducing
> side pressure.  Does that make sense?
>
> Absolutely. I agree completely. It's how to achieve it that's the issue.
>
> >
> > The way to do that is to adjust the engine mounts until the shaft exits
> the stern tube in the center and makes it possible to mount the cutlass
> bearing as I said before.  All assuming that the shaft is true, and doesn't
> wobble when turning.  You will be surprised to see how little movement on
> the engine mounting bolts will greatly impact the shaft exit location
> because of the offset length magnifies movement.  What I did was somewhat
> snuggly tighten the rear mounting bolts (my engine is mounted on rubber
> pads all around) and I then used a car scissor jack to move the front of
> the engine from side to side in controlled manner while someone was
> watching the rear, to get the correct side to side position.  Then I
> tightened or loosened the front bolts to control the up and down motion
> until I got the shaft perfectly centered at the exit.  Slipped the new
> cutlass bearing/housing on the shaft.  Bedded with 5000, screwed it on the
> stern tube and the mounting holes perfectly lined up witho
>  ut causing any pressure on the bearing surfaces.  That's what you want in
> the end.
> >
>
> So you didn't bother to measure alignment at the flanges? You just
> eyeballed the shaft to center it in the stern tube and called it a day?
>
> If that works, it would be a huge help to me because, with my flexible
> coupling in place, there is no way to measure the gap between the flanges.
> It's hard to explain but my gearbox flange is shaped kind of like a
> four-leafed clover and the bolts on the flexible coupling line up with the
> gaps between the leaves. So there's really no way to reliably measure a gap
> between the bolt heads and the gearbox flange, which is what the flexible
> coupling alignment instructions require. Apparently the instructions assume
> that your gearbox flange is round, which mine isn't.
>
> With your method I can forget all about the gap between the flanges.
>
> > Hope this helps.  You might consider doing away with the flex coupling
> and going back to a rigid mount.
>
> I've thought about that. It makes alignment much harder, and provides
> uncertain benefit, at least to me. However, Beta Marine must think it is
> worthwhile because they include it with all their engines. It's not even
> optional.
>
> Thanks for your help.
>
> >
> > Clay
> >
> >
> >
> > On Thu, Jan 31, 2019 at 4:12 PM David Fay <davidfay999 at gmail.com
> <mailto:davidfay999 at gmail.com>> wrote:
> > Clay,
> >
> > Interesting that you mention shaft alignment because, when I removed my
> bearing housing, my shaft was not centered in the stern tube. I'm about to
> tackle that problem. If it is off-center but doesn't wobble, I guess I have
> an alignment problem. If it does wobble, either the shaft is bent or the
> shaft is not aligned properly with the gearbox flange.
> >
> > I think the problem began when I had a bad rope wrap around the prop
> shaft last fall in a squall. It didn't stop the engine but the rope melted
> from the heat. Since the rope was attached to the bow of the boat (a long
> unstowed bow line), it could have exerted some sideways force on the prop
> shaft when the rope tightented. I doubt it would be enough to bend a 7/8"
> stainless steel shaft, but what do I know.
> >
> > Complicating the whole matter is that I have a flexible coupling between
> the shaft flange and the gearbox flange. It could be that the flexible
> coupling was damaged and needs to be replaced. Or perhaps just re-aligned.
> We'll see.
> >
> > David
> > Blue Teal #284
> >
> > > On Jan 30, 2019, at 9:59 PM, Clay Pass via Public-List <
> public-list at lists.alberg30.org <mailto:public-list at lists.alberg30.org>>
> wrote:
> > >
> > > I would check to make sure the shaft is coming directly out of the
> center
> > > of the stern tube and rotate it by hand to make sure it doesn't wobble
> on
> > > rotation.
>
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