[R-390] Autopsy of a bad PTO.

[Barry Hauser]

Hi Jim:

Sorry ... I didn't notice your earlier post(s) in reply to Phil.  Thanks for
the new URL.  Those missing photos help make it clearer about the action
with the "footbridge" riding and straddling adjacent screws.  The
overlap/staggering of the linearity screws is intended to avoid jumping
which could otherwise cause up-down variations in frequency as the unit
rotates.  (and how an abrupt jump from one screw to the next could cause
problems.)  The photos on your site seem to be clearer and larger as well.
Even clearer to me that the little ramp or bridge should be rigid, not
flexible.  If the metal will take to soldering, Phil could solder the broken
weld and perhaps fill the void under the bridge (no boat traffic) with more
solder. Of course, it would have to be reshaped as well as possible first
and the final result deburred and lubricated with a small amount of
non-running grease.

Barry



----- Original Message -----
From: "Jim Miller" <jamesmiller20@worldnet.att.net>
To: "Barry Hauser" <barry@hausernet.com>; "Phil Atchley" <k06bb@elite.net>;
<r-390@mailman.qth.net>
Sent: Saturday, June 28, 2003 8:38 AM
Subject: Re: [R-390] Autopsy of a bad PTO.


> I just noticed some of the photos in my Cosmos PTO article on Dave's page
> have become corrupted.  If anyone's interested, the original article is at
> the following url.  Copy anything you want.
>
> http://home.att.net/~jamesmiller20/cosmos.htm
>
> I tried emailing Dave but his email bounces.  I guess I like the Cosmos
over
> the Colling PTO because it appears to be relatively easier to linearize
> despite Phil's nightmare.  I spent over a week (couple of hours a day)
going
> through the stack (trial and error) in a Collins PTO, taking the cover
off,
> tweaking, putting the cover back on, over and over.  I can see why the
> inventor of the Cosmos approach thought he had a better idea.  Sounds like
> someone in a past life got heavy handed with a screwdriver and jammed some
> of the little screws.
>
> ----- Original Message -----
> From: "Barry Hauser" <barry@hausernet.com>
> To: "Phil Atchley" <k06bb@elite.net>; <r-390@mailman.qth.net>
> Sent: Saturday, June 28, 2003 7:28 AM
> Subject: Re: [R-390] Autopsy of a bad PTO.
>
>
> > Hi Phil & gang:
> >
> > I dunno, there was a time there would have been several posts with
> detailed
> > "been-there-done-that" posts to assist you.  I've held back on replying
> > simply because I've never been that deep into a Cosmos.  Soooo.. .in the
> > absence of any experienced replies .... (maybe some were off-list?)
> >
> > Somehow, I'd also assumed that with all the downloading you mentioned
you
> > had certainly come across Dave Medley's pages or another site with Jim
> > Miller's "A JOURNEY THROUGH THE COSMOS (PTO)".
> >
> > The direct URL on Dave M's site is http://www.davemed.com/cosmos.html
> This
> > includes detailed photos, though there's something wrong with one or two
> of
> > the photos.
> >
> > Gonna try to shed some light on this, but again, bear in mind that I
have
> no
> > hands-on, so take it for what it's worth.
> >
> > >Since there is another PTO coming I went ahead and took this one apart
> all
> > >the way down to the corrector disk to see if I could determine the
> failure
> > >mode (not sure I can get it back together correctly 8^)
> >
> > Somehow, I'm not surprised.  I always take your "quit" and "throw in the
> > towel" declarations with a grain of salt. (about the size of a football
> ;-)
> > While you were tearing into that PTO, I was probably searching for that
> web
> > article (or copping some Z's).
> >
> > >Anyway, I was
> > >right.  The 48 tuning screws are rotated one by one past a round spring
> > >loaded thingy made of spring steel that has a small piece of steel that
> > >looks kind of like a "footbridge" spot welded to it.  As the screw for
a
> > >particular segment of the frequency range (25 KHz steps) walks over the
> > >"footbridge" it presses down on it and pushes the slug under it further
> > into
> > >the coil.
> >
> > The web site photo shows this.  The spot welded piece looks more like a
> > triangle, so I'm not sure what you mean by a footbridge.  (Maybe like
the
> > ones in Japanese gardens and Central Park?) Check out the photo for what
> the
> > shape of it should be.
> >
> > >The "footbridge" was damaged in two ways.  First it had a BIG dent in
it
> on
> > >one side that would make screws riding on that side of the bridge
> (they're
> > >staggered) not press down as hard on the bridge. Secondly, the spot
weld
> > >that holds one end of the bridge is broken loose so that it has lost
some
> > of
> > >its "stiffness" and positive action.
> >
> > >In trying to fix the "dent" in the bridge I'm afraid that I also
weakened
> > it
> > >some more as spring steel doesn't take kindly to dents and trying to
> > >straighten them out.
> >
> > From the photo, it would seem that bridge is supposed to be rigid, not
> > flexible, even if it's made of the same spring steel as the ring. (Just
a
> > manufacturing convenience, vs. a solid piece of something, also
avoidance
> of
> > dissimilar metals. Also, spring steel is fairly hard and less prone to
> > wear.)  Wouldn't it be solid if that other weld weren't broken?
> >
> > >The third problem I found was that some of the screws had apparently
been
> > >turned "counterclockwise" too hard, spreading the slot and screw too
wide
> > to
> > >thread down into the disk far enough to reach the damaged bridge.
These
> > are
> > >the culprits that probably had no adjustment affect as they wouldn't
> reach
> > >the "footbridge".  These are very tiny screws that are really only a
> screw
> > >shaft with a "slot" in the end and a flat head on the backside to "walk
> > >across the bridge".
> >
> > Can't you turn them back down again while it's apart, to make sure
they'll
> > make it through?  Sounds like what you are describing is a false
bottoming
> > out of the adjustment screws.  The "no adjustment" effect might also be
> due
> > to the "bridge" being out of shape, partly flattened and flexing flatter
> due
> > to the broken weld on one end and/or the dent.  Actually, if that bridge
> or
> > arch was flexing, that would have messed up your linearity adjustments
and
> > may have contributed to the other problem.
> >
> > >NOTE:  A couple articles that I read said that there was a flexible
> Teflon
> > >ring that these screws rode on forming kind of a cam.  This one had no
> such
> > >ring, only the spring steel thingy that the screw heads walked across.
> >
> > Was that a Cosmos PTO in the articles?  May have been a different mfr.
> >
> > Again, keeping in mind the nature of the source, (not
> > been-there-done-that) -- here's some partially lame advice:
> >
> > From the photo, it looks like the bridge should be solid, not flexible.
I
> > imagine the screws should just catch the bridge on the rise, not at the
> base
> > near the spot welds.  If so, then you should be able to repair the thing
> > with epoxy, or perhaps a solder repair, if the spring steel will take
> > soldering.  One way would be to fill the void in the bridge -- basically
> an
> > "arch support".  Reshape the arch as best as possible, using the photo
or
> > another unit as a guide.  Cut the corner off a piece of sheet metal --
> > aluminum -- to make a small triangle support with a slightly rounded
apex
> to
> > fit the underside of the bridge and secure the whole business with
epoxy,
> > using a small clamp to hold in place until it sets.  Basically -- an
> > orthothic shoe insert.
> >
> > Then fix the screws somehow, if they actually need fixing.  The photo on
> the
> > website shows that quite a few screws are either not present or in the
> fully
> > backed out positioin.
> >
> > There's another thing.  I was a bit confused as to how this setup could
> work
> > if the screws are riding up and down the arch, with gaps in between,
> causing
> > reciprocating action.  That would mean that it would be nearly
impossible
> to
> > have smooth action in terms of degrees rotation to frequency.  The key
is
> > that the adjacent screws are straddled so that the tip of the
bridge/arch
> is
> > always riding on a screw or none at all.
> >
> > But -- what that means is that the screw setting should end up with
> > reasonably smooth transition from one to the next.  If there were too
much
> > of a differential from one (or a pair) to the next, it could jam or
cause
> > roughness.  I would think that extreme differences between adjacent
screws
> > might be indicative of a problem elsewhere -- worn lead screw or
> something.
> >
> > Also -- does it seem possible to run a screw down far enough to actually
> > crush the bridge piece (or snap a weld)?
> >
> > The photos on that site are of fairly high resolution.  I blew up the
side
> > view of the "bridge" and it certainly appears to me that it should be
> fairly
> > or completely rigid, not flexible, if both welds are intact.  It's
> basically
> > triangular in shape, but with a smooth bend at the apex. With one weld
> > broken, it will flatten under pressure and you won't get a reliable
> > adjustment.  That's my theory anyhow.
> >
> > Of course, as I'm typing the last of this speculative tract, Phil is
> waiting
> > for the epoxy to cure or soldered the thing, or maybe hand carved a new
> ring
> > and "footbridge" out of some spring steel in his junk drawer.
> >
> > "Throw in the towel"  -- yeah, right.  We know you better than that. ;-)
> >
> > Barry
> >
> >
> >
> >
> >
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