[R-390] Autopsy of a bad PTO.

Sat, 28 Jun 2003 09:56:40 -0400

I don't want to sound too negative, but if that bridge is broken, I'm afraid
it's time for plan B (new PTO).  That is spring steel which won't solder
well at all.  Even if repaired, it may never be as smooth and repeatable as
an undamaged one.

Jim

----- Original Message -----
From: "Barry Hauser" <[email protected]>
To: "Jim Miller" <[email protected]>; "Phil Atchley"
<[email protected]>; <[email protected]>
Sent: Saturday, June 28, 2003 9:28 AM
Subject: Re: [R-390] Autopsy of a bad PTO.

> 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" <[email protected]>
> To: "Barry Hauser" <[email protected]>; "Phil Atchley"
<[email protected]>;
> <[email protected]>
> 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" <[email protected]>
> > To: "Phil Atchley" <[email protected]>; <[email protected]>
> > 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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> > > http://mailman.qth.net/mailman/listinfo/r-390
> >
> >
>
>