Mechanical tach failure?

wrote in message
...

Isn't the aircraft tach run off the cam, too? Thus indicating 2500 RPM
would
require the cable at 1250?

When Cy mentioned that, I remembered it. I've got a couple of small
synchronus motors out in the shop. I'll hook one of them up and see if I can
get an accurate reading on the dial.

Whatever you do... don't tell them it's for an airplane...

Learned that lesson well when I was building. Example - an automotive spring
shop refused to sell me some leaf spring stock until I came back the next
day and told them it was for an invention for the handicapped and NOT for an
airplane. Spring stock fer Pete's sake!

Rich S.

DON'T TAKE IT APART !! At least not until you have lubed the drive cable..
when they get dry, they start sticking somewhere, the part in front of the
"stick"
winds up a little, then releases this energy in a quick snap, this causes the
instrument to read high. I know this may sound silly, but it happens every winter
with my trucks
Jerry

wrote:

On Wed, 3 Nov 2004 13:23:05 -0800, "Rich S."
wrote:

So, I was flying back home, happily burping all the shellfish I ate at the
Shelton Oysterfest, when my engine started speeding up. Well, it didn't
really - but my tach said it did. Without touching the throttle, I went from
2400 rpm up to 3400. Funny, my airspeed didn't change.

It looks like my sturdy, simple mechanical tach (bought used at OSH) has
gone South on me. I pulled it out and removed the case. Everything inside
looks great. It is cable driven through a magnetic coupling just like a car
speedo. The needle has a return spring wound in a coil like a clock spring.
I assume the calibration is done by setting the needle on its shaft in a
position relative to the return spring, so a given rpm matches the needle
position on the dial.

I thought I would discover a broken return spring, but it looks okay.
Turning the drive with an 1100 rpm electric drill shows ~2500 on the dial.
If I allow the needle to bypass the stop pin and add an extra full turn of
tension on the return spring, the tach shows the drill turning ~400. It
looks like the needle has slipped on the shaft releasing about a half-turn
of spring tension. Either that or

A. the spring has developed a weak spot or
B. the other end of the spring has come unsoldered from the frame.

I don't think it's "B" because the entire spring would be unwound. Visual
examination of the spring doesn't show any defects.

Any ideas?

How about either a change inmagnetic flux or a change in the position
of the EC (Edy Current) ring?

A weakened magnet would cause the reading to DROP. So something that
causes the aluminum disc to get closer to the magnet would cause the
RPM to read high, or something that causes the air movement from the
spinning magnet to influence the disc - like a bit of oil or grease
from the cable migrating into the instrument.

Rich "May as well take it apart - it don't work" S.

"Cy Galley" wrote in message
news:eAeid.52917$R05.12713@attbi_s53...
You can take it to an automotive speedometer shop and they can clean,
lubricate, and calibrate. Tell them that it is driven off the cam in your
stock car so they get it right and don't panic.

Okay, Cy - here's what I found out. The speedo shop said they would be happy
to look at it and fix it, even if they have to throw away the guts and
replace them with a new SW unit. If they have the parts, it would be $92 for
an overhaul. A completely new unit would run ~$120.

They said the common problem with this type of tach is worn bearings on the
cup shaft which allows interference between the cable shaft and the cup. The
scuff marks in my tach make it appear as if this is the problem.

I think I'll buy another used but operative unit. Prices seem to run in the
$40-60 range. If it lasts another three years I'll be happy.

Rich

"Rich S." wrote in message ...
wrote in message
...

How about either a change inmagnetic flux or a change in the position
of the EC (Edy Current) ring?

A weakened magnet would cause the reading to DROP. So something that
causes the aluminum disc to get closer to the magnet would cause the
RPM to read high, or something that causes the air movement from the
spinning magnet to influence the disc - like a bit of oil or grease
from the cable migrating into the instrument.

Like I said, everything looked fine inside (no dirt or grease). There is a
scuff mark on the inside of the disk which could be new or old. If lubing
the cable doesn't do the trick, I'll probably send it out to be rebuilt;
unless the cost is more than another used unit.

Thanks,
Rich S.

Look closer. The inboard end of the shaft that carries the needle
fits into a tiny bore in the inboard end of the input shaft to keep
the magnet and drum concentric with one another. This close fit is
subject to dried-out lube or a bit of grit interfering with the needle
shaft's freedom and drives the needle up. Or it might be worn to the
point that it's chattering and adding friction, with the same result.

Dan

"Rich S." wrote in message ...

Whatever you do... don't tell them it's for an airplane...

Learned that lesson well when I was building. Example - an automotive spring
shop refused to sell me some leaf spring stock until I came back the next
day and told them it was for an invention for the handicapped and NOT for an
airplane. Spring stock fer Pete's sake!

I like to tell them I'm building a variable oscillating frambulator.
That pretty much results in a blank stare from the other side of the
counter, and an end to the questions.

John Galban=====N4BQ (PA28-180)

"Dan Thomas" wrote in message
om...

Look closer. The inboard end of the shaft that carries the needle
fits into a tiny bore in the inboard end of the input shaft to keep
the magnet and drum concentric with one another. This close fit is
subject to dried-out lube or a bit of grit interfering with the needle
shaft's freedom and drives the needle up. Or it might be worn to the
point that it's chattering and adding friction, with the same result.

Thanks, Dan. If you read my other post to Cy, you'll know that since I
didn't find anything my limited talents could repair I'm choosing the
cheaper option of replacing it with a used unit rather than a rebuild.

Now if I could just get the rest of me repaired or rebuilt!

Rich "My needle is worn to the point of chattering" S.

On 4 Nov 2004 15:35:36 -0800, (John Galban) wrote:

"Rich S." wrote in message ...

Whatever you do... don't tell them it's for an airplane...

Learned that lesson well when I was building. Example - an automotive spring
shop refused to sell me some leaf spring stock until I came back the next
day and told them it was for an invention for the handicapped and NOT for an
airplane. Spring stock fer Pete's sake!

I like to tell them I'm building a variable oscillating frambulator.
That pretty much results in a blank stare from the other side of the
counter, and an end to the questions.

John Galban=====N4BQ (PA28-180)
My experience has, to a large extent, been opposite. When they find
out I'm building a plane they get all interested, and in one case said
they would not sell me the part for the plane but would gladly GIVE it
to me. They just happened to have a spring leaf the size I needed for
my tail-wheel that had been "mis-drilled" and I was welcome to it.

All the wreckers in the area know what I'm doing and will gladly sell
me anything I can dream up.

On 4 Nov 2004 15:35:36 -0800, (John Galban) wrote:

"Rich S." wrote in message ...

Whatever you do... don't tell them it's for an airplane...

Learned that lesson well when I was building. Example - an automotive spring
shop refused to sell me some leaf spring stock until I came back the next
day and told them it was for an invention for the handicapped and NOT for an
airplane. Spring stock fer Pete's sake!

I like to tell them I'm building a variable oscillating frambulator.
That pretty much results in a blank stare from the other side of the
counter, and an end to the questions.

I tell 'em, "An Off-Road Vehicle." It's the truth, too...:-)

Ron Wanttaja

My experience has, to a large extent, been opposite.

Mine has been mixed. Posted previously, updated:

Shop #1
Me: Hey, you got any XXX fittings?
Parts Desk Guy: Whatcha need 'em for?
M: Puttin' an engine in an airplane.
PDG, suddenly very formal: Sir, you'll have to go to an airplane shop
for that. Try the airport.

Shop #2
Me: Hey, you got any XXX fittings?
Parts Desk Guy: Whatcha need 'em for?
M (who doesn't like to lie and doesn't always think fast on his feet):
Uhhhh...an airplane...?
PDG: Cool. Here ya go.

Shop #3
Me: Hey, you got any YYY fittings?
Parts Desk Guy: Whatcha need 'em for?
M (a little smarter and cautious): An RV.
PDG: Must be pretty high performance. Too bad it's
not an airplane, the boss loves that stuff.
M: Um...
PDG: I get it. C'mon in the back, have some coffee, take a look
around.

Shop #4
Me: Hey, you got any YYY fittings?
Parts Desk Guy: Whatcha need 'em for?
M (now fearless): An RV.
PDG: Here y'go. Y'know, a lot of guys
put this stuff in airplanes. Don't bother me none.

Dave 'locals only' Hyde



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"Rich S." wrote in message
...

I think I'll buy another used but operative unit. Prices seem to run in
the $40-60 range. If it lasts another three years I'll be happy.

Rich

Just curious... Why would you want to get a second-hand, unknown history
instrument for $50 when the difference to a new instrument is about as much
as one long flight?!? You are going to rely on that instrument for years to
properly run a $15000 engine in a $40000 plane (guestimates..), why not get
a new one that will probably last the remainder of the plane's life??

Rob