Mechanical tach failure?

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

"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
...

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

"Rob Turk" wrote in message
...

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..............

Several reasons come into play. I am not of the mind set that says, "If it
ain't new, it's junk". My Pappy always told me, "All of the cars on the road
stop with used brake parts".

1. A tachometer is not a critical instrument.
2. I will select a good quality instrument and have it calibrated.
3. At this point in my life's journey, I purchase items which will last as
long as *I* will need them - not for "the remainder of the plane's life".

The philosophy of sparing no expense simply because we are dealing with
airplanes must have limits. Otherwise, we would change oil, filters, and
sparkplugs before every flight - or other equally silly tasks. Every one of
my instruments was purchased used. In five years and four hundred hours of
flight, the tachometer is the first one to present a problem. Who's to say
that an all-new panel wouldn't have had at least one malfunction in that
time?

Rich S.

On Fri, 5 Nov 2004 07:14:45 -0800, "Rich S."
wrote:

"Rob Turk" wrote in message
...

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..............

Several reasons come into play. I am not of the mind set that says, "If it
ain't new, it's junk". My Pappy always told me, "All of the cars on the road
stop with used brake parts".

1. A tachometer is not a critical instrument.
2. I will select a good quality instrument and have it calibrated.
3. At this point in my life's journey, I purchase items which will last as
long as *I* will need them - not for "the remainder of the plane's life".

The philosophy of sparing no expense simply because we are dealing with
airplanes must have limits. Otherwise, we would change oil, filters, and
sparkplugs before every flight - or other equally silly tasks. Every one of
my instruments was purchased used. In five years and four hundred hours of
flight, the tachometer is the first one to present a problem. Who's to say
that an all-new panel wouldn't have had at least one malfunction in that
time?

Rich S.

my airspeed indicator is ww2 vintage and is still calibrated within 1
knot of what it should read.
my black and white AH is out of a dead cessna.
in fact none of the instruments is younger than the airframe. they
were all selected as old trusted mechanisms.
no need to change the approach Rich.
it works the world over.
Stealth Pilot

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.

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.

"Jerry J. Wass" wrote in message
...
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.

TOO LATE !! But I got it back together. Even used all the pieces

Actually, you anticipated my next thought. I got to wondering if I was
assuming too much by blaming the tach head. I'm going to reassemble the unit
in the case and lube the cable.

Thanks, Jerry.

Rich S.

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.

"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