Mechanical tach failure?

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?

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

Or the needle slipped on the shaft?

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?

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

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

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



An old gent I know who has been flying longer than anybody offered the
observation that the reason that old airplanes don't fly as fast as new ones
is that the needle return springs on tachometers and manifold pressure
gauges get weak with age causing the instrument to over-indicate. If pilots
keep setting power at the same gauge indications the airplane will indeed
fly slower as it ages. Re-calibrating the gauges restores the youthful
vigor.

If only that worked for me.

Bill Daniels

"Bill Daniels" wrote in message
news:Kjdid.351704$MQ5.71307@attbi_s52...

An old gent I know who has been flying longer than anybody offered the
observation that the reason that old airplanes don't fly as fast as new
ones
is that the needle return springs on tachometers and manifold pressure
gauges get weak with age causing the instrument to over-indicate. If
pilots
keep setting power at the same gauge indications the airplane will indeed
fly slower as it ages. Re-calibrating the gauges restores the youthful
vigor.

If only that worked for me.

Bill................

You mean that what I got is catching - and my tach come down with it? Oh,
Lordy.

Actually the tach started acting up all of a sudden, where my body took
years to reach its present state of weakness.

Rich "Need some vigor-ah" S.

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.

It is standard old style AC and they uncrimp and re-crimp the bezels all the
time.

--
Cy Galley - Chair,
AirVenture Emergency Aircraft Repair
A Service Project of Chapter 75
EAA Safety Programs Editor - TC
EAA Sport Pilot
"Rich S." wrote in message
...
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?

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.

It is standard old style AC and they uncrimp and re-crimp the bezels all
the
time.

Cy...........

You the man!

Rich S.

Cy Galley wrote:
: 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.

: It is standard old style AC and they uncrimp and re-crimp the bezels all the
: time.

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

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

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

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.

"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)

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.