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Mechanical tach failure?



 
 
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  #1  
Old November 3rd 04, 09:23 PM
Rich S.
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Default 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.


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  #2  
Old November 3rd 04, 10:04 PM
rip
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Default

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.



  #3  
Old November 3rd 04, 10:53 PM
Bill Daniels
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Posts: n/a
Default


"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

  #4  
Old November 3rd 04, 11:04 PM
Rich S.
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Posts: n/a
Default

"Bill Daniels" wrote in message
news:[email protected]_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.


  #5  
Old November 4th 04, 12:19 AM
Cy Galley
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Posts: n/a
Default

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.




  #6  
Old November 4th 04, 12:35 AM
Rich S.
external usenet poster
 
Posts: n/a
Default

"Cy Galley" wrote in message
news:[email protected]_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.


  #7  
Old November 4th 04, 06:01 AM
external usenet poster
 
Posts: n/a
Default

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.


  #8  
Old November 4th 04, 01:48 PM
external usenet poster
 
Posts: n/a
Default

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 *
************************************************** ***********************

  #9  
Old November 4th 04, 03:03 PM
Rich S.
external usenet poster
 
Posts: n/a
Default

"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.


  #10  
Old November 4th 04, 03:08 PM
Rich S.
external usenet poster
 
Posts: n/a
Default

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.


 




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