Mechanical tach failure?

"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

"Stealth Pilot" wrote in message
...

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.

Phew! One flame avoided. . .

Rich S.

You may need to clean and re oil the cable and the input shaft of the
Tach.
If it chatters then the tach will read high for a while, then it will
read ZERO when the cable breaks. You can feel this vibration if you
touch the front of the Tach when it happens. If this does not fix it
you may have too much clearance in the Tach bearings.
john

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?

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

On Fri, 5 Nov 2004 08:22:51 -0800, "Rich S."
wrote:

"Stealth Pilot" wrote in message
.. .

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.

Phew! One flame avoided. . .

Rich S.


had a tacho fault in a cessna once. would periodically read zero in
flight. on the next flight it would be ok. needless to say it would
always work perfectly when the maint guy disassembled it and looked it
over. then it would fail and read zero again.

one day when it failed I took it apart. (there IS a fault here it is
just a matter of looking long enough at it)
the end of the tacho drive shaft (coiled wire inner gubbins) ends in a
ferrule at one end. the gubbins bit had come loose within the ferrule
and worn itself into a conical shape within the ferrule. occasionally
it would lock together like two parts of a morse taper and drive away
perfectly. other times it would work apart.
maint guy looked it over,pulled it apart, pushed it back together,
played with it a while then said "F*** Me, thats nifty, you ought to
patent it :-) "

Stealth Pilot

"Stealth Pilot" wrote in message
...

had a tacho fault in a cessna once. would periodically read zero in
flight. on the next flight it would be ok. needless to say it would
always work perfectly when the maint guy disassembled it and looked it
over. then it would fail and read zero again.

I *hate* intermittent faults. The mechanic always looks at you like you're
nutso.

Rich S.