Subject: [TR] Tachometer calibration

From: "Dave Connitt" <[email-masked]>
Date: Sun, 5 Jul 2015 11:42:23 -0400

This is a multi-part message in MIME format.

--===============8609953761789450672==
	boundary="----=_NextPart_000_006D_01D0B717.A8C8FF20"

This is a multi-part message in MIME format.

------=_NextPart_000_006D_01D0B717.A8C8FF20
	charset="Windows-1252"
Content-Transfer-Encoding: quoted-printable

I had a problem a week ago or so where the tachometer in my TR4A was =
making a lot of screaching noises and figured it was a dry bushing from =
50+ years of life. I figured I would try fixing it myself. What could =
happen??
As I was taking the tach apart, I noticed there was a black paint mark =
on the drag cup that was connected to the needle so I just noted where =
the mark was with the needle "at rest" against a point on the frame and =
continued taking it apart. From reading information on the topic, I knew =
that that little spring attached to the drag cup was very delicate and =
was very careful to hold the tach upside down and lift the "drive" end =
off the drag cup, not the other way around. Once I had separated the =
frame, I just set the drag cup half on the bench in that same =
orientation.=20
The other half of the "frame" contains the magnet, shaft and bushing =
which connects to the tach cable. Sure enough the little shaft that the =
magnet was attached to was dry as a bone.  The bushing and the shaft =
looked OK so I figured it just needed cleaning and lubrication. After =
careful cleaning of the shaft and it's bushing with some carburetor =
cleaner, I lubricated it with a few drops of 3-1 oil and put the magnet =
and shaft back into the bushing. MUCH BETTER.. The shaft would spin very =
smoothly by hand.=20
The assembly went well. Keeping the frame half with the drag cup upside =
down, I dropped the "drive" half back on the drag cup half and =
reinstalled the 5 little screws. It took a little trial and error to get =
the two pieces together where the drag cup would move smoothly around =
the magnet. The two pieces can only go together one way so all the screw =
holes line up and there are two keying pins to help hold the thing in =
alignment but there still is a slight amount of movement so it's not a =
quick assemlby.  Next, I aligned the mark on the drag wheel to the spot =
on the frame I found before taking it apart at pushed the needle back on =
the shaft. By the way, I didn't put any lubrication on the drag cup =
bearings.
The tach is now running smoothly and the best of all, no noise.
The only problem is that somehow the tach is running about 500 RPM below =
what I think it should be. At idle, the needle just floats off the pin =
from "0". I think I can get a pretty accurate reading of the actual =
engine RPM using a Harbor Freight digital tachometer ($39.00). My =
problem is that I am wondering which way I should turn the needle on the =
drag cup? I have to take the tach out of it's case to move the needle as =
I have to hold the drag cup while I twist the needle on it's shaft and =
don't want to end up taking it apart 10 times..
One way I thought about doing the "calibration" was to pick a RPM, say =
2,500. With the tach out of it's case and the cable out of the firewall, =
have someone hold the throttle at 2,500 RPM and paint a new mark on the =
drag cup. Stop the engine and move the drag cup to the new mark and =
install the needle pointing at 2,500 RPM. At least it would be accurate =
at mid-range.=20
What do you guys think??
Dave Connitt
Thanks,
Dave Connitt
------=_NextPart_000_006D_01D0B717.A8C8FF20
	charset="Windows-1252"
Content-Transfer-Encoding: quoted-printable

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META content=3D"text/html; charset=3Dwindows-1252" =
http-equiv=3DContent-Type>
<META name=3DGENERATOR content=3D"MSHTML 9.00.8112.16659">
<STYLE></STYLE>
</HEAD>
<BODY bgColor=3D#ffffff>
<DIV><FONT face=3DArial>I had a problem a week ago or so where the =
tachometer in=20
my TR4A was making a lot of screaching noises and figured it was a dry =
bushing=20
from 50+ years of life. I figured I would try fixing it myself. What =
could=20
happen??</FONT></DIV>
<DIV><FONT face=3DArial>As I was taking the tach apart, I noticed there =
was a=20
black paint mark on the drag cup that was connected to the needle so I =
just=20
noted where the mark was with the needle "at rest" against a point on =
the frame=20
and continued taking it apart. From reading information on the topic, I =
knew=20
that that little spring attached to the drag cup was very delicate and =
was very=20
careful to hold the tach upside down and lift the "drive" end off the =
drag cup,=20
not the other way around. Once I had separated the frame, I just set the =
drag=20
cup half on the bench in that same orientation. </FONT></DIV>
<DIV><FONT face=3DArial>The other half of the "frame" contains the =
magnet, shaft=20
and bushing which connects to the&nbsp;tach cable. Sure enough the =
little shaft=20
that the magnet was attached to was dry as a bone.&nbsp; The bushing and =
the=20
shaft looked OK so I figured it just needed cleaning and lubrication. =
After=20
careful cleaning of the shaft and it's bushing with some carburetor =
cleaner, I=20
lubricated it with a few drops of 3-1 oil and put the magnet and shaft =
back into=20
the bushing. MUCH BETTER.. The shaft would spin very smoothly by hand.=20
</FONT></DIV>
<DIV><FONT face=3DArial>The assembly went well. Keeping the frame half =
with the=20
drag cup upside down, I dropped the "drive" half back on the drag cup =
half and=20
reinstalled the 5 little screws. It took a little trial and error to get =
the two=20
pieces together where the drag cup would move smoothly around the =
magnet. The=20
two pieces can only go together one way so all the screw holes line up =
and there=20
are two keying pins to help hold the thing in alignment&nbsp;but=20
there&nbsp;still is a slight amount of movement so it's not a quick =
assemlby.=20
&nbsp;Next, I aligned the mark on the drag wheel to the spot on the =
frame I=20
found before taking it apart at pushed the needle back on the shaft. By =
the way,=20
I didn't put any lubrication on the drag cup bearings.</FONT></DIV>
<DIV><FONT face=3DArial>The tach is now running smoothly&nbsp;and the =
best of all,=20
no noise.</FONT></DIV>
<DIV><FONT face=3DArial>The only problem is that somehow the tach is =
running about=20
500 RPM below what I think it should be. At idle, the needle just floats =
off the=20
pin from "0". I think I can get a pretty accurate reading of the actual =
engine=20
RPM using a Harbor Freight digital tachometer ($39.00). My problem is =
that I am=20
wondering which way I should turn the needle on the drag cup? I have to =
take the=20
tach out of it's case to move the needle as I have to hold the drag cup =
while I=20
twist the needle on it's shaft and don't want to end up taking it apart =
10=20
times..</FONT></DIV>
<DIV><FONT face=3DArial>One way I thought about doing the "calibration" =
was to=20
pick a RPM, say 2,500. With the tach out of it's case and the cable out =
of the=20
firewall, have someone hold the throttle at 2,500 RPM and paint a new =
mark on=20
the drag cup. Stop the engine and move the drag cup to the new mark and =
install=20
the needle pointing at 2,500 RPM. At least it would be accurate at =
mid-range.=20
</FONT></DIV>
<DIV><FONT face=3DArial>What do you guys think??</FONT></DIV>
<DIV><FONT face=3DArial>Dave Connitt</FONT></DIV>
<DIV><FONT face=3DArial>Thanks,</FONT></DIV>
<DIV><FONT face=3DArial>Dave Connitt</FONT></DIV></BODY></HTML>

------=_NextPart_000_006D_01D0B717.A8C8FF20--

--===============8609953761789450672==
Content-Transfer-Encoding: 7bit
Content-Disposition: inline

** [email-masked] **

Archive: http://www.team.net/archive

--===============8609953761789450672==--

---

From: "Randall" <[email-masked]>
Date: Sun, 5 Jul 2015 09:31:22 -0700

> One way I thought about doing the "calibration" was to pick a 
> RPM, say 2,500. With the tach out of it's case and the cable 
> out of the firewall, have someone hold the throttle at 2,500 
> RPM and paint a new mark on the drag cup. Stop the engine and 
> move the drag cup to the new mark and install the needle 
> pointing at 2,500 RPM. At least it would be accurate at mid-range. 
> What do you guys think??

What if you just note how far the needle needs to move (eg in 100's of rpm) then take it apart and move the needle that far?  For
example, hold the throttle at 2500 rpm (as indicated by the digital tach) and note the original tach reading, perhaps 2000 rpm.
Then disassemble, hold the cup so the needle reads 2000 rpm; make a reference mark there, then move the needle to indicate 2500 rpm.
You can use the new reference mark to double-check the reading after moving the needle.  I would probably use pencil though, and
erase it afterwards to avoid confusion for the next technician :)

I wanted something a bit more sophisticated, so I could adjust the strength of the magnets as well as the position of the needle, so
I rigged up a cable to the lathe and used it as a variable speed source.
http://i258.photobucket.com/albums/hh260/TR3driver/Speedometer%20repair/DSCF0050.jpg~original

Thought of doing something similar with an old sewing machine motor, but the lathe was easier to link to the cable.

I measured lathe shaft rpm in various gears (with the HF tach), then used a simple spreadsheet to translate that to mph
http://i258.photobucket.com/albums/hh260/TR3driver/Speedometer%20repair/Atlas%20Lathe%20speeds%20rpm%20vs%20mph.jpg~original

Randall