Rick:
Because the tach is not geared directly to the cam. The tach
*CABLE* is geared to the cam, but from the cable to the needle it is not
physically connected AT ALL.
The cable spins a magnet inside the tach. The magnet is directly
connected to nothing except the cable. The needle on the tach is
connected to something called a "drag cup". It looks sorta like a
chewing tobacco can lid. The drag cup is lightly spring loaded to force
the tach needle to 0. The magnet is very close proximity to, but not
actually touching, the drag cup.
When the engine is running, the cam turns the cable, the cable
turns the magnet. The magnet motion creates a moving magnetic field
which penetrates the metal drag cup, inducing "eddy currents". These
eddy currents are themselves magnetic, so in effect the drag cup becomes
magnetized, and it then tries to follow the motion of the magnet. This
moves the needle on the tach. This motion is opposed by the spring.
The calibration then becomes a matter of balancing the drag cup
force with the spring, such that the needle is pointing to the correct
RPM numbers on the dial. Obviously, this requires a precise balance of
the spring with the eddy current forces, hence the need to calibrate the
tach.
Cheers,
Vance
Here is a nifty science demo - take an aluminum pie tin. Place
it upside down on the tip of a pin, and dimple the center the tin such
that the pie tin is balanced on the tip of the pin. Take a horse shoe or
bar magnet and suspend it on a string, centered above the pie tin with
both the north and south pole of the magnet the same distance from the
pie tin. Spin the magnet. Slowly, the aluminum pie tin will begin to
rotate to follow the magnet, even though aluminum is not magnetic. Why?
The aluminum is temporarily magnetized by the eddy currents induced in
the aluminum by the moving magnetic field. It is magnetic as long as the
magnet is motion and close to the aluminum.
-----Original Message-----
From: trinitygadget@alltel.net [mailto:trinitygadget@alltel.net]
Sent: April 19, 2005 11:07 AM
To: Navarrette, Vance; Steve Lindquist; 6pack@autox.team.net
Subject: Re: RE: Speedometer out of calibration?
Help me out here . . . how can our mechanical tach not be spot-on
"calibrated" when it's geared directly to the cam via the distributor
drive?
Rick O.
72 TR6
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