proof meter

[GERRY]

i 2 profmeters both working , the needels go anti clockwise ,
and they swing L/R at the bottom of the clock. they are both the same except
one says 1412rpm and the other 1450rpm , is one of these for a super
and other for standered and if so which is correct for S/D mybe someonewould know why the
differance . GERRY

[Jerry Coles]

Both proofmeters indicate engine RPM for both Super and standard Dexta as they both use the same Fuel Injection pump and it is driven from the engine crankshaft.
What you are seeing is a minor difference in tolerance between the two. As the internals comprise of a spinning disk which 'drags' the needle round I wouldn't worry about the indicated difference as it's probably due to friction/age/wear. your only loking at a difference of 38 revs per min which is a matter of 1.58 revs every sec.
From Wikepedia: For Speedometer read Proofmeter/tacho
The eddy current speedometer has been used for over a century and is still in widespread use. Until the 1980s and the appearance of electronic speedometers it was the only type commonly used.
A small permanent magnet affixed to the rotating cable interacts with a small aluminum cup (called a speedcup) attached to the shaft of the pointer on the analogue instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field. The effect is that the magnet "drags" the cup, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.
The pointer shaft is held toward zero by a fine torsion spring. The torque on the cup increases with the speed of rotation of the magnet (which is driven by the car's transmission). Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. The cup and pointer will turn until the torque of the eddy currents on the cup is balanced by the opposing torque of the spring, and then stop. Since the torque on the cup is exactly proportional to the car's speed, and the spring's deflection is proportional to the torque, the angle of the pointer is also proportional to the speed. At a given speed the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial.

[Lesfen]

Gerry,

From the ones I have seen the Smith models are rated at 1450rpm and the AC are 1412rpm to rollover one hour on the clock.
Are your tachs possibly from early Dextas (pneumatic pumps) with the hours at the top?

Our '62 Super Dextas have the hours at the bottom and the needle sweeps the top CCW from about 4 o'clock to 8 o'clock. They are AC and take 1412 revs to register an hour on the meter. (maybe all the minimec Dextas had the hours at the bottom?)

Les

Now I just came across one with hours at the bottom and needle sweeps the bottom!

[GERRY]

Thanks for you replies , one of the clocks sweeps from about 10,oclock to roughfuly 2,oclock
l/r bottom sweep
with window for hrs worked at the top , 1450rpm, the other
the other one is the same sweep L/R but is higher about 11oclock to 1.oclock
with hrs window at bottom . the window is longer on this one, it is 1412 rpm
both came off dextas but idont know if they were standered or super, ithought
id like to put the right one on my super if there is a differance,

[Bensdexta]

There's some more info in these threads:
http://www.fordsontractorpages.nl/phpbb ... ter#p19293
http://www.fordsontractorpages.nl/phpbb ... o&start=25

Here a pic of my tacho:

1961 Dexta pneumatic fuel pump

[GERRY]

That is the same as one of them Ben the other is as lesfen says ac wrote on it,
an 1412 rpm , as Gerry Coles says it is just amakers differance , this will mean that both are
off the standered pn/goverened dexta. was there a different one for the super dexta,
or was there no change fror super. Gerry

[Bensdexta]

Have a look at the threads I mentioned and the Dexta Parts List.

I think you will see a number of different part nos for the Dexta/ Super D tachos over the years. Sorry I can't tell you the what the differences are - might be worth contacting Speedograph Richfield who are the acknowledged experts.

[Lesfen]

Our '62 Super Dexta tach (not working!):




Les

[bobfaecullen]

Hi All,

Here is another version of the proofmeter. This one has the pto speed at 1600 instead of the later 1800, it is on an early 1958 dexta.
It's an AC unit with service hours at 1412rpm

[GERRY]

The last picture of the proof meter by bob face cullen is the same as my other one ,
this confirms that they are both from the earlier type standered dexta still leaves the ?
was there another for the s/d an mybe another for the nps/dexta, or did they all use these
two types right through from 57/64.

[Jerry Coles]

I think we have to go back to the production days and Fordson would probably buy in a match from different manufacturers when available, a sort of "just in time" planning, hence the different styles and manufacturers.
My tuppenywoth.
Jerry
Bath
UK

[Bensdexta]

was there another for the s/d an mybe another for the nps/dexta, or did they all use these
two types right through from 57/64.

Not according to the parts list (see Stefan's site). Standard & Super Ds have the same parts no, but there is was change for the standard in 1960. The tacho cable changed I guess when the mech pump came in, in April 1962.