The Washington herald., 1921-06-12
MAGNETIC SPEEDOMETERS GIVE
LITTLECAUSEFOR REPAIR WORK
DESPITE DELICATE MECHANISM
knowledge of Different Types Needed by Auto
Owner When Anything Goes Wrong.

Among the subsidiary mechanical
| units of the modern motor car
which only gives trouble on occa
sion, must be included the speed
ometer. In spite of the delicate ma
ture Of this instrument it is not
often that the motorist has occasion
to attempt repairs of it. However
when such an occasion does arise,
some knowledge of the device is
essential.
At the present time there are in
service four definite types of speed
ometer; magnetic. centrifugal, hy-
draulic and that driven by air cur
rent. The magnetic type Of speed
ometer is probably the most popu-
lar type, as far as the number of
makes of cars on which it finds a
place. A typical construction in
this class employs but one moving
part-a circular magnet. Over this
magnet, but not touching it, is an
inverted metallic cup, on the rim Of
which Is a row of figures, indicating
the miles per hour. The magnet Is
revolved by means of a flexible
shaft, it's field acting on the dial
cup. As the speed of the magnet in
creases So does the magnetic pull,
and the dial cup Is drawn around to
indicate the increasing speed. The
dial Is mounted on a steel shaft
with a jewelled bearing at the top.
A hairspring is fitted to resist the
pull of the magnet and this also
serves to return the cup to zero
when the car stops. s1n general a
temperature compensation is fitted in
this type to prevent the hairspring
being affected by climatic changes.
Within the past year or so the
air current type Of speedometer has
been achieving considerable popu-
larity. This type of instrument
employs a stream of air to turn the
dial. which indicates the mileage.
A typical construction in this
class has a flexible shaft driving an
air circulstor consisting of two
elongated aluminium gears placed
in the head of the instrument and
opening into an air chamber. This
latter has two openings. one an en-
trance for the air. the other an
exit. A dial fitted with a vane is
riveted in the chamber. A current
of air is forced into the chamber by
the revolutions of the gears. and
this stream striking the vane re-
wolves the dial to show approx
mate figures. A hairspring Is used
to return the dial to zero when the
car stops
The centrifugally operated speed
ometer is actuated on the same
principle found in the fyball gov
ernor. Weights are mounted on the
moving shaft by means of levers or
cranks. so that the movement OF
the weights by centrifugal force is
through the shaft to
the dial. When the car speed in
creases these weights tend to fly
out. and being connected with the
dial through the shaft. pull the for
mer around to the right and dis
play the correct figures for the
miles per hour.
The hydraulic type of speedome-
ter has not gained any considerable
as yet, though its many
excellent qualities may eventually 1
win il recognition.
All speedometers have certain
in common: the va
riations in type generally refer to
the means employed of actuating
the mileage dial. All these instru-
nents have a connection in the
shape of a rotating shaft that is
iexible and is housed in a flexible
housing running to some running
part of the mechanism. usually the
'front.' wheel. On the end of this
'haft is a gear which meshes with
gear on the moving part en-
'loyed. This shaft is driven from
he cars mechanism and in turn II
irives the mechanism of the speed
'meter through some sort of joint. |
he's in the past been the practicg |
lily universal custom to connect the |
Ipeedometer with front wheel, but |
recently the idea Of utilizing the |
propeller shaft has come into |
favor. Now some makers
re beginning to connect the speed |
'meter with the transmission unit.
md claims of maximum quiet, per- |
ect fabrication and protection are
nsde for this location.
When something goes wrong with
he internal economy of the speed- |
meter. itself the car owner can DO
Iothing but take 1t to the service
tation. He would stand just as
much chance Of repairing his watch
s this instrument. However. there
re other arrangements that he can
epair. For instance. the coupling
I the bottom of the speedometer
cad sometimes shakes loose. so that
he shaft sail revolves but the dial
oes not register anything. Totfx
MS. the end of the shaft should be
'osed from the head. pushed firmly
place and frightened again. Again,
,e flexible shaft may bind In the |