The Washington herald., 1921-07-03

PART OF EVERY
CARS MAKEUP
Four Definite Types Now
In Service in
America.
AIR MODEL POPULAR
Magnetic Model Leads in Use
But Has Some
Defects.
AIR MODEL POPULAR

4
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
place. \ typical construction in this
class employs but one moving part
"a circular magnet. Over this
magnet. but not touching IL, is an
inverted metallic cup, on the rim
of which is a row of figures, indi-
eating the miles per hour. The mag
net is revolved by means of a fiex-
ible shaft, its field acting on the
dial cup. As the speed of the mag
net increases, 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. In gen.
eral. a temperature compensation
Is fitted in this type to prevent the
hairspring being affected by eli
matte changes.
Air Model Popular.
Within the past year or so the air
current type of speedOfeter has
been achieving considerable popu-
larity. This type of instrument en-
ploys 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 circulator 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 piv-
oted 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 appropriate
hsures- A hairspring is used to
return the dial to zero when the
car stops
The centrifugally operated speed
emeter is actuated on the same
principle found in the hyball 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 COM
municated through the shaft to the
dial. When the car speed increases
these weights tend to fly out, and
being connected with the dial
through the shaft, pull the former
around to the right and display the
correct figures for the miles per
hour.
Hydraulic Type.
The hydrauli type of speed
ometer has not gained any consider
able following as yet, though its
many excellent qualities may ev-
entually win II recognition.
All speedometers have certain
characteristics in common. The va
ration in type generally refers to
the means employed of actuating
the mileage dial. All these instru-
ments have a connection in the
shape Of a rotating shaft that is
flexible and is housed in a flexible
housing running to some running
front wheel On the end of this
shaft is a gear which meshes with
a gear on the moving part en-
ployed. This shaft is driven from
the cars mechanism and in turn II
drives the mechanism of the speed |
ometer through some sort of joint. |
1t has in the past been the practi- |
caily universal custom to connect
the speedometer with a front wheel, |
but recently the idea of utilizing the
propeller shaft has come into
marked favor. Now some makers
are beginning to connect the speed |
ometer with the transmission unit.
and claims of maximum quiet, per-
feet fabrication and protection are
made for this location.
To Service station.
When some thing goes wrong with4
the internal economy of the speed
ometer itself, the car owner can do
nothing but take il to the service |
station. He would stand just as
much chance of repairing his watch
as this instrument. However, there |
are other arrangements that he can
repair. For instance. the coupling
at the bottom of the speedometer
head sometimes shakes loose, so |
that while the shaft still revolves,
the dial does not register anything. |
To fix this, the end of the shart |
should be loosened from the head,
pushed firmly in place and frightened
again. Again, the flexible shaft
may bind in the housing due to
abrupt bends and then the speed
ometer will fail to register. When
trouble occurs look first at the
shaft and see that it has as few
bends in it as possible. and these
long and sweeping.
Another thin gto watch is in
making a change from one size tire
to another on the wheel that drives
the speedometer. This will make
the reading inaccurate. Even a
change to oversize will cause inac-
curacy. For instance, on one make
OF instrument the makers announce
that IL a change is made from SKI
inch tire to 35x4h, the sixty-eight-
toothed gear used in the former
will have to be changed to a sew
anty-tooth gear.
way to stop Nei.eh.
1t sometimes happens, where the
driving gears are of fiber. that a
noise is caused by loosening of
the parts By using a little strong
laundry soap the gears may be
swollen again. so that they mesh
better. II is a good thing to give
he moving parts a little oil from
lime to time, but where the gears
run exposed. this will not be very
effective.
In some cases after the speedo-
meter has been in service for soune |