TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
play is necessary to prevent excessive friction.
bearing may be caused by undue wear, corrosion, or
When the pinion or worm is short, or the worm is
rubbing. Clean and lubricate or replace parts as
concave to match the curvature of the worm wheel,
required. The lower bearing assembly ordinarily
the height of the moving elements should be set so
consists of a stationary sapphire jewel with a pivot
that the worm and pinion do not bind. For cleaning
on the rotating element, or two jewels with a steel
the pinion or worm, a small stiff brush or a sharp-
ball between them. Excessive friction here may be
ened piece of soft wood may be used.
caused by damaged or grooved jewels, worn or de-
(d) If the light load test indicated friction,
fective balls or pivots, or the presence of foreign
clean all bearings and replace if necessary.
material on the jewel. When replacing a sapphire
(e) Check the position of the disk in relation
bearing, pivots and balls should both be replaced
to the magnet poles. Center and align if necessary.
since they have probably been damaged by rough
(f) Calibrate the meter, making all necessary
bearings. When the jewel is still perfect, the pivot or
adjustments described previously.
ball may be worn. Do not use your hands to manipu-
(g) Clean the cover and examine for defects.
late balls and pivots which can be damaged by such
Replace gasket washers if necessary.
handling. Pivots can be inserted with a wrench pro-
(h) When work is complete, take a reading,
vided for that purpose. Balls may be inserted into
known as an "as left" reading, and record the data.
bearings by use of a ball dropping device. The lower
b. Lubrication. Use lubricants on electromechani-
bearing should be oiled by filling the bearing cup
cal units with extreme caution.
with an oil recommended by the manufacturer, but
(1) Instruments. The shafts of indicating in-
lower bearings of the ball type should never be
oiled. Care should be taken no to mix oils of differ-
struments are usually supported by pivot bearings,
which normally do not require lubricating.
ent characteristics. Several manufacturers make oil
(2) Meters. If the bearings of meters become
and special oilers for jewels and synchronous tim-
ers. Cleaning fluids are also available from several
defective, friction will be increased causing the
commercial sources to remove dirt and grit from
meter to run slow. The function of the top bearing is
bearings and balls.
to act as a centering guide. Excessive friction on this
12-15. Temperature influence on instruments
Hence, these ammeters are virtually independent of
ordinary temperature changes. Electrodynamom-
eter type instruments have controlling springs that
Almost all electromechanical instruments are influ-
are the only important element with respect to the
enced by temperature changes to some degree. Take
temperature coefficient. Such instruments will read
this into account when recording instrument read-
slightly lower at temperatures below that at which
they are calibrated. The temperature effect on mod-
a. Direct-current instruments. The temperature
ern ac portable standard watthour meters is negli-
error of dc voltmeters is principally due to changes
gible throughout the normal temperature range.
in the tension of the springs. The change in electri-
12-16. Stray-field influence on instruments
usually negligible, since this circuit consists of re-
sistance wire having an extremely low temperature
coefficient. Direct-current ammeters usually have
Stray fields produced by other instruments, by con-
higher temperature coefficients than voltmeters, be-
and motors may cause an appreciable error in in-
cause their electrical circuits are composed chiefly
of low-resistance copper.
strument readings. Even nonmagnetized masses of
b. Alternating-current instruments. The movable
iron may influence the flux in an instrument. When
iron voltmeter designed for use at commercial volt-
it is necessary to make tests in places subject to
ages has a very small temperature coefficient,
strong stray fields, the instrument should be read,
then turned 180 degrees, read again, and an aver-
which normally does not need to be taken into ac-
count. An increase of temperature lowers both the
age taken. Some instrument of the electrody-
permeability of the iron and the torque, and at the
namometer type are made astatic (independent of
same time reduces both the strength of the spring
position), in order to avoid the errors caused by
and the counter torque by nearly the same amount.
stray fields. Two movable coils are connected so that