Tachometers, or speed
indicators, indicate the speed directly and thus include the time
element. The principal types are centrifugal, liquid, reed, and
electrical. In the centrifugal type, a revolving weight on the end of
a lever moves under the action of centrifugal force in proportion to
the speed, as in a flyball governor.
This movement is indicated by a pointer
which moves over a graduated scale. In the portable or hand type, the
tachometer shaft is held in contact with the end of the shaft being
measured, and in the stationary type, the instrument is either geared
or belted.
In the liquid tachometer of the Veeder
type, a small centrifugal pump is driven by a belt consisting of a
light cord or string. This pump discharges a colored liquid into a
vertical tube, the height of the column being a measure of the speed.
Reed tachometers are similar to
reed-type frequency indicators, the reeds being set in resonant
vibration corresponding to the speed of the machine. The instrument
may be set on the bed frame of the machine, where any slight
vibration due to the unbalancing of the reciprocating or evolving
member will set the corresponding reed in vibration.
Some forms are belted to the revolving
shaft and the vibrations imparted by a mechanical device. Electrical
tachometers may be either reed instruments operated electrically from
small alternators geared or belted to the machine being measured or
ordinary voltmeters connected to small permanent magnet dc generators
driven by the machine being tested.
Chronographs are speed-recording
instruments in which a graphic record of speed is made. In the usual
forms, the record paper is placed on the surface of a drum which is
driven at a certain definite and exact speed by clockwork or weights,
combined with a speed-control device so that 1 in on the paper
represents a definite time.
The pens which make the record are
attached to the armature of electromagnets. With the pens in contact
with the paper and making a straight line, an impulse of current
causes the pen to make a slight lateral motion and, therefore, a
sharp indication in the record.
This impulse can be sent automatically
by a suitable contact mechanism on the shaft of the machine or by a
key operated by hand. The time per revolution is then determined
directly from the distance between marks.
Stroboscopic methods are
especially suitable for measuring the speed of small-power rotating
machines where even the small power required to drive an ordinary
speed counter or tachometer would change the speed, also for
determining the speed of machine parts which are not readily
accessible or where it is not practicable to use mechanical methods
or where the speed is variable.
One convenient form of stroboscopic
tachometer employs a neon lamp connected to an oscillating circuit
supplied from a 60-Hz circuit, which is adjusted to “flash” the
neon lamp at the frequency necessary to make the moving part that the
lamp illuminates appear to stand still. Speeds from a few hundred to
many thousands of revolutions per minute can be very conveniently
measured.
No comments:
Post a Comment