2-9
Figure 2-5.—Centrifugal tachometer.
Each portable centrifugal tachometer has a small rubber-covered wheel and a number of hard rubber
tips. You fit the appropriate tip or wheel on the end of the tachometer drive shaft, and hold it against the
shaft to measure speed of rotation. As the drive shaft begins to rotate, the fly weights rotate with it.
Centrifugal force tends to pull the fly weights away from the center, causing the lower collar to rise and
compress the spring. The lower collar is attached to a pointer, and its upward motion, restricted by the
spring tension, causes an increase in the indication on the dial face.
When properly used, a centrifugal tachometer will indicate correct shaft speed as long as it is in
contact with the machine shaft under test. A portable centrifugal tachometer has three ranges: low (50 to
500 rpm), medium (500 to 5,000 rpm), and high (5,000 to 50,000 rpm).
CHRONOMETRIC TACHOMETER.—The chronometric tachometer (figure 2-6) is a
combination watch and revolution counter. It measures the average number of revolutions of a shaft per
minute. The chronometric tachometer also comes with hard rubber tips, which must be inserted over the
drive shaft.
