SM0486
(8) Substituting
This formula gives the following statement: Radians per second equal the velocity
of a point on the circumference divided by the radius where the velocity is in the
same units (per second) as the radius. The formula (3) can be solved for v, giving
v = r
This is a convenient expression for arriving at the speed of a point on a rotating
body, such as the tip of an airplane propeller.
To relate to rpm, the following
expression may be used:
where
is radians per second. Now that you have looked at the basic principles of
rotation, look at the most simple of rotary measuring devices, the mechanical
tachometer.
(3) The Mechanical Tachometer.
A mechanical tachometer is a device which
normally consists of a shaft which is physically pressed against the center of a
rotating shaft.
By means of gearing or a pendulum arrangement, information is
translated to an RPM indicator.
a. The pendulum type of mechanical tachometer operates on the principle
shown in Figure 3. You may recognize it as the old governor principle.
Figure 3.
The basic principles of a pendulum type mechanical tachometer
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