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