Froude Pendulum

(also friction pendulum), one of the simplest self-oscillating mechanical systems. The Froude pendulum (Figure 1) consists of a compound pendulum (1) that is rigidly fastened to a sleeve (2) mounted on a rotating shaft (3).

Figure 1. The Froude pendulum

At any moment in time, the angular velocity of the shaft exceeds that of the pendulum. Hence, in contrast to the case of a conventional suspension, the friction torque acting on the pendulum has a constant direction and slows down the motion of the pendulum during one half-period, when the pendulum and shaft move in different directions (that is, when the relative velocity of the sleeve is higher), and speeds up the motion during the other half-period, when the pendulum and shaft move in the same direction. If the frictional force is such that it decreases in some velocity range as the velocity increases, the accelerating torque is, on the average, greater than the decelerating torque. This inequality leads to the build-up, or self-excitation, of oscillations. As a result, self-oscillations may be established in the system under appropriate conditions.

The Froude pendulum is named after the English scientist W. Froude (1810–79).