Page 167 - Thomson, William Tyrrell-Theory of Vibration with Applications-Taylor _ Francis (2010)

154 Systems with Two or More Degrees of Freedom Chap. 5

a wheel of rotational inertia:
m{R + r)
‘^eff — (5.7-8)
1 - rco^/Rn^
which can become infinite at its natural frequency.
This poses some difficulties in the design of the pendulum. For example, to
suppress a disturbing torque of frequency equal to four times the rotational speed
n, the pendulum must meet the requirement co^ = (4n)^ = n^R/r, or r/R =
Such a short effective pendulum has been made possible by the Chilton bifilar
design (see Prob. 5-43).

5.8 VIBRATION DAMPER

In contrast to the vibration absorber, where the exciting force is opposed by the
absorber, energy is dissipated by the vibration damper. Figure 5.8-1 represents a
friction-type vibration damper, commonly known as the Lanchester damper, which
has found practical use in torsional systems such as gas and diesel engines in
limiting the amplitudes of vibration at critical speeds. The damper consists of two
flywheels a free to rotate on the shaft and driven only by means of the friction
rings b when the normal pressure is maintained by the spring-loaded bolts c.
When properly adjusted, the flywheels rotate with the shaft for small oscilla­
tions. However, when the torsional oscillations of the shaft tend to become large,
the flywheels do not follow the shaft because of their large inertia, and energy is
dissipated by friction due to the relative motion. The dissipation of energy thus
limits the amplitude of oscillation, thereby preventing high torsional stresses in the
shaft.

Figure 5.8-1. Torsional vibration
damper.