Application of Dynamic Vibration Absorption - Mechanical Engineering PDF Download

Application of Dynamic Vibration Absorption
n the last lecture, we have discussed about the limitation of vibration neutralizers as they are designed to reduce the vibration response of the primary system at a specific frequency only. Self tunable neutralizers are attractive for applications where there is a possibility of change of excitation frequency.

Self-Tuned Pendulum Neutralizer
It is known that in a multicylinder engine, the time period of the variation of the turning moment depends on the following parameters:

This dynamic turning moment gives rise to torsional oscillations of the crankshaft. In this lecture, we shall discuss how a pendulum-type neutralizer can remain self-tuned at all rotational speeds of the engine and thus control the torsional oscillations of the crankshaft.​
 

Self - Tuning
The most dominant or the primary exciting frequency of a multicylinder engine is given by
w = aNn₀ , where a = 1 for two stroke cycle and 1/2 for a two stroke cycle      (17.1)
N = number of cylinders, and = rotational speed of the engine, neglecting all the higher harmonics of the system. Now, the natural frequency of a gravity pendulum of length " l " is  where is the acceleration due to gravity.


Figure 17.1: Simple pendulum

If the pendulum moves in a horizontal plane with its hinge point O' (Fig. 17.1)) rotating at a speed n₀ with radius r, then the gravity field (g) is replaced by the centrifugal field
( ). The natural frequency (w_n) of this centrifugal pendulum becomes
                                                        (17.2)
Comparing Eqns. (17.1) and (17.2), we see that the necessary condition for self-tuning is

Torsional vibration of crankshaft has been controlled in industrial applications by using a secondary mass in the form of a loose flywheel which is coupled to the vibrating shaft by
1. pendulum damper
2. coulomb friction ( Lanchester damper)
3. viscous fluid friction (Houdaille damper)

Tuned mass dampers are largely used in vibration control of crankshafts, hand-held devices and transmission cables. A few such applications are discussed here:

Houdaille Damper

Fig 17.2 : Houdaille Damper

In this type of dynamic vibration absorber, a flywheel (as secondary mass) is coupled to the primary crankshaft with fluids as shown in the figure above. The damping constant is given by:

Where, μ is the viscocity of the fluid, the other dimensions are indicated in the figure above.


Damping of hand-held devices
Electromagnetic Motors are used extensively to power hand held devices such as Hair clipper, Dry Shaver and similar instruments. Usually, the motors operate at a fixed frequency such as 60 Hz. 

                Figure 17.3: Electric hair clipper

The figure above shows a typical electric hair clipper. In such hair-clippers, an electro-magnet is used to develop vibrating force for cutting. However, this also generates an unpleasant vibration of the housing. This vibration is neutralized by the application of a pair of mass dampers fixed to the housing at two different points.


TMD for floor vibration control
Floor vibration in a building are considered harmful due to several reasons:

• impair the operation of sensitive instruments
• affect human psychology for acceleration level even greater than 0.005g
• motion having peak amplitude greater than 1 mm is not desirable in active environments.

​Traditional methods for improving the environment include

• adding extra columns
• adding extra thickness to the floors and
• increasing structural stiffening

In a new application, TMDs are used bellow the floor of a building as shown in the figure.

         Fig 17.4 : Houdaille Damper

The TMDs are tuned at a frequency of 6Hz which is the first resonating frequency of the floor. The change in floor vibration (acceleration response) after the application of the TMD is evident in the green curve in comparison to the blue curve showing under-damped response before the application of TMD.


            Figure 17.5: Vibration suppression by TMD