Energy and Motion Control

Elastomers are frequendy subjected to dynamic loads where heat energy and motion control systems are required. One of the serious dynamic loading problems frequently encountered in machines, vehicles, moving belts, and other products is vibration-induced deflection. Such effects can be highly destructive, particularly if a product resonates at one of the driving vibration frequencies.

One of the best ways to reduce and in many cases eliminate vibration problems is by the use of viscoelastic plastics. Some materials such as polyurethane plastics, silicone elastomers, flexible vinyl compounds of specific formulations, and a number of others have very large hysteresis effects (Chapter 3). By designing them into the structure it is possible to have the viscoelastic material absorb enough of the vibration inducing energy and convert it to heat so that the structure is highly damped and will not vibrate.

In each case the plastic is arranged in such a way that movement or flexing of the product results in large deflections of the viscoelastic materials so that a large hysteresis curve is generated with a large amount of energy dissipated per cycle. By calculating the energy to heat it is possible to determine the vibradon levels to which the structure can be exposed and still exhibit cridcal damping.

Plastics exhibit a spectrum of response to stress and there are certain straining rates that the material will react to almost elastically. If this characteristic response corresponds to a frequency to which the structure is exposed the damping effect is minimal and the structure may be destroyed. In order to avoid the possibility of this occurring, it is desirable to have a curve of energy absorption vs. frequency for the material that will be used.

The same approach can be used in designing power transmitting units such as moving belts. In most applications it is desirable that the belts be elastic and stiff enough to minimize heat buildup and to minimize power loss in the belts. In the case of a driver which might be called noisy in that there are a lot of erratic pulse driven forces present, such as an impulse operated drive, it is desirable to remove this noise by the available damping action of plastics and obtain a smooth power curve. This is easily done using a viscoelastic belt that will absorb the high rate load pulses.

Making one gear in a gear train or one link in a linear drive mechanism an energy absorber can use the same approach as the belt. The viscoelastic damping is a valuable tool for the designer to handle impulse loading that is undesirable and potentially destructive to the product.

There is another type of application that has a long history where the damping effect of plastic structures can be used to advantage. The early airplanes used doped fabric as the covering for wings and other aerodynamic surfaces. The cellulose nitrate and later cellulose acetate is a damping type of plastic. Consequently, surface flutter was a rare occurrence. It became a serious problem when aluminum replaced the fabric because of the high elasticity of the metal surfaces. The aerodynamic forces acting on the thin metal coverings easily induce flutter and this was a difficult design problem that was eventually corrected for minimizing the effect.

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