Isolating And Damping Vibrating Elements

In all but the simplest machines, the vibrational energy from a specific moving part is transmitted through the machine structure, forcing other component parts and surfaces to vibrate and radiate sound—often with greater intensity than that generated by the originating source itself.

Generally, vibration problems have two parts. First, energy transmission must be prevented between the source and surfaces that radiate the energy. Second, the energy must be dissipated or attenuated somewhere in the structure. The first part of the problem is solved by isolation. The second part is solved by damping.

The most effective method of vibration isolation involves the resilient mounting of the vibrating component on the most massive and structurally rigid part of the machine. All attachments or connections to the vibrating part, in the form of pipes, conduits, and shaft couplers, must have flexible or resilient connectors or couplers. For example, pipe connections to a pump that is resiliently mounted on the structural frame of a machine should be made of resilient tubing and mounted as close to the pump as possible. Resilient pipe supports or hangers may also be required to avoid bypassing the isolated system (see Figures 6.6.7 and 6.6.8).

Damping material or structures are those that have some viscous properties. They tend to bend or distort slightly, thus consuming part of the noise energy in molecular motion. The use of spring mounts on motors and laminated galvanized steel and plastic in air-conditioning ducts are examples.

When the vibrating noise source is not amenable to isolation, as in ventilation ducts, cabinet panels, and covers, then damping materials can be used to reduce the noise.

The type of material best suited for a particular vibration problem depends on several factors such as size, mass, vibrational frequency, and operational function of the vibrating structure. Generally speaking, the following guidelines should be observed in the selection and use of such materials to maximize vibration damping efficiency (see Figure 6.6.9):

Damping materials should be applied to those sections of a vibrating surface where the most flexing, bending, or motion occurs. These areas are usually the thinnest sections.

For a single layer of damping material, the stiffness and mass of the material should be comparable to that of the vibrating surface to which it is applied. Therefore, single-layer damping materials should be about two or three times as thick as the vibrating surface to which they are applied.

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