One of the design decisions he discourages is the use of housed springs, which eventually can lead to metal-to-metal contact. "We prefer open springs with nothing on the side," he says. "You can handle lateral control with a device called a 'snubber.'"

Care must also be taken to match the spring to its load so it doesn't bottom out and lose its ability to compress. Installation and ongoing maintenance are also important to keep springs aligned correctly and functioning as intended.

While effective for small, high-frequency pumps, stiff pads—even those of laminated neoprene and cork--are not appropriate for large devices. They can cut down on the equipment noise, but they don't afford adequate protection in vibration-sensitive areas.

Successful vibration control typically reflects scrupulous attention to detail. For example, to isolate the vibration generated by water flow through pipes installed in the subfab under the Duffield Hall cleanroom, a metal frame around the pipe cluster attaches to a small box containing a spring suspended from the ceiling by a connecting rod.

Elsewhere, a device called a twin-sphere pipe flex cuts down on the vibration transmission from one end of a pipe to another via a rubber pipe connector, stabilized by rings outside the rubber to keep the rubber from expanding outward and lateral rods that control expansion in the long direction of the pipe.

Pumps in the Duffield Hall basement sit on inertia bases (metal frames filled with concrete), while spring isolators, attached to the bases at the corner, rest on neoprene disks. "The neoprene keeps higher-frequency vibrations from going into the floor, while the springs handle low frequencies," says Amick, adding that inertia bases are almost always used on pumps and sometimes on fans.

Despite the need for meticulous workmanship, there is some latitude in installation techniques. For instance, in one area where space is tight, springs have been installed in cut-out sections of the base's corners so they don't take up much room. In another installation where space is not an issue, the springs are located several inches out from the base, extending the footprint of the motor assembly whose vibrations they isolate.

NZS

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