Less noise and vibration with foam

Nov. 22, 2002
Innovative foams not only damp noise and vibration they also filter particles and act as heat shields, cushions, and seals.

Foam is versatile. Through innovative manufacturing and fabrication techniques, foam can take on a variety of properties. Foam can be water resistant, absorbent, porous, and flame retardant either through formulation or by cutting, laminating, impregnating, felting, and thermoforming.

Foam consists of individual cells or pores which have been completely polymerized and solidified to form a skeletal structure. Varying foam-cell structure alters physical properties to meet specific applications. This lets foam improve machinery performance or help equipment meet a variety of health and safety standards.

Foam's versatility was key in the design of a generator set that provides emergency and standby power to schools and hospitals. The generator uses several different foams to meet requirements for filtering, gasketing, cushioning, noise and vibration-absorption, and heat shielding.

Outfitting the generator
Reticulated polyurethane provides a two-part foam filter that protects the generator engine from harmful particles. A prefilter of open-cell foam (10 parts-per-inch (ppi)) catches leaves, dirt, and other particulates that enter the air intake. A fine-grade final filter (72 ppi) is oiled to help trap dust and other fine particles. Specially fabricated in metal-framed sheets, the two-part filter protects the engine from tiny contaminants but doesn't impede airflow.

Polyethylene gaskets ensure a proper seal. Dense and semirigid, closed-cell polyethylene creates a housing seal between the motor and the generator, preventing moisture and particles from entering the moving assemblies. Polyethylene used in this application is a special variety. During manufacture, the material is "crosslinked." Crosslinking makes the foam more durable, flexible, and prolongs its life. Soft and pliable yet dense and strong, closed-cell foams seal and conform to irregular surfaces better than open-cell foams.

A proprietary melamine-based foam called willtec serves as thermal insulation. It withstands up to 302*F (maximum sustainable). The foam is also Class 1 fire-rated (ASTM E-84) for flame spread and smoke density, which makes it an ideal heat shield when applied next to the generator and engine. The open-cell structure gives the foam a low density (0.62 lb/ft3), making it easy to fabricate for special applications. Additionally, willtec foam resists organic solvents and a variety of diluted acids and alkali.

Vinyl composite dampers serve a dual purpose by both absorbing sound and damping vibration. Viscoelastic damping materials applied directly to the surface of the vibrating structure absorb and distribute vibration or shock while helping reduce any noise present.

Damping materials, about 1/8-in. thick, can substantially reduce structure-borne vibrations before they produce annoying sound or destructive resonance. When bonded to inch-thick open-cell polyurethane foam, the composite provides a nonreflective surface that absorbs air noise from gears, linkages, fan blades, and motors. It also reduces vibration. Vibration dampers and sound-absorbing foam should be selected so they both absorb in the same low-frequency range produced by the machinery.

Polyurethane foam is perhaps the most widely used foam material. Open-cell polyurethane foams are odorless, breathable, fire retardant, and resist bacteria, dust, and oils. They also absorb sound. When applied on the inside walls of the generator, the polyurethane foam absorbs noise before it reaches outside. Hospitals, schools, telecommunications, and other buildings that rely on standby power can operate the generator with minimal interference from operating noise -- at 23 ft, the sound pressure level of a 150-kW generator is reduced to approximately 72 dB. Circle 622

Self-lube silicone valves and seals

A new family of self-lubricating liquid silicone rubbers comes from Nusil Technology Inc., Carpinteria, Calif. (www.nusil.com). The new line was developed in part to prevent vulcanized silicone surfaces that touch from sticking or blocking to each other. The new silicones will serve as valves, seals, and gaskets as well as O-rings, septums, and stoppers in a variety of health-care applications. The family of self-lube silicones is currently being graded based on Durometer (15 to 60 Shore A) and lubricant loading (high or low).

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