Grommets are eyelets that are used to strengthen or protect an opening, or to insulate and protect an object passing through an opening. They are made from metal, plastic, or rubber materials and are usually round, oval, oblong, square or rectangular. Sizes vary by application. For nonstandard shapes and sizes, the grommet material is often furnished in a strip or roll. Most rubber grommets feature a one-piece construction. Metal and plastic grommets are usually made of two pieces. There are three types of metal grommets, each of which mates with a washer. Plain grommets and washers consist of a domed grommet and flat washer that does not have teeth or spurs. By contrast, rolled rim grommets and washers have interlocking spur teeth to grasp the material. They are less common than plain grommets and washers, but designed for heavier duty applications. Self-piercing grommets and washers are designed to pierce the material without the need for pre-punching a hole.

Materials

Grommets are made from a variety of metal, plastic, and rubber materials. Brass provides good strength, excellent high temperature ductility, reasonable cold ductility, good electrical conductivity, excellent corrosion resistance, good bearing properties and low magnetic permeability. Nickel alloys consist of nickel and one or more other metals, such as copper. Steel, a commercial iron that contains carbon in any amount up to about 1.7 percent, is malleable under suitable conditions and distinguished from cast iron by both its malleability and lower carbon content. Stainless steel is chemical and corrosion resistant and can have relatively high pressure ratings. Butyl, a common term for the isobutylene isoprene elastomer, is resistant to water, steam, alkalis, and oxygenated solvents. Butyl has low gas permeation and is capable of providing both high-energy absorption (dampening) and good hot tear strength. Nitrile and styrene butadiene rubber (SBR) are used with most oils, hydraulic fluids and alcohol. A variety of proprietary compounds are available for specific applications.

How to Select a Grommet

Selecting grommets requires an analysis of physical specifications. Inside diameter (ID) is the diameter of the hole through the center of the grommet. Groove width corresponds to the thickness of the material into which grommets are inserted. Groove diameter is the inside diameter of the groove in the grommet. Other physical specifications for grommets include overall thickness and outside diameter (OD) or length. Width is an important measurement for oval, oblong, square and rectangular grommets. Some grommets are measured in inches (in). Others are measured in centimeters (cm).

Standards

ADS AS1902 - Cable grommets

ADS AS44404-406 - Elastomeric grommets (various), for aeronautical purposes

AN 231 - Grommet & drainage, plastic

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Advanced Antivibration Components - AAC | S-B Group