A grommet is a ring or edge strip inserted into a hole through thin material, typically a sheet of textile fabric, sheet metal and/or composite of carbon fiber, wood or honeycomb. Grommets are generally flared or collared on each side to keep them in place, and are often made of metal, plastic, or rubber. They may be used to prevent tearing or abrasion of the pierced material or protection from abrasion of the insulation on the wire, cable, line being routed through the penetration, and to cover sharp edges of the piercing, or all of the above.
A small grommet may also be called an eyelet, used for example on shoes, tarps and sails for lacing purposes. In electrical applications these are referred to as "insulating bushings". Most common are molded rubber that are inserted into small hole diameters up to 2" in diameter. There are many hole configurations from standard round to assorted U-shapes. Larger penetrations that are irregular in shape as well as long straight edges often use extruded or stamped strips of continuous length. These Continuous length materials are referred to as "grommet edging".
These are quite common in applications that range from telecom switches and data center cabinets to complex and dense wire/cable and even hydraulic tubing in aircraft, transportation vehicles and medical equipment.
Grommets are used to reinforce holes in leather, cloth, shoes, canvas and other fabrics. They can be made of metal, rubber, or plastic, and are easily used in common projects, requiring only the grommet itself and a means of setting it with a punch, a metal rod with a convex tip. A simple punch, often sold with the grommets, can be struck with a hammer to set the grommet. It can also be set with an electronic, pneumatic, or gas-powered machine. There are also dedicated grommet presses with punch and anvil, as shown in the picture, ranging from inexpensive to better-quality tools, which are somewhat faster to use. They are used to strengthen holes; in footwear for boot and shoe laces, in laced clothing such as corsets, and in curtains and other household items that require hanging from hooks, as when they are used in conjunction with tensioner rods for shower curtains. The grommet prevents the cord from tearing through the hole, thereby providing structural integrity. Small grommets are also called eyelets, especially when used in clothing or crafting. Eyelets may be used purely decoratively for crafting. When used in sailing and various other applications they are called cringles. Sometimes field workers may refer to them as grunyons.
If metal or another hard material has a hole made in it, the hole will probably have sharp edges. Electrical wires, cord, rope, lacings, or other soft vulnerable material passing through the hole can become abraded or cut, or electrical insulation may break due to repeated flexing at the exit point. Rubber, plastic or plastic coated metal grommets are used to avoid this. The grommet could also protect the wiring/cabling from contamination from dirt, air, water, etc. The smooth and sometimes soft inner surface of the grommet shields the wire from damage.
Grommets are generally used whenever wires pass through punched/drilled sheet metal or plastic casings for this reason. Molded and continuous strip grommets, also known as edge grommets, are manufactured in a wide variety of sizes and lengths expressly for this purpose; they are usually a single piece which can be inserted by hand. Two-piece hard plastic devices are available which also grip the wire that passes through. These are called strain relief bushings and are often used to insulate, anchor, and protect power cords where they enter panels. Preventing a tug or twist on the wire from stressing the electrical connections inside the connected equipment. Sleeved grommets have a flexible extension (sleeve), usually tapered or moulded to flex increasingly towards the free end in order to reduce fracturing of electrical insulation.
Grommets made of rubber or other elastic material are also used to minimize the transmission of vibration. They were widely used for mounting shock-sensitive computer disk drives, particularly in equipment subject to vibration or jarring, but are not usually used with more robust modern drives. The screws that hold the drive in place pass through grommets that decouple it acoustically from the chassis. Grommets are used in a similar way to acoustically isolate electronic circuit components that are susceptible to microphonism caused by mechanical vibration or jarring.
In chronic cases of otitis media with effusions present for months, surgery is sometimes performed to insert a grommet, called a "tympanostomy tube" into the eardrum to allow air to pass through into the middle ear, and thus release any pressure buildup and help clear excess fluid within.
This is also a correcting measure for a patulous Eustachian tube (when air moves to and from the middle ear with each breath making the eardrum flap).