A Complete Guide to Rubber Grommets

Let’s start with the basics - just what is a rubber grommet? They are attachments used in electrical engineering and manufacturing to cover sharp edges. This may have functional benefits - for example, ring-shaped grommets help to avoid cuts and friction damage to wires, cables and pipes.

They also provide additional protection against dust and water. Such grommets usually feature flared collars to hold them in place. Alternatively, edge strip rubber grommets may simply provide a more pleasing appearance to the device where they are attached.

They are typically black, but you will also find transparent and grey varieties.

Rubber grommets have several alternative names, including:

• Edging
• Eyelets
• Bushings or insulating bushings, when used with electrical appliances (up to 51 mm in size)

Amongst the industries to make routine use of rubber grommets are:

• Electronics
• Construction
• Automotive engineering
• Aeroplane engineering and avionics
• Telecommunications
• Internet data centres
• Medical equipment manufacturers

Rubber is a resilient substance and offers several benefits in comparison to other grommet materials. It is resistant to:

• Weather
• Moisture
• Heat
• Low temperatures
• Ozone
• Ultraviolet light (UV)
• Hydrochloric, saltpetre, phosphoric and sulfuric acid
• Alkali environments
• Strain

Rubber does not dry out, crack or come loose easily. Rubber grommets do not require screwing, so they can be inserted more easily, and also do not need locking nuts so can be inserted from one side only. Rubber grommets are push-fit, meaning that they are held in place by internal pressure and expansion only and so require no adhesive.

Rubber grommets are available in several different types. Here are the principal ones:

Rubber Blanking Plugs

Rubber blanking plugs are used to seal holes and are designed to hold and control pressure coming from above or below, making them a good grommet choice for high-pressure UK devices and tubing.

Some feature a small hole into which a wire can be inserted for a firm hold. These are called diaphragm grommets.

Open Rubber Grommets

Open rubber grommets are the most basic design, featuring a central hole. They are used to protect cables and pipes when they pass through holes in metal plating.

Closed Rubber Grommets

Also known as blind rubber grommets, closed grommets provide air-tight seals when applied, and can be a good choice for sealing holes not currently in use to prevent the build-up of dust.

Unlike similarly shaped round grommets, blind rubber grommets have no central hole. They are widely used by the UK electronics industry.

A further variant is the semi-blind grommet. These feature a central membrane that is thin enough to be pierced by wires or cables. This means that semi-blind grommets can take the place of both open and closed grommets.

Tapered Rubber Grommets

Tapered rubber grommets are principally used to blank (seal) holes in metal panels and plates. The tapered design allows them to be used with thicker panels that might be too large for a closed grommet

Stepped Rubber Grommets

Stepped rubber grommets typically have a tapered cone shape, marked at intervals. These lines indicate where the grommet can be cut to different sizes. This versatile design makes them ideal for industries that use multiple cable sizes.

Custom Rubber Grommets

Some manufacturers offer custom rubber grommets, also known as special shape grommets, made to a customer’s unique specifications. This can include particular rubber materials, sizes and designs.

Like most manufactured components, grommets are available in a variety of designs for different tasks. These include:

• Square rubber grommets
• Oblong rubber grommets
• Round rubber grommets
• Rectangular rubber cable grommets
• D-shaped rubber grommets
• Oval rubber grommets

Most grommets are made from rubber, but plastic grommets are available in the UK and elsewhere, along with models made from other materials, including:

• Metal
• Polyvinyl chloride (PVC)
• Nylon

Different types of grommet rubber are also used. These include:

Natural Rubber

This offers good tensile strength and resistance to strain and tears, along with strong electrical insulation.

Nitrile Rubber

Also known as Buna-N, nitrile rubber is highly oil-resistant. This makes it the first choice for grommets when exposure to oil is a possibility. Electrically, it is a semiconductor (a substance with electrical conductivity halfway between an insulator and a metal) and is also antistatic.Additionally, if the grommets are made from NBR nitrile rubber, there is a mechanical industry standard that indicates a shelf life of 4-5 years.

Fluorocarbon Rubber

Also known as FKM or Viton, fluorocarbon rubber is a particularly stable rubber effective that can withstand high temperatures and strongly acid or alkali environments.

Neoprene

This resilient material is resistant to heat, flame, ozone corrosion and weather, making it a good choice for use outdoors and in areas of fire risk.

Silicone

Silicone offers bright colours and it is a friendlier material than some other rubbers. It is odourless and perfectly safe when in contact with food and skin, so silicone grommets are frequently used in kitchen devices, food processing machines and medical equipment.

Rubber grommets are available in a variety of sizes suited to different tasks. Various metrics can be used to measure grommets but for round grommets, the most fundamental are:

1. The maximum cable diameter - i.e. the largest cable that will pass through the central hole
2. The panel hole diameter - the size of the hole into which the grommet can be inserted
3. The maximum panel thickness - i.e. the depth of the metal plate into which the grommet can be inserted, a measurement which corresponds to its length

The latter metric will vary according to design; larger grommets tend to be shallower.

The following rubber grommet chart lists some of the most common sizes in the UK, small to large:

Maximum Cable Diameter	Panel Hole Diameter	Maximum Panel Thickness
3mm	6mm	2mm
4mm	6.4mm	2.5mm
6mm	9mm	4mm
10mm	12mm	2mm
25.5mm (1 inch)	25.5mm	2.4mm
31mm	40mm	2.5mm
48mm	60mm	2.5mm

Grommet strips are more straightforward. When selecting, simply ensure that they fit the width and breadth of the edge to be covered.