AN931 Series Grommets

线套 AN931 系列

Our AN931 Series Grommet Size list

The AN931 is one of the most classic and common Army-Navy Aeronautical Standards in the US aerospace sector. Although it is often superseded by MS35489 in modern designs, AN931 remains synonymous with "rubber grommets" in General Aviation, vintage aircraft restoration, and general industrial applications.

Here is a deep technical analysis of the AN931 series:

1. What is an AN931 Grommet?

The AN931 is an elastic grommet made from synthetic rubber.

  • Primary Function: When wires, cables, or tubing pass through drilled holes in metal bulkheads or firewalls, the AN931 is installed to cover the metal edges. It prevents the wire insulation from being worn down or cut by sharp metal edges (Chafing Protection), while also providing vibration dampening and sealing capabilities.

  • Structure: It features a typical "donut" shape with a Groove around the perimeter designed to snap onto the edge of the metal sheet.

2. Material Specifications

For a rubber manufacturer, this is the most critical section. The AN931 must meet specific physical property standards.

  • Applicable Specifications: Historically corresponds to MIL-G-3036 or AMS 3205/3208.

  • Common Materials:

    • NBR (Nitrile/Buna-N): The most common material. Since it is used in aircraft and mechanical environments, it must have excellent oil resistance (specifically against aviation hydraulic fluids and fuels) and coolant resistance.

    • CR (Neoprene/Chloroprene): Also frequently used due to its good weather resistance, ozone resistance, and flame retardancy.

  • Hardness: Typically soft to facilitate manual installation. The range is generally between 40-60 Shore A. This is significantly softer than standard industrial O-rings (70 Shore A), allowing for greater elastic deformation to fit into holes.

  • Color: Typically Black.

  • Temperature Range: Generally -40°C to +120°C (depending on the specific compound).

3. Sizing & Designation

The naming convention for AN931 typically follows the AN931-Dash Number format. Understanding the logic behind the Dash Number is crucial for selection.

The dimensions of an AN931 are determined by the following parameters:

  1. ID (Inside Diameter): The size of the hole for wires to pass through.

  2. OD (Outside Diameter): The total outer diameter.

  3. GW (Groove Width): Corresponds to the thickness of the metal plate.

  4. GD (Groove Diameter): Corresponds to the diameter of the hole drilled in the metal plate.

Naming Examples & Logic: While Dash Numbers do not represent direct inch values, they follow a sequence:

  • AN931-3: ID 3/16"

  • AN931-4: ID 1/4"

  • AN931-6: ID 3/8"

  • AN931-8: ID 1/2"

Note: Most AN931s are designed for specific sheet metal thicknesses (usually thin sheets, around 1/16" to 3/32"). If the plate is too thick, a special specification or the MS series may be required.

4. Relationship between AN931 and MS35489

As an expert, you must know the distinction between these two:

  • AN931 (Legacy Standard): Army-Navy Standard. In many modern military procurement drawings, it is marked as "Inactive for new design."

  • MS35489 (Current Standard): Military Standard. This is the direct replacement for the AN931.

  • Interchangeability: In most cases, MS35489 and AN931 are completely interchangeable in terms of dimensions and function. For example, an AN931-4 is functionally equivalent to an MS35489-4.

  • Differences: MS35489 material specifications are often more modern, with more explicit requirements for environmental resistance (such as ozone aging). Many "AN931" products on the market today are actually manufactured to MS35489 standards and dual-marked.

5. Manufacturing & Application Troubleshooting

If you are manufacturing or selling these products, you may encounter the following issues:

  1. Blooming: Due to excess wax or antioxidants in the formula, a white film may appear on the surface after long-term storage. While this does not affect performance, customers may mistake it for aging.

  2. Cracking: This is caused by ozone aging. If a cheap NBR formula is used without sufficient antiozonants, cracks will appear after a few months when installed in a stretched state. It is recommended that the formula design explicitly passes the ASTM D1149 ozone test.

  3. Tolerance Control: Because this is a low-hardness product, controlling mold shrinkage is difficult. If deflashing/trimming is excessive after vulcanization, the Groove Diameter (GD) may become too small, causing the grommet to be loose or fall out after installation.

The AN931 is a classic, low-hardness, oil-resistant synthetic rubber grommet. While it looks like a simple rubber ring, it plays a vital role in preventing electrical shorts and mechanical abrasion.