Aircraft Washer Usage

Rule of Flat Washers:

All washers shall be made from a material which is capable of accepting the peak fastener load without deformation.

 Incorrect washer used on NAS148 high tensile strength bolt led to the loss of N76195 and its occupant

A washer can provide multiple functions, the two most important ones are:

1. Spreads the clamping force over a larger area to avoid compressive yielding, and

2. Hard, smooth, consistent material for good preload (clamping) control.

Other functions are to:

1. Prevent galling of the nut face or surface during tightening.

2. Reducing the external load carried by the bolt by increasing the effective pressure area. This stiffens the joint members and the stiffer the joint members the smaller the fraction of external load the bolt will "see".

3. Prevent galvanic corrosion by separating dissimilar metals. Example would be using an aluminum washer under a steel bolt head tightened against an aluminum crankcase. Any galvanic corrosion occurs between the washer and bolt head rather than between the crankcase and bolt head. A washer is cheaper to replace then the crankcase.

4.  Increase energy stored in bolt by using a longer bolt. This helps retain clamping force.

5. Adjusting grip length.

Washer Strength:

WASHER compressive strength MUST be matched to the BOLT/NUT combination! 

Pictured above is a low-yield strength hardware store washer placed under a propeller bolt.  Low-yield strength washers that score/crush in-service under high strength BOLT heads or NUTS relieves the clamping force, eventually resulting in propeller detachment during operation.

High-Strength Aircraft Washers MS20002

MS20002 smaller inside diameter for closer fit to bolt shank

Standard AN960 has larger inside diameter  - less bearing surface area

A size comparision of the common AN960 washer with the harder MS20002 reveals that the MS20002 washer has a smaller diameter inner hole and a slightly larger outside diameter. The non-chamfered version offers approximately 20% more surface area to the nut. For example, a 1/4 inch AN960 has a surface area of .13989 sq. in. compared to .16998 sq. in. for the MS20002, thereby reducing the stress per square inch on the washer by spreading the load over a larger surface area. This helps prevent washer or faying surface crushing and reduces joint embedment relaxation. A quick calculation shows that when a AN4 bolt is fully torqued the stress per square inch on the washer reduces from 18,000 psi with the AN960 washer to 15,000 psi for the MS20002 washer

Comparing Thickness - high-strength washer next to head is thicker than standard washer

MS20002C - bevel to clear radius at bolt shank to head

Aircraft Washer Usage Chart

When not to use a washer

Incorrect - no washer needed or desired here

When NOT to use a washer. The built-in washer under the head of a flange head bolt acts to distribute the clamping load over a greater area. No washer is needed or desired. This aircraft starter is assembled with washers under the flanged bolt head.  Notice that the bolt head overhangs the washer more on the left side. Bolt now has prying tension.

Calculating Surface Pressure:

Not only the washer but the joint (faying) surfaces must have adequate compressive strength.

Crushing - tightening beyond compressive yield strength of bearing material.

To calculate bearing-stress (surface-pressure), you take the bolt-tension and divide by the contact area between the bolt head and the part. You then compare this value to the allowable surface pressure for the joint material. The allowable contact stress for material is usually about equal to the ultimate tensile strength due to the nature of localized forces on solid bodies.

A rule of thumb is that the allowable surface pressure is approximately equal to the material's ultimate tensile-strength (due to elastic and plastic constraint from the surrounding material). Even if you reach the pressure limit, that just means you begin indentation of the part, which does not necessarily mean part failure. You will need to decide what the part limits are with respect to static and cyclic loading, temperature exposure, etc.

Non-Metallic Washers

Aircraft Spinner with non-metallic washer

Some aircraft applications use a non-metallic washer such as under propeller spinner screws. Typically, these washers are made from high compressive strength phenolic or sometimes nylon so that the screw can be tightened without crushing the washer.  Some have suggested using a "Teflon" washer.

PTFE "Teflon" is has a tendecy to creep under compression (cold flow). In other words "it runs away from the stress" and leaves the screw loose. This low compressive strength can result in loose fasteners and  joints when used as a washer under screw heads. To illustate the low compressive strength of PTFE, the chart below compares Nylon's compressive strength to PTFE. Care needs to be exercised when substituting materials that the substitute has suitable mechanical properties to function as well as the original.

Plastic washer material strength