Thursday, February 03, 2011

When to Use a Washer

Flat Washer with Split Lock

A washer is often used under the nut or bolt, whichever is turned during the tightening operation. When both nut and bolt can be turned, washers are commonly used under both. All washers shall be made from a material which is capable of accepting the peak fastener load without deformation.

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 compressive strength must be matched to the  bolt/nut clamping force.

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, will relieves clamping force, eventually resulting in propeller detachment during operation.

Aircraft Accessory as Designed!

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. 

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