Lug Sizing Theory

Lugs are connector elements used to transfer structural loads with pins. Historically, lugs have been over-designed since weight and size requirements were for the most part, unrestricted. However, the desire for lightweight structures requires lug joints be designed in an optimized manner that reduces weight while remaining structurally sound.

Double Shear Lug Joint

Numerous methods have been devised to calculate lug joint strength. The first methods were simple cross sectional area techniques that resulted in robust, but over-designed lugs. In the 1950's, the work of M. A. Melcon, F. M. Hoblit, and F. P. Cozzone from the Lockheed Aircraft Corporation improved upon the original cross section techniques to utilize efficiency factors that permitted more exact lug allowables. The new efficiency factor method produced safe lug joints with reduced weight.

F. P. Cozzone, M. A. Melcon and F. M. Hoblit Original Bearing Efficiency Curves

 In 1969, the Air Force Research Laboratory, published a version that did away with specific material curves by relying on easily obtained material properties, including, ultimate strain, ultimate strength to yield strength ratio, and other geometric constants.

Specially Developed Material Curves for Lug Analysis

Today, the methods of  M. A. Melcon, F. M. Hoblit, and F. P. Cozzone, and the Air Force Research Laboratory method are all widely used.

Improved, Non-Material Specific Air Force Research Laboratory Tensile Efficiency Curves

It has been asked if closed form (hand calculations) lug joint design have become obsolete with the advent of finite element techniques. The aerospace industry uses both. Traditional closed form lug joint techniques allow for very fast lug sizing that include non-linear effects. Running comparable analysis with finite elements techniques is possible but much more costly and time intensive. Today, many aerospace designs are modeled with finite elements but structurally justified with engineering reports that default to the closed form method used by LugCalc.

Finite Element Lug Analysis

LugCalc software is designed for use by those familiar with structural engineering and typical assumptions made in structural design. It is recommended the user be familiar with the content and methods detailed in the texts listed on the references page. These references provide step-by-step explanations documenting the analysis process. LugCalc uses the Air Force Research Laboratory method of calculation. All lug calculations assume uniform pin loading. As explained in the reference material, non-uniform loading (pin bending) will contribute to premature lug failure and invalidates any result from LugCalc or the published methods.

Any form of lug analysis, including LugCalc, is purely a theoretical estimation. All lug designs require full scale testing to ensure adequacy.

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