Loads • Section 9.3
Lug Loads, Angles, and Fitting Factors
Axial forces, transverse cross-loads, oblique angles, Fitting Factors (FF), and Load Factors (LF)
1. Load Vector Representation in Lug Analysis
Lugs rarely carry pure axial tension alone; in real aircraft mechanisms, landing gear drag struts, and crane attachments, forces act at oblique angles. LugCalc™ supports two interchangeable load input representations:
| Input Mode | Parameters | Formulas | Typical Application |
|---|---|---|---|
| Component Form | Axial force ($P_{ax}$ or $F_{ax}$) Transverse force ($P_{tr}$ or $F_{tr}$) |
$P_{app} = \sqrt{P_{ax}^2 + P_{tr}^2}$ $\theta = \arctan(P_{tr} / P_{ax})$ |
Output from FEA beam/element coordinate extraction or 2D free-body diagrams. |
| Vector / Polar Form | Resultant force ($P_{app}$) Load angle ($\theta$) |
$P_{ax} = P_{app} \cos\theta$ $P_{tr} = P_{app} \sin\theta$ |
Known cable/strut pull vector acting at an angle to the lug symmetry axis. |
2. Safety Multipliers: Fitting Factor & Load Factor
To ensure structural integrity under uncertain joint contact tolerances and dynamic shock, aerospace specifications mandate regulatory multipliers:
- Fitting Factor (FF): Mandatory in aerospace engineering under FAR § 25.625 / 14 CFR § 25.625. Fittings whose strength is not proven by extensive limit and ultimate joint test programs must use a minimum fitting factor of 1.15. The factor accounts for contact stress concentrations, pin tolerance slop, and localized wear.
- Load Factor (LF): Ratio of ultimate design load to limit (operational) load. Standard aerospace practice specifies a minimum LF = 1.50.
- Combined Multiplier ($k$): In typical aerospace analyses, $k = 1.15 \times 1.50 = 1.725$.
3. Load Distribution in Double-Shear Clevis Joints
When the Include Female Lugs toggle is checked in the web app, the total joint load $P_{app}$ is reacted across the two symmetric female ears (clevis prongs):
Each female lug is evaluated individually for bearing, net tension, and transverse capacity under its half-share load. The joint assembly is subsequently evaluated for pin shear and pin bending across the gap spacing $g$.
4. Oblique Load Interaction Preview
Under combined axial and transverse loads ($0^\circ < \theta < 90^\circ$), the lug does not fail at its pure axial or pure transverse capacity. Instead, failure follows the empirical interaction exponent ($1.6$) established in USAF Equation 9-20:
Full mathematical derivation and margin-of-safety calculations for oblique loading are detailed in the Margin of Safety documentation.