Geometry • Section 9.2
Calculated Lug Geometry & Section Dimensions
Derived geometric parameters, net section areas, and material ligament checks
1. Overview of Derived Geometric Parameters
While the user enters basic nominal dimensions ($w, t, D, e$), the structural stress equations in USAF AFFDL-TR-69-42 operate on precise local section measurements at the hole boundary. In LugCalc™, the geometric solver calculates these derived parameters automatically:
| Derived Dimension | Symbol | Mathematical Formula | Governing Analysis Role |
|---|---|---|---|
| Transverse Width across Bore | $W$ | $W = h_0 + D + h_2$ | Used in the net-section ratio $D/W$ and net tension allowable ($P_{nu,L}$). For straight lugs, $W = w$. |
| Tip Edge Ligament | $a$ | $a = e - \frac{D}{2}$ | Forward material ligament resisting shear tear-out. Modifies bearing capacity when $e/D < 1.5$. |
| Projected Bearing Area | $A_{br}$ | $A_{br} = D \cdot t$ | Projected contact area for pin bearing and bushing load calculations. |
| Net Tensile Section Area | $A_{net}$ | $A_{net} = (W - D) \cdot t$ | Cross-sectional area resisting axial hoop tension across the bore centerline. |
| Minimum Edge Clearance | $\text{Clr}_{min}$ | $\min\Big(\text{dist}(\text{bore},\; \text{boundary})\Big) - \frac{D}{2}$ | Ensures physical integrity; guards against hole breakout through tapered side edges. |
| Effective Transverse Dimension | $h_{avg}$ | Harmonic average of radial sections $h_1 \dots h_4$ | Governs transverse load efficiency factor $K_{tru}$ (USAF Fig. 9-8). |
2. Why the True Transverse Width ($W$) Differs from Nominal Width ($w$)
In a standard straight lug with parallel side edges, the width across the hole center is identical to the nominal plate width ($W = w$). However, when a lug is designed with side tapers ($\alpha, \beta > 0$) or the hole is axially offset ($e_{cca} \ne 0$), the true width measured along a line perpendicular to the longitudinal axis passing through the bore center is:
Here, $h_0$ is the material ligament on the upper side of the hole and $h_2$ is the material ligament on the lower side. If the hole is shifted forward toward the narrow tapered tip, $W$ is significantly less than the nominal shank width $w$, which properly reduces the net-section tension allowable $P_{nu,L}$.
3. Viewing Calculated Properties in the Web App
In the LugCalc™ web app, switch to the Component Properties tab in the right-hand panel. This table displays the exact computed values for both the Male Lug and Female Lugs (if enabled):
- Total lug volume and estimated component weight.
- Exact net width $W$ and minimum ligament $a$.
- Radial edge dimensions $h_0, h_1, h_2, h_3, h_4$.
- Harmonic average $h_{avg}$ and non-dimensional design ratios.