Assembly • Section 9.4
Double-Shear Joint Assembly & Pin Analysis
Pin double shear ($P_{us}$), plastic bending ($P_{ub}$), balanced weak-pin design, and clevis gap effects
1. Double-Shear Clevis Joint Architecture
A standard aerospace structural pinned joint consists of a single center male lug (tongue, thickness $t_2$) inserted between two symmetric female cheek lugs (clevis ears, thickness $t_1$) separated by an assembly clearance gap $g$ on each side, joined by a transverse shear pin of diameter $D_p$.
Per USAF AFFDL-TR-69-42 Section 9.4, the combined joint capacity depends on the interaction between pin shear strength, pin bending stiffness, and lug bearing plasticity.
2. Pin Shear Strength (Equation 9-12)
In a symmetrical double-shear clevis, the pin must fail across two simultaneous transverse cross-sections located in the gaps between the male lug and female ears:
where $D_p$ is the nominal pin diameter and $F_{su,pin}$ is the pin material ultimate shear allowable (typically $0.60 \times F_{tu}$ for high-strength steel pins like 4340 or 17-4PH).
3. Pin Plastic Bending Strength (Equations 9-14 & 9-15)
Because the male lug and female lugs are spaced apart by clearances ($g$), the applied load produces a severe transverse bending moment across the center of the pin. The ultimate bending capacity accounts for full plastic hinge development:
where $k_b$ is the pin plastic bending coefficient (Section 9.4.3):
- $k_b = 1.56$: Standard aerospace coefficient for ductile steel pins ($\varepsilon_u > 5\%$). Represents the plastic shape factor of a solid circular cross section ($1.70$) reduced slightly for shear deflection coupling.
- $k_b = 1.00$: Brittle pins ($\varepsilon_u \le 5\%$), restricted to elastic section modulus ($Z = \pi D_p^3 / 32$).
The nominal pin bending load allowable $P_{ub}$ across the nominal span is:
4. Strong Pin vs. Balanced Weak-Pin Behavior
Section 9.4 classifies joint assemblies into two distinct structural regimes:
| Classification | Physical Behavior | Governing Joint Capacity ($P_{u,J}$) |
|---|---|---|
| Strong Pin ($P_{ub} \ge P_{u,LB}$ or $P_{ub} \ge P_{us}$) |
The pin is thick and stiff relative to the lugs. It deflects negligibly under load; bearing pressure remains evenly distributed across the full thickness of male and female lugs. | $P_{u,J} = \min\left(2 P_{u,LB1},\; P_{u,LB2},\; P_{us}\right)$ |
| Weak Pin (Balanced Design) ($P_{ub} < P_{u,LB}$) |
The pin flexes significantly under load. This flexibility concentrates bearing contact near the gap edges, peeling away from outer faces. The joint reaches a plastic equilibrium governed by effective contact widths $b_1$ and $b_2$. |
Evaluated using USAF Equations (9-16) and (9-18a/b): $P_{ub,max} = 2 C \left[\sqrt{\frac{P_{ub}}{C}\left(\frac{t_1}{2} + \frac{t_2}{4} + g\right) + g^2} - g\right]$ |