Components • Section 9.2.4
Bushing Design, Materials & Compressive Allowables
AFFDL-TR-69-42 Section 9.2.4 bushing bearing equation (9-9), interference fits, and bronze alloy allowables
1. Why Bushings are Critical in Aerospace Lugs
In production aircraft and mechanical mechanisms, high-strength pins are rarely installed directly against parent aluminum or titanium lug holes. Instead, thin-walled cylindrical bushings are press-fitted into the bore to serve essential functions:
- Wear & Fretting Protection: Prevents direct metal-to-metal galling between hard steel pins and softer aluminum lugs.
- Serviceability & Field Overhaul: Worn bushings can be pressed out and replaced during scheduled maintenance without scrapping expensive structural forgings.
- Fatigue Life Enhancement: Controlled radial interference fits ($0.001 - 0.003\text{ in/in}$) introduce beneficial compressive residual hoop stress fields around the lug hole, significantly delaying fatigue crack initiation.
- Corrosion Barrier: Provides galvanic isolation between dissimilar metals (e.g. steel pins and aluminum airframes).
2. USAF Bushing Bearing Formulation (Equation 9-9)
Per USAF AFFDL-TR-69-42 Section 9.2.4, the ultimate compressive bearing allowable of a cylindrical bushing under pin contact is:
where:
- $F_{cy,bush}$ is the compressive yield strength of the bushing material. (If compressive yield is not documented, tensile yield $F_{ty}$ is commonly used as a conservative proxy).
- $D_p$ is the inner bore diameter of the bushing (pin diameter).
- $t$ is the effective lug/bushing thickness.
- The factor $1.304$ accounts for triaxial compressive constraint and localized plastic strain hardening within the cylindrical sleeve.
3. Standard Aerospace Bushing Alloys
LugCalc™ includes standard high-performance aerospace bushing alloys in its built-in materials library:
| Bushing Material | Specification | $F_{tu}$ / $F_{ty}$ | $F_{cy}$ (Compressive) | Application Profile |
|---|---|---|---|---|
| Aluminum Nickel Bronze | AMS 4640 / C63000 | 110 ksi / 68 ksi | ~75 ksi (517 MPa) | Standard for heavy landing gear pivots and high-load articulation joints. Excellent corrosion and galling resistance. |
| Beryllium Copper | AMS 4533 / C17200 | 165 ksi / 140 ksi | ~150 ksi (1034 MPa) | Extreme load joints where maximum compressive yield is required to prevent bore ovalization. |
| Phosphor Bronze | SAE 660 / C93200 | 35 ksi / 20 ksi | ~22 ksi (152 MPa) | General-purpose low-stress bearings and guide sleeves with continuous lubrication. |
| 17-4PH Stainless Steel | AMS 5643 (H1025) | 155 ksi / 145 ksi | ~160 ksi (1103 MPa) | High-strength corrosion-resistant bushings in titanium or composite lug fittings. |
4. Assembly Interaction in LugCalc™
When bushings are included, LugCalc checks both the lug plate and the bushing sleeve for both axial and transverse loading:
If the bushing material compressive allowable is lower than the lug material bearing allowable, the bushing governs, and LugCalc flags "Bushing bearing" as the critical failure mode.