Geometry • Section 9.2
Basic Lug Geometry & Dimensional Parameters
Fundamental dimensional inputs for straight and concentric pinned lugs
1. Primary Dimensional Inputs
In the LugCalc™ web calculator, a straight pinned lug is defined by four fundamental physical dimensions entered in the Male Lug panel (and duplicated for Female Lugs when a clevis joint is analyzed):
| Symbol | Web App Label | Physical Definition | Valid Range / Constraints |
|---|---|---|---|
| $w$ | Lug Width | Overall width of the lug plate across the bore centerline. Also defines the outer radius of the rounded tip ($R = w/2$). | $w > D$ |
| $t$ | Lug Thickness | Plate thickness in the out-of-plane direction. | $t > 0$ |
| $D$ | Bore Diameter | Inner diameter of the hole (or outer diameter of the bushing sleeve if bushed). | $D < w$ |
| $e$ | Edge Distance | Distance measured from the hole center to the outermost free edge of the rounded tip along the longitudinal centerline. | $e \ge 0.5 D$ (USAF bounds: $0.5 \le e/D \le 2.5$) |
Figure 5.1: Plan and edge views defining the basic geometric parameters ($w, t, D, e, a$).
2. Tip Profile & Concentricity
In a standard straight concentric lug:
- The free tip is semicircular with radius $R = w / 2$.
- When the hole is concentric with the outer radius, the edge distance is $e = R = w / 2$.
- The minimum material ligament forward of the bore is the tip edge distance ($a$):
$$a = e - \frac{D}{2}$$Equation 5.1: Minimum tip material ligament.
If the hole is offset toward or away from the free tip, or offset laterally across the width, the lug is considered eccentric, requiring additional geometric calculations.
3. Standard Aerospace Design Guidelines
Based on extensive aerospace testing compiled in USAF AFFDL-TR-69-42 and NASA design standards, optimum strength-to-weight efficiency occurs within specific dimensional windows:
- Edge Distance Ratio ($e/D$): Ideal range is $1.2 \le e/D \le 1.8$. Values below $1.0$ risk premature shear-out; values above $2.0$ yield diminishing returns while adding parasitic weight.
- Thickness Ratio ($D/t$): Values of $D/t \le 5.0$ are considered "thick" lugs and provide peak bearing efficiency ($K_{bru}$). When $D/t > 5.0$, thin-plate instability reduces allowable bearing stress.
- Width Ratio ($w/D$): Typical values range from $2.0$ to $2.5$. If $w/D < 1.5$, net-section tension severely governs; if $w/D > 3.0$, bearing failure governs and outer material remains under-stressed.