A lug of width b and thickness h is used to carry a load P using a pin passing through a hole of diameter d as shown in Figure. The lug is fabricated from 0.375 in. thick 2024-T351 plate [see Table MH3.2.3.0(b1) on page MH3-71 for mechanical properties]. The load is applied in the L direction of the plate and B basis design values are assumed. For ultimate stress design, the tensile stress in the lug is assumed to be s = P/ [(b-d)h] and the bearing stress in the lug along the pin-lug interface is sb = P/(d h):
(a) Determine the minimum width b of the lug to carry a load P = 15.000 lb using a pin diameter d = 0.50 in. Design for ultimate stress.
(b) Suppose the lug has a width b of 1.25 in. and a pin diameter d = 0.50 in. Deter- mine the load P required to cause bearing failure (using the bearing ultimate stress) for an edge margin e/D = 2.0 (typical design). Repeat the calculation for an edge margin e/D = 1.5 (short edge margin). Note that d = D.
(c) For certain choices of width b and diameter d, the design of the lug becomes "bearing critical," meaning that the bearing stress (sb) reaches its ultimate stress (Fbru) before the lug tensile stress (s) reaches its ultimate stress (Ftu). De- termine an equation for b at which this change occurs; it will be a function of Ftu Fbtu and d.
