Lug Analysis

In lug analysis & sizing we consider both lug & pin. Below shown are the three different types of loads generally applied on lugs.

a) Axial load case (α = 0)b) Transverse load case (α = 90°)c) Oblique load case (α between 0 & 90°)

In analysis & sizing of the lug following are the points to be covered.

A) Design Consideration1. Symmetrical double shear & multiple shear lugs are only used for fail-safe

conditions

2. Ratio of lug thickness to hole diameter should be greater than 0.33. Grain direction of lug material must be carefully oriented as this drives the

lug strength.4. Forged material & press fit bushings should be used to improve fatigue

life.

B) Case I – Axial Load

The failure modes of the lug in this load case are.1. Shear bearing failure

Pbru = Kbr Ftux Abr

Abr = Dt2. Tension failure

Ptu = Kt Ftux At

At = (W-D) t3. Yield Failure – Lug

Py = C (Ftyx/Ftux) (Pu) min

4. Yield Failure – Bushing

Pbry = 1.85 Fcy Abrb

5. Pin shear-off failure

Pps = Fsu (πD2/4)Pps = 2Fsu (πD2/4) Lug design for double shear

6. Pin bending failure

C) Case II – Transverse Load

1. Compute

Abr = DtAav = 6/ [(3/A1) + (1/A2) + (1/A3) + (1/A4)]

2. Ultimate load is obtained by below equation

Ptru = Ktru Abr Ftuy

3. Yield Load

Py = Ktry Abr Ftyy

D) Case III – Oblique Load

1. Ultimate Load

MS = [1/ (Ra,u 1.6 + Rtr,u

1.6)0.625] – 1

2. Yield Load

MS = [1/ (Ra,y 1.6 + Rtr,y

1.6)0.625] – 1

E) Case IV – Tubular

Tubular pin preferable over large dia. Pins where high concentrated loads are acting, as tubular pins has greater moment of inertia. Even distribution of bearing/compressive stress is obtained in addition to reduction in wear & improvement in fatigue life of lug.

1. Tubular pin is used for landing gear trunion & engine pylon mount fittings.

2. Double pin design (Small pin inside tubular pin) provides fail safe design

Design criteria:5 < D/t < 10 D – Tubular pin outer dia., 0.5 > t1/D < 1.0 t – Tubular wall thickness, t1 – Female lug wall thickness

a. Ultimate Load

MS = [1/ (Rs,y2+ Rb,y)] – 1

b. Yield LoadShear Stress (fsy) = [(Plimit/2) /Atube]

F) Combined Bolt Load

The MS of bolt in combined shear & tension may be determined by above curve

G) Lug Clamp-UpFemale lug (Clevis) should be checked for excessive residual stresses under clamp-up.

Use of spherical bushing, flanged bushing & cross sectional area of bushing should be greater than or equal to bolt shank for rotating hinges design.