### NUT DESIGN

§        The nut is fastened to the body through retaining pins. The collar of the nut rests on the top of the body.

§        The nut is subjected to same stresses as the screw except bearing stress between the nut and the body.

§        We can assume a uniform load distribution on the nut threads as we are considering a lubricated screw-nut assembly. This will give us the length of the nut.

§        Calculate stresses for the worst case i.e. 0.38 of the load is carried by the first thread.

Nut will be subjected to following stresses:

§        Bearing stress

§        Transverse shearing stress

§        Torsional shear stress

§        Tensile stress (Direct)

# Length of Engagement:

§        Calculate length of engagement by limiting bearing stresses to a value given in the Table 8-4 (remember, multiply bearing pressure by 2).

Where :

d= major diameter

dr= root diameter

Twice of bearing pressure

Le= length of engagement

§        Calculate length of engagement by considering bending of the thread as cantilever beam.

Take

for ductile material or  for brittle material.

§        Calculate lengths of engagement by considering transverse shear stress both on the screw as well nut.

,

,

for ductile material and

for brittle material.

Ssu= shear strength of the material (shear modulus of rupture).

The Length of engagement will be the maximum of the four values calculated above.

# Stress Analysis:

§        Calculate direct tensile stress

.

Assume

§        Calculate torsional shear stress

.

T= torque required to raise the load.

§        Calculate bending stress, σbn with F=0.38F and nt=1

(same as bending stresses on the screw)

o       Depending on the type of material (ductile or brittle), apply appropriate failure theory to determine the safety of the nut.

o       For ductile nut material, apply either maximum shear stress theory or Von-Mises Theory.

Determination of dimension ‘a’ of the nut:

The main load in this case is shearing due to axial load.

§

§

Determination of diameter ‘do2’ of the nut:

The main load in this case is bearing (compression) stress between the nut and the body.