§ 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)

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

Where :

*
F=*
load on each screw

*
P=*
pitch of the thread

*
d=*
major diameter

*
d _{r}=*
root diameter

Twice of bearing pressure

*
L _{e}=*
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.

*
t=*
Thread thickness.

*
S _{su}=*
shear strength of the material (shear modulus of rupture).

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

** **

§ Calculate direct tensile stress

** **

.

Assume

§ Calculate torsional shear stress

.

*
T=*
torque required to raise the load.

§
Calculate bending stress, *σ _{bn}* with

(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 ‘ d_{o2}’ of the nut:__

** **

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