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 :

 

F= load on each screw

 

P= pitch of the thread

 

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.

 

t= Thread thickness.

 

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.