§ 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).
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.
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.
§ Calculate direct tensile stress
§ 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.