__
Screw Design__

__
__

__
Buckling Analysis__

Select suitable material
for the screw from Table E-20. This will give you S_{y}. Suggested
material is Carbon Steel (AISI-1035 to AISI-1060). Cold Drawn steel provides a
good surface finish, which improves fatigue strength. Machining of the screw is
easier and more accurate. Improved surface hardness provides high surface wear.

Select a suitable Service Factor for the screw.

Assume screw as Johnson’s column and take critical load

where ‘n’ is safety (service) factor and P is the load

Take

(Conservative design)

where,

Root diameter or the minor diameter

(Radius of gyration)

Select end condition co-efficient for the column from Table 4-6. Assume one end free, one end fixed.

Calculate the unsupported length

Calculate using formula

From the table attached
with the problem sheet, select standard thread size, ‘*d’*

Where,

*
d*
= major diameter

Calculate .

Calculate =

If , then screw is a Johnson’s column.

If > , then it is a Euler’s column and apply Euler’s formula to

get .

Calculate .

Calculate lead, .

Where , for single threads, for double threads and for triple threads.

Select a suitable material for the nut.

From Tables given find out coefficients of friction for the screw – nut combination and for the collar.

Check whether screw is self locking or not > .

Calculate torque,

Where

Co-efficient of friction between screw and nut.

Collar friction.

Half of the included angle.

Collar mean diameter.

Calculate , by substituting in the torque equation.

Calculate efficiency, .

__
Stress Analysis__

When a nut engages a thread, theoretically all the threads in engagement should share the load.

Some experiments show that the first engaged thread carries 0.38 of the load, the second 0.25, and the third 0.18, and the seventh is free of load.

To estimate thread stresses, we substitute , and set number of threads . This gives us the largest level of stresses in thread-nut combination.

Calculate direct stress, .

Calculate torsional shear stress, .

Calculate bending stress, .

With,

,

,

,

,

,

,

Calculate either using maximum shear stress or Von-Mises failure theories.

Compare with the yield strength of the material and calculate factor of safety.