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.