Screw Design

 

Buckling Analysis

 

Select suitable material for the screw from Table E-20. This will give you Sy. 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 Johnsonís column and take critical load

 

 

 

where Ďní 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, Ď

 

Where,

 

d = major diameter

 

Calculate .

 

Calculate =

 

If  , then screw is a Johnsonís column.

 

If    >   , then it is a Eulerís column and apply Eulerís 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 Ė 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.