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
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, Ďdí
d = major diameter
If , then screw is a Johnsonís column.
If > , then it is a Eulerís column and apply Eulerís formula to
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 > .
Co-efficient of friction between screw and nut.
Half of the included angle.
Collar mean diameter.
Calculate , by substituting in the torque equation.
Calculate efficiency, .
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, .
Calculate either using maximum shear stress or Von-Mises failure theories.
Compare with the yield strength of the material and calculate factor of safety.