ME – 308 – MACHINE DESIGN II
Fall Semester. 2002 – 2003 (021)
2. Different parts of a Hand Screw Press
· Nut or Bushing
· Pressure Plate
· Foundation Bolts
· Assume the material for the screw : AISI Steel 1040 CD
· Assume the material for the nut : Bronze
Reasons for selecting AISI 1040 CD for the screw and Bronze for the Nut
· It has a good surface finish which improves fatigue strength
· It can be machined easily and resists wear.
· Cold Drawing results in a large increase in yield strength, ultimate strength and hardness.
· It has high strength and high wear resistance
· It is used with the nut to reduce friction with the screw
Assuming that it is a Johnson’s Column with central loading and assuming that both ends are rounded or pivoted:
is Critical Load
is Radius of Gyration
is Unsupported Length of the column
= Root Diameter
= Pitch Diameter
Using equation (3 – 58) and equation (3 – 56) check for Johnson’s column. If it is not a Johnson’s column then repeat the above steps for the Euler’s column.
Finalizing the value of dr :
From the Table given along with the handout, try to assume a starting value for major diameter (d), and get the corresponding Pitch (P). Check whether the value of
dr = d - P
is approx. equal to the calculated value of dr .
dr (from the table) = dr ( Calculated)
Sometimes when the load is large or the friction is low, the load will lower itself by causing the screw to spin without any external effort.
If this doesn’t happen then it is said to be self locking.
Condition for self locking:
Select from Table –2.
Finding the Torque to raise the load:
is the Torque required to overcome the thread friction and to raise the load.
Where n = 1 and P is the pitch of the threads
Stress Analysis for the Screw
Carry out the stress analysis for the screw for two cases:
Case 1: Above the Nut where the torque will be equal to
T total = T + Tc
Case 2: Below the Nut where the total torque will be
T total = Tc
In both the cases calculate the three stresses which are the
1. Axial Stress
2. Bending Stress
3. Torsional Shear Stress
By using the equations given to you in the class.
Check for the safety factor by using either the Max. Shear Stress Theory or Von-Mises theory.
Max. Shear Stress Theory:
σ’ = 0.5 Sy / n
σ’ = 0.577 Sy / n
Efficiency of the power screw: