ME – 308 – MACHINE DESIGN II
Fall Semester. 2000 – 2001 (001)
Allowable bearing pressure = 2 X (Bearing pressure in table 1)
Starting Friction = 1.3 X (Running Friction)
2. Different parts of a Screw Jack
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.
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.
The area of the screw, which is at the root diameter, is subjected to a biaxial stress caused by both the compressive stresses on the screw and the shearing stress caused by the applied torque.
Max. Shear Stress Theory:
Distortion Energy Theory:
Select the largest as a safe root diameter.
Efficiency of the power screw: