**ME – 308 – MACHINE DESIGN II**

**Fall Semester. 2002 – 2003 (021)**

2. Different parts of a Hand
Screw Press

·
Screw

·
Nut or Bushing

·
Pressure Plate

·
Frame

·
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.

Bronze

·
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:

_{}_{}

where

_{}_{} is Critical Load

_{}_{} is Radius of Gyration _{}_{}

_{}_{} is Unsupported Length of the
column

_{}_{} = _{}_{} = _{}_{}_{}

where

_{}_{}= 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 d_{r} :

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

d_{r} = d - P

is approx. equal to the calculated value of d_{r
}.

d_{r } (from the table) = d_{r} ( 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 + T_{c
}

_{ }

Case 2: Below the Nut where the total torque
will be

T _{total }= T_{c
}

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 S_{y }/ n

Von-mises theory:

σ’ = 0.577 S_{y }/
n

Efficiency of the power screw:

_{}