ME – 308 – MACHINE DESIGN II

Fall Semester. 2002 – 2003 (021)

DESIGN PROJECT # 1: Design of a Hand Screw Press

Design Analysis

1. Data Given

Load to be lifted = 1 Ton = 2000 lb

Lifting Height = 25 Inches

Safety factor = 2.5

2. Different parts of a Hand Screw Press

· Screw

· Nut or Bushing

· Pressure Plate

· Frame

· Foundation Bolts

Screw Design

· 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

AISI 1040 CD steel

· 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)

Checking Self – Locking Condition

What is Self – Locking ?

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: