ME – 308 – MACHINE DESIGN II

Fall Semester. 2000 – 2001 (001)

 

 

DESIGN PROJECT # 1: Design of a Screw Jack

 

 

Design Analysis

 

 

 

 

1.        Data Given

 

 

Load to be lifted = 50 KN

 

Lifting Height =           300 mm

 

Square Threaded Screw with single thread

 

Safety factor = 2

 

Allowable bearing pressure = 2 X (Bearing pressure in table 1)

 

Starting Friction = 1.3 X (Running Friction)

 

 

2.    Different parts of a Screw Jack

 

 

*   Screw

*   Nut

*   Turning Lever

*   Base

*   Cup

*   Retaining Pins

 

 

 

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.

 

 

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.

 

 

 

Combined Torsional and Axial Stress on the Screw

 

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:

 

  where 

 

 

 

 (Check)

 

Distortion Energy Theory:

 

 

 

  (Check)

 

Select the largest  as a safe root diameter.

 

Efficiency of the power screw:

 

 

 

 

 

 

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