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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