1. Power Screw and Springs

2. Objectives Define and label the parts of a Power screw
Identify various Power screw thread forms
Draw detailed, schematic, and simplified threads of Power screw
Define typical thread specifications

3. Objectives (cont.)
Identify various types of Power screw and describe their use
Define springs types
Identify springs nomenclature , main dimensions and functions
Draw springs

4. Uses of Power Screws
Obtain high mechanical advantage in order to move large loads with a minimum effort. e.g screw jack.
Generate large forces e.g tensile testing machine, compactor press.
Obtain precise axial movements e.g. camera calibration rigs.

5. A power screw is a device that is common to tools or machinery that are used to change angular motion into translation. It is also capable of developing a large amount of mechanical advantage. Familiar applications include clamps or vises, presses, lathes lead screws, and jacks.

6. Screw Jacks

7. Power Screw Loads

8. Car Jack

9. Vises devices

10. X-Y Precision Table

11. Advantages of power screws
Compact design and takes less space
Large load carrying capability
Simple to design and easy to manufacture
Can obtain a large mechanical advantage
Precise and accurate linear motion
Easy maintenance
Self-locking feature

12. Advantages of power screws
Compact design and takes less space
Large load carrying capability
Simple to design and easy to manufacture
Can obtain a large mechanical advantage
Precise and accurate linear motion
Easy maintenance
Self-locking feature

13. Thread Terminology
Pitch is the distance from the crest of one thread to the next.
When the screw rotates by one revolution the screw advances by its pitch.

14. Types of Thread

15. Ball Screws (1)

16. Ball Screws (2)

17. Advantages of a ball screw
Have very high efficiency (Over 90%)
Could be used in applications which require precise and repeatable movement
Could be easily preloaded to eliminate backlash
Smooth movement over full travel range
Can use a smaller size for same load
Has a longer life for thread

18. Disadvantages of a ball screw
Requires higher levels of lubrication
Possibility of the screw to contaminate
Additional brakes have to be used if locking is required

19. Power Screw Selection

20. Springs
A spring is a mechanical device designed to store energy when deflected and to return the equivalent amount of energy when released
Springs are classified as:
Helical springs
Flat springs

21. Function of Helical spring
What is helical spring
Helical spring is a spiral wound wire with
a constant coil diameter and uniform pitch.
Function of Helical spring
Used to store energy and subsequently release it
To absorb shock
To maintain a force between contacting surfaces

22. Design consideration of helical spring

23. Helical Springs Helical springs have three types: Compression springs
Extension springs
Torsion springs

24. Helical Springs

25. Nomenclature of Helical spring
C = Spring Index D/d d = wire diameter (m) D = Spring diameter (m) Di = Spring inside diameter (m) Dil = Spring inside diameter (loaded ) (m) E = Young's Modulus (N/m2) F = Axial Force (N) G = Modulus of Rigidity (N/m2) K W = Wahl Factor = [(4C-1)/(4C+5)}]+ (0,615/C) L 0 = Free Length (m) L s = Solid Length (m) n t = Total number of coils n = Number of active coils p = pitch (m) y = distance from neutral axis to outer fibre of wire (m) τ = shear stress (N/m2) τ max = Max shear stress (N/m2) θ = Deflection (radians)

26. Spring material Phosphor Bronze (Grade A) - ASTM B159
High carbon steel
Music Wire (ASTM A228)
Hard Drawn (ASTM A227)
High Tensile Hard Drawn (ASTM A679)
Oil Tempered (ASTM A229)
Carbon Valve (ASTM A230)
Alloy spring steel
Chrome Vanadium (ASTM A231)
Chrome Silicon (ASTM A401)
Stainless steel
AISI 302/304 - ASTM A313
AISI ASTM A313
17-7 PH - ASTM A313(631)
4. Copper based alloy
Phosphor Bronze (Grade A) - ASTM B159
Beryllium Copper - ASTM B197
Monel 400 (AMS 7233)
Monel K500 (QQ-N-286
Nickel based alloy
A286 Alloy
Inconel 600 (QQ-W-390)
Inconel 718
Inconel X-750 (AMS 5698, 5699)

27. Effect of End treatment.

28. Compression Springs

29. Extension Springs

30. Torsion Springs

31. Flat (Leaf) Spring

32. Self-dampening
The reason for using multiple leaves clamped together rather than a single piece of metal cut to the same shape has to do with what happens when a load is applied to the spring.
Interleaf friction
Interleaf friction provides a self-dampening characteristic to the spring pack. Two factors ensure a spring pack retains its self dampening.
First, when a spring pack is assembled, the individual leaves must never be lubricated or painted. This would reduce interleaf contact friction.
Second, the function of the center-bolt that clamps the leaves is critical. The tension it loads the leaves under helps define the self-dampening ability of the spring assembly. In the event of a broken center-bolt, much of the self- dampening properties of a spring pack are lost.
Shock absorbers not necessary
The advantage of the multi-leaf spring pack is that shock absorbers can be eliminated.

33. Flat Springs