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Power Screw and Springs

Power Screw and Springs. 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. Objectives (cont.).

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Power Screw and Springs

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  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. 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 coilsn = Number of active coilsp = 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 • 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) • 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 316 - ASTM A313 • 17-7 PH - ASTM A313(631)

  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

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