Power Screw and Springs
Dec 14, 2015
Power Screw and Springs
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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
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Objectives (cont.)
• Identify various types of Power screw and describe their use
• Define springs types • Identify springs nomenclature ,
main dimensions and functions • Draw springs
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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.
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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.
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Screw Jacks
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Power Screw Loads
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Car Jack
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Vises devices
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X-Y Precision Table
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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
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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
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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.
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Types of Thread
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Ball Screws (1)
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Ball Screws (2)
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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
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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
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Power Screw Selection
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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
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Helical spring is a spiral wound wire with a constant coil diameter and uniform pitch.
What is helical spring
Function of Helical spring
Used to store energy and subsequently release it To absorb shock To maintain a force between contacting surfaces
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Design consideration of helical spring
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Helical Springs
• Helical springs have three types:• Compression springs• Extension springs• Torsion springs
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Helical Springs
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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)
Nomenclature of Helical spring
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Spring material
1. 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)
2. Alloy spring steel
• Chrome Vanadium (ASTM A231) • Chrome Silicon (ASTM A401)
3. Stainless steel
• AISI 302/304 - ASTM A313 • AISI 316 - 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
5. Nickel based alloy
A286 Alloy Inconel 600 (QQ-W-390) Inconel 718 Inconel X-750 (AMS
5698, 5699)
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Effect of End treatment.
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Compression Springs
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Extension Springs
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Torsion Springs
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Flat (Leaf) Spring
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• 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.
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Flat Springs