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