2Objectives Define and label the parts of a Power screw Identify various Power screw thread formsDraw detailed, schematic, and simplified threads of Power screwDefine typical thread specifications
3Objectives (cont.)Identify various types of Power screw and describe their useDefine springs typesIdentify springs nomenclature , main dimensions and functionsDraw springs
4Uses of Power ScrewsObtain 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.
5A 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.
11Advantages of power screws Compact design and takes less spaceLarge load carrying capabilitySimple to design and easy to manufactureCan obtain a large mechanical advantagePrecise and accurate linear motionEasy maintenanceSelf-locking feature
12Advantages of power screws Compact design and takes less spaceLarge load carrying capabilitySimple to design and easy to manufactureCan obtain a large mechanical advantagePrecise and accurate linear motionEasy maintenanceSelf-locking feature
13Thread TerminologyPitch 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.
17Advantages of a ball screw Have very high efficiency (Over 90%)Could be used in applications which require precise and repeatable movementCould be easily preloaded to eliminate backlashSmooth movement over full travel rangeCan use a smaller size for same loadHas a longer life for thread
18Disadvantages of a ball screw Requires higher levels of lubricationPossibility of the screw to contaminateAdditional brakes have to be used if locking is required
20SpringsA spring is a mechanical device designed to store energy when deflected and to return the equivalent amount of energy when releasedSprings are classified as:Helical springsFlat springs
21Function of Helical spring What is helical springHelical spring is a spiral wound wire witha constant coil diameter and uniform pitch.Function of Helical springUsed to store energy and subsequently release itTo absorb shockTo maintain a force between contacting surfaces
25Nomenclature 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)
26Spring material Phosphor Bronze (Grade A) - ASTM B159 High carbon steelMusic Wire (ASTM A228)Hard Drawn (ASTM A227)High Tensile Hard Drawn (ASTM A679)Oil Tempered (ASTM A229)Carbon Valve (ASTM A230)Alloy spring steelChrome Vanadium (ASTM A231)Chrome Silicon (ASTM A401)Stainless steelAISI 302/304 - ASTM A313AISI ASTM A31317-7 PH - ASTM A313(631)4. Copper based alloyPhosphor Bronze (Grade A) - ASTM B159Beryllium Copper - ASTM B197Monel 400 (AMS 7233)Monel K500 (QQ-N-286Nickel based alloyA286 AlloyInconel 600 (QQ-W-390)Inconel 718Inconel X-750 (AMS 5698, 5699)
32Self-dampeningThe 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 frictionInterleaf 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 necessaryThe advantage of the multi-leaf spring pack is that shock absorbers can be eliminated.