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Machines

Explore the mechanics of screws, from ideal and actual mechanical advantages to their various applications. Learn about screw threads, left-handed screws, lead vs. pitch, and practical examples. Practice with lever devices to enhance understanding.

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Machines

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  1. Machines Practice #7—Screws Mr. Burleson geaux15@hotmail.com

  2. Agenda • Review • What are Screws • IMA for Screws • Machines Practical • Homework

  3. Basics of Simple Machines • Lever • Inclined Plane • Wheel and Axle • Wedge • Pulley • Screw (not included in Machines [B])

  4. What is an Inclined Plane? • An inclined plane is a flat supporting surface tilted at an angle, with one end higher than the other, used as an aid for raising or lowering a load

  5. Frictionless Inclined Plane Mechanical Advantage • Frictionless inclined plan means there is no friction between the object and the plan. • Object could be rolling or on wheels • Surface could be low friction/slippery like ice or oil covered • IMA = Length/Rise = 1/sinθ • The longer the ramp the lower the rise, the higher the IMA Where θ is the angle of the plane

  6. Friction Inclined Plane Mechanical Advantage • When there is friction between the plane and the load (sliding something up the ramp) some of the work applied is dissipated as heat by friction (Wfrict). • Win = Wfrict + Wout • Ffric = f = μFnormal = mg sinθ , where μ is the coefficient of friction • Fnormal = N = mg cosθ is the normal force between the load and plane Where θ is the angle of the plane

  7. What is an Screw? • A screw is a mechanism that converts rotational motion to linear motion, and a torque (rotational force) to a linear force. It is one of the six classical simple machines. ... Geometrically, a screw can be viewed as a narrow inclined plane wrapped around a cylinder. https://en.m.wikipedia.org/wiki/Screw_(simple_machine)

  8. ScrewIdeal Mechanical Advantage • Fout is the axial or linear force • Fin is the rotational/torque/rotational force • Lead and Pitch (how close the screw threads are to each other) • Lead (l) and Pitch (P) are the same in single-start screws, but are different in multi-start screws • Lead (l) is defined as the axial distance (parallel to the screw's axis) the screw travels in one complete revolution (360) of the shaft. • Pitch (P) is defined as the axial distance between the crests of adjacent threads • Radius ( r ) or Length of Lever (L) is how far from the center of the rotation the input force is applied • The greater the L or r the greater the IMA, but the longer distance Fin must move • Lead (or Pitch) is l or P • The smaller the l or P the greater the ideal mechanical advantage, but the slower the linear movement and higher the friction (when friction is present) https://en.wikipedia.org/wiki/Screw_(simple_machine)

  9. ScrewActual Mechanical Advantage • Even well lubricated screws like jack screws only have an efficiency of 15-20% • Because of their low efficiency, in powered machinery screws are not often used as linkages to transfer large amounts of power but are more often used in positioners that operate intermittently • η is the efficiency https://en.wikipedia.org/wiki/Screw_(simple_machine)

  10. Other Screw Uses • Because of its self-locking property, the screw is widely used in threaded fasteners to hold objects or materials together: the wood screw, sheet metal screw, stud, and bolt and nut. • Screws are self-locking if η < 0.5 or 50% • The self-locking property is also key to the screw's use in a wide range of other applications, such as the corkscrew, screw top container lid, threaded pipe joint, vise, C-clamp, and screw jack. • Screws are also used as linkages in machines to transfer power, in the worm gear, lead screw, ball screw, and roller screw. Due to their low efficiency, screw linkages are seldom used to carry high power, but are more often employed in low power, intermittent uses such as positioning actuators. • Rotating helical screw blades or chambers are used to move material in the Archimedes' screw, auger earth drill, and screw conveyor. • The micrometer uses a precision calibrated screw for measuring lengths with great accuracy https://en.wikipedia.org/wiki/Screw_(simple_machine)

  11. ScrewDistance moved • Linear distance moved (d) after rotating a screw degrees (α) is given by the pitch (l) and the following formula • Using distance ratio to calculate the mechanical advantage gives the same answer (but efficiency doesn’t matter) https://en.wikipedia.org/wiki/Screw_(simple_machine)

  12. Practical • Now practice with your lever device and see if you can get within 99% accuracy of the actual ratios • Do not time until you have achieved 99% accuracy 3 times in a row

  13. Homework #7 Screws • Add all of these to your binder • Explain the different types of Screw threads including • V thread • American National • Whitworth or British Standard • Square • Acme • Buttress • Knuckle • Explain difference between left handed and right handed screws and find at least one example of a left handed screw • Explain difference between Lead and Pitch, Single Start, Double Start, and Triple Start—include when you would use each • Find examples of screws in tools, cars, bicycles, and the kitchen

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