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5. wire rope and sample problem

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ME 6601 - DESIGN OF TRANSMISSION SYSTEMS
WIRE ROPES AND SAMPLE PROBLEMS

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5. wire rope and sample problem

  1. 1. DEPARTMENT OF MECHANICAL ENGINEERING PROBLEMS ON DESIGN and selection wire ropes using psg design data book By Mr. B.Balavairavan Assistant Professor Mechanical Engineering Kamaraj College of Engineering and Technology Virudhunagar
  2. 2. A history • It evolved from the steel chains and wooden rods . • As the wire ropes are made of strands of twisted metal wires it cant easily fail. • Originally developed for hoisting in the 1830s, mining engineer Wilhelm Albert twisted together several strands of metal wire around a hempen core, resulting in a superior hoisting cable for use in vertical shafts. • Later used for hoisting in cranes, elevators and for power transmission. • Widely used for application like, Bowden cable, in suspension bridges, in tramways and in spinning mills.
  3. 3. Wire Rope Wire Ropes are made by twisting thin metal wires which increases the strength of the rope. Wire ropes have diameter larger than 9.52 mm. If the diameter is less than 9.52 mm its called as cable or cords. The common material used to make wire rope is steel. It is used when large amount of power is to be transmitted from one pulley to another pulley.
  4. 4. Advantages of wire ropes • Ropes furnish smooth, continuous action. • Rope Drives are low in cost of installation and high in efficiency. • The ratio of revolutions can easily be changed by a new small pulley. • Changes of pulley alignment which are so destructive to gears or chains do not affect ropes. • Freedom from shut downs by accidents is assured. The breaking of a tooth is serious; a temporary overstraining of ropes does very little harm. • Rope Drives are noiseless. • The engine or motor may be located away from the grit and dust of the mill. • Freedom from shocks is obtained, as ropes are elastic. • Efficiency of Rope Drives varies from 87 % to 97%.
  5. 5. Wire Rope Construction
  6. 6. Strand and core Strand is two or more wires wound concentrically in a helix. They are usually wound around a centre wire called core.
  7. 7. Lay – 3 types • The direction strands lay in the rope right or left. • The relationship between the direction strands lay in the rope and the direction wires lay in the strands.  In regular lay, wires are laid in the strand opposite the direction the strands lay in the rope.  In lang lay, the wires are laid the same direction in the strand as the strands lay in the rope. • The length along the rope that a strand makes one complete spiral around the rope core
  8. 8. Code Number Composition
  9. 9. Choosing the right wire rope To increase fatigue resistance by selecting a rope with more wires, the rope will have less abrasion resistance because of its greater number of smaller outer wires. For a wire rope with greater abrasion resistance, one choice is a rope with fewer (and larger) outer wires to reduce the effects of surface wear. But that means the rope’s fatigue resistance will decrease. You must consider all operating conditions and rope characteristics.
  10. 10. Basic characteristics of wire rope are, Strength (Tensile strength) Fatigue Resistance (a rope made of many wires will have greater fatigue resistance than a same-size rope made of fewer, larger wires because smaller wires have greater ability to bend as the rope passes over sheaves or around drums.)
  11. 11. Fatigue Resistance (a rope made of many wires will have greater fatigue resistance than a same-size rope made of fewer, larger wires because smaller wires have greater ability to bend as the rope passes over sheaves or around drums.)
  12. 12. Crushing Resistance (Crushing is the effect of external pressure on a rope, which damages it by distorting the cross- section shape of the rope, its strands or core – or all three.) Crushing resistance therefore is a rope’s ability to withstand or resist external forces, and is a term generally used to express comparison between ropes.
  13. 13. Resistance to metal loss and deformation/Abrasion resistance (ability to withstand metal being worn away along its exterior). Resistance to rotation (In rotation resistant ropes, the lay of the outer strands is in the opposite direction to the lay of the inner strands)
  14. 14. Lubrication
  15. 15. Other types of ropes used for power transmission Torque Wire Rope Torque coil
  16. 16. Wire Rope Selection using PSG design data book
  17. 17. Procedure 1. Selection of wire rope type 2. Calculation of design load 3. Selection of wire rope diameter 4. Calculation of sheave or drum diameter 5. Selection of useful cross sectional area 6. Calculation of wire diameter 7. Selection of weight of rope 8. Calculation of Effective Load 9. Calculation of factor of safety 10.Calculation of number of wires 11.Sheave dimensions
  18. 18. Problem 1 Problem Statement: Design a wire rope for an elevator in a bulding 60 m high and for a total load of 20 kN. The speed of the elevator is 4 m/sec and the full speed is reached in 10 seconds. Given Data: Load (W) = 20 kN Velocity (v) = 4 m/s Time (t) = 10 s Height (H) = 60 m
  19. 19. Step 1 “Selection of wire rope” Given that a wire rope has to designed for a elevator, from Page No. 9.1 for the given application, Wire rope 6 x 19 Class 3 is selected
  20. 20. Step 2 “Calculation of Design Load” Design Load = 2.5 x Load to be lifted x Assumed factor of safety Considering the given application and the selected wire rope 6 x 19 Class 3, from Page No. 9.1, Design Load = 2.5 x (20 x103 ) x 5 Design Load = 250 x 103 N Factor of safety = 5
  21. 21. Step 3 “Selection of wire rope diameter (d)” Assuming the design load as breaking load select the wire rope diameter from PSG Design Data Book P. No: 9.4 to 9.5 Design Load = 250 x 103 N Design Load = 25.48 Tonnes Rope dia (d) = 22 mm
  22. 22. Step 4 “Calculation of sheave or Drum Diameter (D)” Using the selected type of wire rope select D/d ratio from PSG Design Data Book P. No: 9.1 and using the ratio D (Drum diameter) is calculated. For the selected wire rope 6 x 19 Class 3, D d = 23 D 22 = 23 D = 506 mm
  23. 23. Step 5 “Selection of useful cross sectional area (A)” Using the formula for "A" in PSG Design Data Book P. No: 9.1 cross sectional area is calculated, π 4 d 2A = 0.4 x π 4 22 2A = 0.4 x A = 152.05 mm 2
  24. 24. Step 6 “Calculation of wire diameter (dw)” d w = d 1.5 d w = 1.373 mm d w = 22 1.5 i = Number of strands x number of wires in each strand
  25. 25. Step 7 “Selection of weight of rope (Wr)” For the selected diameter of wire rope in PSG Design Data Book P. No: 9.4 to 9.5 select the weight of rope. W r = 1.84 kgf/m =18.05 N/m For 60 m height, Wr x 60 =1083 NW r
  26. 26. Step 8 “Calculation of Effective load” Wea - Effective Load Wd - Direct Load (load to be handled) Wb - Bending Load Wa - Acceleration Load E' - Take it from PSG Design Data Book P. No: 9.1 v1 = 0 (initial velocity)
  27. 27. Wea - Effective Load Wd - Direct Load (load to be handled) Wb - Bending Load Wa - Acceleration Load E' - Take it from PSG Design Data Book P. No: 9.1 v1 = 0 (initial velocity)
  28. 28. Wea - Effective Load Wd - Direct Load (load to be handled) Wb - Bending Load Wa - Acceleration Load E' - Take it from PSG Design Data Book P. No: 9.1 v1 = 0 (initial velocity)
  29. 29. Wea - Effective Load Wd - Direct Load (load to be handled) Wb - Bending Load Wa - Acceleration Load E' - Take it from PSG Design Data Book P. No: 9.1 v1 = 0 (initial velocity)
  30. 30. Step 9 “Calculation of factor of safety” Working factor of safety = Working factor of safety = 250 x 103 56129 The working factor of safety is not greater than the recommended factor of safety hence the design is not safe. Hence number of wire ropes is calculated as in next step. 4.454
  31. 31. Step 10 “Calculation of number of wires” Number of wires = 1.122 = 2 wires Number of wires= 5 4.454
  32. 32. Step 10 “Sheave Dimensions” From Page No. 9.2 and 9.3

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