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Design of Steel Cables Wes Chappell Barrett Blackburn Kelly de Montbrun
Design Strategy • as a response to the high failure rate of chain links or solid steel bars used in mining. • The use of multiple intertwined strands of wire distribute the load and the friction between the individual wires and strands (because of their twist) further compensates for any flaws. • A larger number of wires increases the flexibility of the cable and decreases abrasion resistance.
Structure Individual wires are first twisted into a strand, then six strands are twisted around a core which is used to cushion off stress forces when bending the rope. The core can be made from steel or a natural fiber core consisting of hemp, sisal, manila, henequen, or jute and is used to cushion off the stress forces when bending the cable.
Basic Construction 3*7 Contains a total of 21 wires; in smaller diameters, it is very flexible. 7*7 Contains 49 wires, allowing for abrasion resistance and flexibility through a large range of diameters. 7*19 Contains 133, allowing for increased flexibility along a range of diameters. 7*49 Contains 343 wires giving this cable extraordinary flexibility.
Design Consideration/Applications Elevator Wire Rope Rotation Resistant 19x7 Construction (1-6) WC, preformed, extra improved plow steel. 8x19 (Seale) Traction Steel with Natural Fiber Core. Black Powder Coated Galvanized Aircraft Cable Fewer wires=less flexible Very Flexible Flexible
Type of Lays • Right Hand Lay and Left Hand Lay: Describes the way the strands are laid to form the rope. • Ordinary and Lang’s Lay • Describe the way the wires are laid to form a strand of the wire rope. • On ordinary lay, the outer wires approximately follow the alignment of the rope: with Lang's lay they are cross at an angle of about 45°. • Alternate Lay Lay alternates between Ordinary and Lang’s Lays
Cable Braider Completed Cable Core Strand
Disadvantages to Type of Lay Lang's laid rope is able to flex over sheaves more easily (with less damage) but it has the disadvantage of having a high torque tendency (it tends to untwist when tension load is applied) compared with ordinary laid rope. Untwisting can be dangerous with a steel-cored rope: load is shed from the strands and may cause the core to fail as it becomes higher loaded.
Flexibility Cable flexibility depends on the number of wires in the cable (more wires=more flexible) and the diameter of the wires (thinner wire=more flexible) The flexibility of the cable is important in cables used in machinery such as cranes and elevators as well as transportation uses (in cable cars, cable railways, and aerial lifts). Cable flexibility is less important for cables used in suspension bridges.
Corrosion Resistance Methods • Galvanized • Galvanized to finished size • Has a strength 10% lower than standard Bright Wire. • The finished cable is run through the galvanized process • This increases the diameter of the cable, making it less flexible. • Drawn Galvanized Wire • Is galvanized before cable is braided • Equal in strength and size to Bright Wire • Stainless Steel • High resistance to any corrosive conditions • Used extensively in yachting and control cables • Plastics (polyethylene coating/tube)
Cable Termination • Steel cable must be terminated in certain methods to prevent unraveling of the cable. • Most common termination style is to turn the end of the rope back and secure it to rest of the cable making a loop. • Making the loop requires tools to create the loop • Thimble-prevents too tight of a bend in the cable • Clip/Clamp-fixes the loose ends back to the rest of the cable
Termination Methods • Swaged Terminates two wire rope ends together or terminates one wire rope end to something else. • Wedged Sockets Used when a fitting needs to be replace frequently. • Potted Ends Creates a high strength permanent termination. • Eye Splice (Flemish Eye) May be used to secure the loose ends of the cable when forming a loop
Rope Labeling Wire ropes are labeled based on a coding system that describes the number of strands in the rope, number of wires in the strand, the material in the core, and the lays of the rope. i.e. 6*19 FC RH OL FSWR
6 Common Uses • Bridges • Cranes • Elevators • Supports • Cargo Derricks • Tightrope Walking
Refrences • . Mechanical Cable Construction. 21 November 2010 <http://www.savacable.com/pages/applic_01.html>. • . Military Cables and Assemblies. 22 November 2010 <http://www.picwire.com/military.html>. • . Wire rope. 27 November 2010. 21 November 2010 <http://en.wikipedia.org/wiki/wire_rope>. • . Wire Rope Hardware. 2010. 22 November 2010 <http://www.gbgindustries.com/standardhardware.htm>.