Wire and Rope Drives Explained
- 1. Wire Ropes © Dr.V.R Deulgaonkar 2018
- 2. Construction of wire ropes Prof.Deulgaonkar V.R University of Pune
- 3. • Wire rope consists of number of “Strands”. • Each strand comprises several steel wires. • The no. Of wires in each strand is 7,19,37 while the number of strands is usually six • Individual wires are twisted into the strand and then strands are twisted around fibre or steel core. Prof.Deulgaonkar V.R University of Pune
- 4. • Specification of wire rope includes two numbers as 6X7 or 6X19. • First number indicates the number of strands while the second number gives the number of steel wires in each strand. • Popular constructions of steel wire ropes are 6X7 (6/1) , 6X19 (12/6/1), 6X37 (18/12/6/1) Prof.Deulgaonkar V.R University of Pune
- 5. • Central portion of the wire rope is called core • Cores are of three types : fibre, wire and synthetic material . • Fibre Core : it includes natural fibers like sisal, hemp, jute or cotton. It is flexible & suitable for all normal working conditions, except under high load. • Wire core is used for severe heat or crushing conditions. Prof.Deulgaonkar V.R University of Pune
- 6. • Rope Lay : It refers to the manner in which the wires are helically laid into strands and the strands into the rope. • If the wires in the strand are twisted in the same direction as the strands, then the rope is called Lang’s Lay rope. • When the wires in the strand are twisted in a direction opposite to that of strands, then the rope is said to be “regular or ordinary lay” Prof.Deulgaonkar V.R University of Pune
- 7. Prof.Deulgaonkar V.R University of Pune
- 8. • Regular lay ropes are more popular than the other one as it offers following advantages: 1) More structural stability. 2) More resistance to crushing and distortion. 3) Less tendency to rotate under load. 4) Less possibility of kinking. 5) Easy handling during installation. Prof.Deulgaonkar V.R University of Pune
- 9. Stresses in wire ropes • The individual wires are subjected to direct tensile stress & bending stresses due to load being raised. • The bending stress in one individual wire is given by σb = Mb y/I ; y = dw/2 dw = wire diameter in (mm) σb = Mb dw/2I ----------- (a) Prof.Deulgaonkar V.R University of Pune
- 10. • The elastic curve equation is Mb/EI = 1/r. • Radius of curvature in the above equation = radius of sheave. Mb/EI = 2/D -------- (b) D is dia of sheave From (a) & (b) σb = Edw/D -------- (c) Prof.Deulgaonkar V.R University of Pune
- 11. • The modulus of elasticity is replaced by effective modulus of elasticity of wire rope Er • So equation (c) becomes σb = Erdw/D ----------- (d) To design wire ropes, it is convenient to bending stress into equivalent bending load, that would induce the same bending stress. Prof.Deulgaonkar V.R University of Pune
- 12. • The equivalent bending load is Pb is Pb = σb A = A {Erdw/D} --------- (e) A is the area of metallic cross-section in the wire rope. (refer table 23.7 :Bhandari V.B) Failure of wire rope is mainly due to fatigue or wear. Bending and straightening of wire as it passes over the sheave results in fluctuating stresses. Prof.Deulgaonkar V.R University of Pune
- 13. • Amount of wear depends upon the pressure between the rope and sheaves. The force per unit length is pdr • Considering the equilibrium of forces in vertical direction we have 2P = D p dr : p = 2P/D dr Prof.Deulgaonkar V.R University of Pune
- 14. Selection of wire ropes • Guidelines for selection of wire ropes are 1) Referring to table 23.4 {Design of machine elements by V B Bhandari} wire ropes of designation 1960 have higher load capacity than those with designation 1570. Use of steel cores in place of fibre cores increases the strength of wire ropes to some extent. Prof.Deulgaonkar V.R University of Pune
- 15. 2) Flexibility of wire rope is one of the important consideration. The wire rope of 6X7 construction consists of a few wires of relatively large size. It is too stiff for hoisting purposes. It is suitable for haulage and guy ropes. The 6X19 or 6X37 constructions are flexible wire ropes and are commonly used in hoists. Prof.Deulgaonkar V.R University of Pune
- 16. 3) Where wire rope is likely to drag through gritty material or across stationery object abrasion resistance is of concern. Large diameter wires with 6X7 gives better wear resistance. Factors of safety for wire ropes for different applications are given in tables as Prof.Deulgaonkar V.R University of Pune
- 17. F.O.S in wire ropes for general applications Application Class 1 Class 2 & 3 Class 4 Fixed guys, jib cranes , ancillary applications as lifting beams . 3.5 4.0 4.5 Hoisting and luffing systems of flexible cranes as mobile derrick(Shock absorbing devices are incorporated in the system) 4.0 4.5 5.5 Cranes and hoists 4.5 5.0 6.0 Prof.Deulgaonkar V.R University of Pune
- 18. FOS for wire ropes in mining applications Application Factor of safety a) Mining ropes For shafts of varying depths Upto 300 mm 10 300-500 9 500-700 8 700-1000 7 b) Haulages ropes 7 Prof.Deulgaonkar V.R University of Pune
- 19. Rope drum construction and design • Two types of constructions for rope drum are available viz. a) Drums with helical grooves b) Plain cylindrical drums without grooves Preference is given to grooved drums rather than plain drums for most hoisting installations. Prof.Deulgaonkar V.R University of Pune
- 20. • The machined grooves increase the bearing surface of the drum and prevent friction between adjacent turns of rope. This reduces wear and increases the life of rope. • Drums are made of grey cast iron of Grade FG200. Sometimes steel is used. Prof.Deulgaonkar V.R University of Pune
- 21. Prof.Deulgaonkar V.R University of Pune
- 22. • A grooved drum and the grove profile are shown in fig. below . Drum is provided with helical grooves so that the rope winds up uniformly on the drum. • Radius of the helical groove should be selected so as to prevent jamming of the rope. • Drums designed for two rope members are provided with two helical grooves. R.H & L.H Prof.Deulgaonkar V.R University of Pune
- 23. • The pitch of the groove is given by t = dr + (2 to 3 mm) The shell thickness of the cast iron drum is given by t1 = 0.02D +(6 to 10 mm) where dr = nominal dia of rope (mm) D = drum dia (mm) Prof.Deulgaonkar V.R University of Pune
- 24. Prof.Deulgaonkar V.R University of Pune