Technical Information

Recommended Minimum Diameter of Pulleys

·  If wire ropes run over pulleys, the working life can be extended by the right choice of pulley and suitable wire rope.

·  Pulley Diameter in mm for 7x7 construction wire rope = 42 x Rope Diameter (mm)

·  Pulley Diameter in mm for 7x19 construction wire rope = 25 x Rope Diameter (mm)

Stretch in Wire Ropes

·  Stretch is a characteristic of all wire ropes; constructional stretch initially (as the individual wires 'bed down') and then as conventional elastic stretch.

·  This permanent stretch can be as much as 0.1% for a 1x19 strand.

Elastic Stretch

·  Once a cable has bedded down it will obey Hookes Law; elastic stretch will be proportional to the load applied. Resistance to this stretch is determined by the modulus of elasticity.

Pre-Stressing

·  Where stretch is critical to the application, constructional stretch can be eliminated by pre-tensioning or pre-stressing. This can be carried out during manufacture or swaging.

·  Pre-stressing is usually only applicable on large bridge structures and is not necessarily appropriate on structures where rigging screws and tensioning devices are used to take up any stretch.

Elastic stretch can be calculated by the following formula:

·  Elastic Stretch = (W x L) / (E x A)

·  W = Applied Load ( kN )

·  L = Cable length ( mm )

·  E = Strand Modulus ( kN/mm²)

·  A = Area of Cable = (D2 x pi) / 4 (where D= Dia of cable mm)

Typical values for E are:

·  1x19 = 107.5 kN/mm²

·  7x7 = 57.3 kN/mm²

·  7x19 = 47.5 kN/mm²

·  Dyform = 133.7 kN/mm²