Basic data on mechanical calculation of wires

Permissible tension for wires’ stretching

The calculation is performed based on permissible tensions method.

Stress in the wire is the force present at any point of the wire measured in kg ().

Tension is the force affecting a unit of cross sectional area of the wire measured in kg/mm2 ():

(Stress divided by tension). The tension in the wire at any climatic conditions should not exceed the permissible tension. There should be some margin of safety ()

.

- margin of safety;

- permissible tension.

 

According to EIR (old edition)? The permissible tension is actually used instead of margin of safety (n), the values are expressed in shares or percent from the tensile strength.

. In actual calculation the tension in the wire material is limited by three options:

1.      For maximum load.

2.      For minimum temperature.

3.      For average annual conditions.

(The three permissible tensions (for maximum load, for minimum temperature and average annual temperature) was taken up untill 1975 for steel-aluminum wires (high for ice, lower for minimum temperature) for mono-metal wires the values were the same. In 1975 the tension for the lowers temperature was established as well as for the maximum load.)

 

The approach towards sorting out the permissible tension will be slightly different for wires made of one material and for those made up of combined materials.

Mono metal wires.

1.                 Permissible tension at low temperatures and maximum load are accounted for by and tensile strength coefficient.

2.                 The permissible tension at average annual conditions are limited by the aspiration avoid breaking of the wires due to vibration (there is no ice, low temperatures or wind). These tension is accounted for as the stress in the wire and the tension of bending in case of vibrations. The summary tension should not exceed the permissible fatigue stress.

Composite wires.

The summary tension of two constituent parts:

1.      Own weight and external load on the wires.

2.      Additional tension – additional temperature tension that appears at temperatures different from the temperature of manufacturing of the wire.

=23·10-6

=12·10-6 - Additional tension – additional temperature tension that appears at temperatures different from the temperature of manufacturing of the wire.

 

Let the manufacturing temperature for a steel-aluminum wire be .

The expansion of aluminum and steel is different (more for aluminum, less for steel). But they are rigidly connected, so the elongation will be something average between steel and aluminum.

Steel will experience stretching force, aluminum will experience contracting force (additional forces). So the coefficient of linear expansion will be somewhat average – .

If , aluminum will experience stretching force, steel will experience contracting forcev.

So the wire will also experience both elongation and contraction with .

зависит от , и от «».

The value of α0 could be found from the equation of stress balance

где

- are modulus of elasticity measured in kg/mm2.

Hence:

» is inserted)

Let us define the modulus of elasticity for the entire wire ().

Summary stress in the wire:

- modulus of elasticity for the entire wire.

The values characterizing wires are partially presented in EIR, reference catalogs and Technical Conditions for wires:

 

Type of wire

23·10-6

6300

16

0.5

0.5

0.3

А

23·10-6

20000

70

0.5

0.5

0.35

ПС

23·10-6

8450

29

0.37

0.42

0.25

АС

 

 

 

Additional tension at minimum, maximum temperatures, average annual conditions

АСУ

 

 

 

АСО

 

Steel-aluminum wires experience some influence at maximum and minimum temperatures. There are some additional temperature tension. At minimum temperatures the influence is stringer. So .

The calculation of composite wires is performed based on permissible tension in the wire material that has the lowest mechanical strength. When calculating, the temperature of the wire should be taken as ambient temperature.