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Mast Specifications - the same size is not always the same strength!

“I will Huff and Puff and blow your mast down!”

The weather conditions have seen significant amounts of damage to the installed base of aerials and reception equipment.  As a result, of which over the last few year we have taken numerous calls about the potential failure point of product (especially aluminium masting) and given advice on the correct choice of product.

The mast diameter and gauge are always the main factors when it comes to installers choosing masting but the aluminium grade can also have a significant effect on the strength and suitability of the mast for the given installation.

There are typically two types of masting available in aluminium on the market. Seam welded and extruded tubing. The manufacturing processes for these types of tubing are completely different.

  1. Seam welded tubing (SWT) starts its life as an ingot of aluminium (5 tonne). It is then rolled and tempered into strips and coils. These strips are then rolled again, pre-formed and formed into the mast shape. A welding process, using a high inductance coil, forms the secure seam. The tubes are deburred, cut, cleaned and packed.
  2. Extruded tubes also starts life as an ingot of aluminium, which is then formed into blanks that are pre-heated and forced (using a ram) into the extrusion process. This process plasticises the aluminium under pressure and forces it through a die to form the shape. The lengths are then cut and packed.

Extruding has the advantage to create very complex cross-sections and to work with materials that are brittle. The disadvantage is that the extruded aluminium is less dense than the seam welded and may contain air pockets which weakens the wall. Both processes have to use different aluminium grades and this is also how the strengths vary. The rolling and forming of the seam welded tube also tempers the material to give additional strength.

The important facts and figures are the alloy and temper details which can be compared. These mechanical properties of both types vary quite a bit:

 

Alloy type

Tensile strength

Yield

Elongation A50

Aluminium Extruded Tube 2” masts

6063 T6

< 215 N/mm²

< 170 N/mm²

< 6%

Aluminium Seam Welded Tube 2” masts

5449 H195

< 290 N/mm²

< 275 N/mm²

< 3%

 

Comparing both grades, it can be seen that the seam welded tube is a lot stronger:

● Tensile strength is about 34% higher.
● Yield about 60% higher.

  

The Yield property is decisive for permanent deflection. In other words, assuming that tube dimensions are identical, in stormy weather an extruded tube in alloy 6063 T6 will bend and deform a lot sooner and more than the SWT.

Other issues are that the metal structure is more homogeneous and dimensional tolerances of the SWT are a lot tighter. On wall thickness extruded tube has +/- 0.25mm tolerance whereas SWT has +/- 0.08mm tolerance. The accuracy may be of importance for the mounting and fitting into castings or clamping.

The CAI understand the differences and recommends using SWT in the Code of Practice for for the ‘Installation of Aerials/Antennas & Receiving Equipment in the Single Dwelling Unit’ and ‘Installation of Terrestrial and Satellite TV Reception Systems (MDU & Commercial)’.

Typically, an extruded tube (Fig. 1) is a dull with a matt finish and may be individually wrapped in plastic. On the other hand, a seam welded tube (Fig. 2) has a brighter finish and you can just see the seam weld running the length of the mast internally and externally. If you are unsure about whether the masting you are fitting is seam welded or extruded please ask your supplier.

Extruded TubeSeam Welded

 

In conclusion:

When aluminium masts are used they should be of a seam welded construction.

     

Note: Please refer to the CAI Codes of Practice for further reading and confirmation of specifications.

Click here for guidelines on the size of bracket for the relevant mast size for the range of brackets.