Non-compliant rod in close-up.
Threaded rod is by far the most popular product used to suspend M&E and HVAC systems and as such has been the obvious target for some manufacturers to increase profitability and/or gain competitiveness. Our policy for the past 10 years has been to ensure that our threaded rod is always sourced from manufacturers that produce threaded rod to the correct standards and weights. Despite our efforts to heighten awareness regarding poor quality threaded rod, there are ever increasing volumes of under-weight rod being imported into the UK from less quality-conscious manufacturers.
These manufacturers have developed a production technique whereby under-sized wire is used in conjunction with special thread forming equipment that produces thinner, “sharper” threads with a much lower thread angle. The resultant rod has the correct outside dimension but the reduced core dimension combined with the elongated threads will heavily affect its load bearing properties and will not achieve the DIN ISO 898/1 stated loadings.
The close-up view of the non-compliant rod (circle B) shows the following characteristics ;
- The lower thread angle which produces longer thinner threads.
- The flat bottom space between the base of each thread.
- The reduction of the internal diameter of the solid core of the rod.
Thread Stripping under load.
It has been found that threaded rod with a thread angle less than the standard 60° will actually have a lower weight per metre.
For example, a rod with a thread angle of 50° has a weight which would be 10% lower than the stated weight for that product. We have even seen weights up to 20% less. This reduction in weight saves the manufacturer not only on the raw material cost, but also the plating and carriage costs too, as these are all directly linked to the products total weight.
It is also evident that nuts used on this under-weight rod have a tendency to appear loose with a slight axial movement. This is a result of the thinner threads not locating correctly within the female thread of the nut. In addition to this, the weight bearing portion of the nut is concentrated onto the ends of the threads, which highly increases the possibility of failure due to thread stripping.
The affects of thread stripping can be clearly seen on a rod that we tested in-house that failed before reaching it’s maximum loading.
The compliance weight test.
The accepted standard for threaded rod is DIN976-1 and DIN 13-20 with tolerance field 6g.
It is possible to confirm whether a rod meets the above standards by performing a simple weight test of a 1mtr length. The table below shows the recognised weights for each popular size.
|M6||4.8||1000 mm||167 g|
|M8||4.8||1000 mm||306 g|
|M10||4.8||1000 mm||484 g|
|M12||4.8||1000 mm||703 g|