Equipment for tensioning of dywidag threaded bars
FPT – Fluid Power Technology – over the years has developed experience and expertise in various fields of industrial and civil engineering. Historically the hollow hydraulic cylinders were born for the extraction of pins. The same devices have proven to be essential for stressing some bars specifically designed for civil purposes.
For the same bars are available ranges of FPT hydraulic tensioners that allow to reduce to the minimum axial distances of installation and extra lengths for stressing the bars.
The bars mentioned above are usually known as Dywidag. Dywidag® is the trade name of a product developed in Germany by the company named in the past as Dywidag (today DSI acronyms of Dywidag System International).
The Dywidag bars correspond to the class Y1050 in the catalogue SAS and identify the product with the higher resistance values (yeld strength fp0,1k= 950 N/mm2, tensile strength fpk= 1050 N/mm2), over the mentioned bars there are other bars ranges with lower resistance values that cover different applications, the most common ones are:
- Bars B500 (corresponding to DSI “Gewi”): yeld strength fp0,2k= 500 N/mm2, tensile strength fpk= 550 N/mm2
- Bars S670 (corresponding to DSI “Gewi Plus”): yeld strength fp0,2k= 670 N/mm2, tensile strength fpk= 800 N/mm2
- Bars for formworks: yeld strength fp0,2k= from 750 to 950 N/mm2, tensile strength fpk= from 875 to 1100 N/mm2
The mentioned bars are threaded on all the length and produced by hot-rolling process, the thread has a width such as to allow it to be screwed in all conditions of use especially in the presence of dirt or mechanical damage of the teeth.
The thread, realized during the rolling process, not interested in the whole circumference of the bar but only two diametrically opposed sectors as can be clearly seen from the drawing below.
A limitation of this thread is the need to tension the bars by putting them in traction and then tighten the anchor nuts. It can also be stretched using torque wrenches (manual or hydraulic), but the losses due to friction system (friction between the thread of the bar and the thread of the nut) and the high efforts of twists that act on the bar (part of the torque applied because mortgage friction between bar and nut is transferred as a torsion on the bar instead of as a traction on the same bar) they discourage use when values are desired tightening in line with normal technical requirements (both geotechnical and structural uses).