Wire drawing machines
Modern trends regarding large double twist bunchers
Concept
While the concept of double twist technology was initially an American invention from the 60s, Italian machinery companies were among the first to develop large double twist bunchers for bobbins up to 2,000 mm (72 inch).
Since the ‘90s, double twist bunchers have been used to manufacture unilay constructions for up to 37 wires or up to 300 mm2 (600 MCM). All these lines featured an external rotating capstan, which provided the main pulling force. This driver capstan also operated as a kind of pre-twister, giving a lay-length very close to the final lay-length of the conductor. Therefore, after the second twist, the conductor was already smooth and with an acceptable quality surface. The surface quality is of utmost importance, since those conductors are not only for Low Voltage cables, but for Medium Voltage cables as well.
The main advantages of the double twisting lines are the following:
• high production speed, more than 100 m/min for the bigger sections;
• individual wires are fed from stem packs. If dual stem packs are used, the line can run continuously, drastically reducing the loading time. The operator welds the ends in parallel with the loading process, which is very fast. For safety reasons, the unwinding stem pack areas are protected with fences with safety interlocks;
• smooth surface quality, thanks to the unilay process;
• up to 13-14% diameter reduction;
• bobbins free environment in the payoff/stem pack area. The only bobbin is the take-up bobbin.
The key concept here is that when the line is operating with the external capstan in use, then the operation is in single-twist mode. It is a kind of hybrid mode between single twist and double twist—but more similar to the single twist. This means that inside the cradle we do not have a complete second torsion, but only a part of it.
Trends
1. Increase of the production speed.
New models have already been introduced and commissioned in the US market, such as the innovative CDT1800S model. With some ingenious engineering solutions, MFL machinery can achieve higher linear speeds than the current standards. The first CDT1800S produced in the MFL GROUP factory in Barcelona has been in operation since mid-2019.
2. Increase of the cradle dimensions.
Over the last 20 years, the primary use of these large double twist bunchers was to produce conductors up to 37 wires (300mm2). The main conductors manufactured are:
- copper and aluminum conductors for LV cables (also for ABC conductors);
- aluminum and copper conductors for MV cables.
Recently, the corporate machines have been used for compacted aluminum conductors up to 500 mm2 (1000 MCM) at 60 m/min, with a DTO2000/CE1000. New models are under development for aluminum compacted conductors up to 630 mm2, with 2500 mm bobbins and new external capstans with a pulling force of 6,500 kg. In the next few years, the usage of double twist bunchers with 2000 mm and 2500 mm bobbins is expected to rise, since the request of MV is in continuous growth (for example, for New Wind Farms and a lot of other applications).
Specifically, for MV cables, the big double twist bunchers of MFL GROUP (OM LESMO/CM CABALLE Brands) can guarantee the eccentricity required by the relevant specifications. Big double twisters are the most efficient solution to produce conductors up to 61 wires. The aforementioned trends for bigger bobbins and external capstans indicate that the industry is not far from implementing big double twist bunchers in the manufacturing of conductors for even higher voltages than the standard 33 kV.
