what  is the difference between single twist & double twist buncher machine ?

The basic difference between a single twist buncher, strander or cabler is that a single twist machine gives one 360 degree twist in the wire or cable for one 360 degree turn of the bows. A double twist machine gives two 360 degree twists in the wire or cable for every 360 degree turn of the bows. Thus a double twist machine gives twice the productivity at the same RPM of the bows.

There is however an issue with double twist machines and a great deal of work has been by double twist machine manufacturers to make them much more equivalent.

The fundamental way a double twist machine works is that one gets a twist going on to the bows and a second twist coming off of the bows. This means that the length of lay at the first twisting position is twice the final length of lay and thus at the second twisting position, the strands of wire must be twisted around themselves to half the initial length of lay.

If you have a number of layers of strands, the strands in each layer require a different extra length to achieve that second twist.

Now you see the problem as each layer must pull in a different extra length and this means that if it isn't done properly, the final conductor will be "bumpy" and that can lead to major problems at the next operation (say extrusion). The more layers of strand, the greater the potential problem. The diameter of the strand layers is also an issue because the the larger the diameters, the greater the effect.

Always be cautious when purchasing double twist stranders. Make sure the machine manufacturer clearly understands what you you want to do and make sure to see an identical machine elsewhere making the product you wish to manufacture before purchasing one.

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