Handbook  of  Cable & Wire  Producer

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DoubleTwist buncher is the fastest and compact solution among other twisting/bunching machines. Layout of Double twist buncher generates  two twists per rotation. Cores are pulled through twisting plate over bow (also known as flyer arms). Then, wound on take-up spool. Twisting occurs first at the entrance of bow and second leaving the bow. Doubletwist machine may be used for several different applications.  Bare conductors and insulated cable cores can be twisted at comparingly high speed level.  Twist per minute is twice the rotation of bow.A big advantage over not only Single-twist machine and also other twisting applications.  Latest models can reach 8000 twists per minute.  In General, double twist machines are referred by the size of Take-up spool. "XTC630" for example, points that take-up spool flange size is 630mm.

Various types of bows are used for bare wires, solid or flexible insulated cores. Use of correct bow is important for vibration problem and tension control over twisted material.  Light-weight bows are more productive.

Double twist is compact , fast and reliable but still not best solution for large solid bare conductors. Construction of bundle is not as good as a rigid strander.  Total weight of take-up spool is limited.  Therefore, over 1600mm flange size is not productive. Also,  Production speed is decreased drastically by spool size.  Vibration is an obstacle for doubletwist bunching. Machine manufactuers use various solutions to that.

Best For:  A doubletwist machine is best for bunching fine strands of bare conductors and flexible insulated cores.

Versatility: DoubleTwist buncher has versatility.  Number of strands has no limit as long as finished overall diameter stays within in the range of machine.  Total weight of take-up spool is limited. Size of take-up spool is fixed.  Application of  foil or other wrapping materials is not possible while twisting process. Changing processed material may require bow change.

There are two main types of doubletwist.

(1) Direct pull-up.

(2) Double twist with capstan.

(1)Direct pull-up double twist uses take-up spool as pulling force. Take-up spool and bow driven by two seperate motors. Lay-length is adjusted by means of changing take-up spool rotation versus bow speed electronically controlled.  Advantages of direct pull double twist machine are precise and constant lay-length, highest speed for twisting, minimum mechanical friction and use of gears minimized. 

Therefore maintenance cost and frequency are recuded as well.  Precise electronic control of take-up speed also results in precise pitch.  Disadvantage of direct pull doubletwist is that requires use of motor driven pay-off units.  Without motor driven pay-off units may result unwanted twist construction.  Second disadvantage is that "flower effect" is still not solved.  Rotating and motor driven pay-off units are necessary to maintain overall quality of final product.

 (2) Double twist with capstan use only one main motor. Rotation of bow unit is transferred to captsan via a gear set. Then, take-up spool by means of a clutch.  Cores are pulled by capstan.  Lay-length is fixed by changing gears that adjust the ratio between bow and capstan. 

Low initial cost is an advantage.  However, low speed , tension control problem, wearing gears,  non-stable pitch problems make this type undesirable.  Not to mention "Flower effect" is still there.


Flower Effect :


When bunching insulated cores,  round shape of insulation is ruined because of torsion. In this case,  insulation thickness is not constant and cores not circular anymore.  Impedance and capacity of the final assembly would not be a constant value by length.  Such default bears impedance, capacity and cross talk problems for Communications cables working over 600Mhz.  Solution is back-twist or pre-twist  depending on what type of twisting machine is used.