Does any one know how to calculate the percentage of back twist in a cable; we are serving 4 stranded wires on a coated core, using a planetary machine. We have the ability to control the amount of back twist with a plc. Full planetary just takes the twist out of the stranded wire.We can get it to work by Experimenting
but it changes with every wire size.
Thanks Brian Sundquist

Hi Brian,

You are going to have to tell us exactly how this planetary machine is geared or designed because, if I understand you correctly, somehow the diameter of the finished cable is having an effect and that suggests the capstan is somehow involved.

If you are doing precision stranding with variable backtwist, you must also keep in mind that the amount of backtwist you are introducing is also added or subtracted (algebraically) to length of lay in the stranded wires you are closing over the coated core. (Rigid strander effect.) This will change the final cable diameter slightly and that may be the effect you are detecting.

Kindest regards,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

The machine has one motor driving the capstan and the main head, and one motor driving the payoff spool rotation, we can set any ratio between the two we want. the cable we are standing has a very short lay (.080). Brian

Hi again Brian,

OK, thanks for the information. Obviously this machine is very small. It is also evident that you are doing precision stranding.

If I understand your description fully, you have a conventional rotating pay-off frame (main head) or carousel on which the pay-off cradles are mounted and that these cradles are independently, electrically, gang geared and driven to provide any degree of backtwist from 0% (Full rigid.) to 100% (Full planetary.) Please correct me if I am wrong.

We also understand that there is just one main motor driving both the capstan and the carousel (main head) so there must be some mechanical means such as gearing to obtain the required length and direction of lay. It also means that there is a full mechanical disconnect between the carousel-capstan control and the cradle back twist control.

Do I now understand that you can only get 100% planetary operation by experimentation with the plc settings and that the settings change with every "every wire size"?

That would be very strange indeed and perhaps could have something to do with different weights of wire in the pay off cradles. (For example, increased drag on the cradle bearings.) Perhaps there is slippage of the drive bands (Belts?) or some other component of that cradle gear train. This slippage could be a function of different pay off cradle weights. Perhaps the cradle motor is too small and the rotor is slipping as a function of the weight in the cradles. The plc could also be incorrectly programmed or malfunctioning. You will have to examine the situation carefully and do some precise experiments to learn exactly what is going on.

Solve the full planetary situation first and all the other back twist settings should just fall into place.

Kindest regards,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Hi Peter,

Thank you for the information. You are right on with the machine description. We use change gears to set the lay ratio so that it doesn’t change,

But I was not explaining the problem right.

What happens is;

When we change the percent of back twist the wrap angle seems to change So we keep changing the percentage of back twist so the strands look like their is no torsion in them, and the lay is right.

Seems happy at 30% on some sizes but 50% seems better on others. I was looking for a way of calculating it without experimenting. Does the back twist percentage change with lay angle?

Sincerely Brian

Hi again Brian,

In a mechanical system, the amount of back twist has nothing to do with the cable lay as it is nothing more than the relationship between the rotation of the carousel and the axis of rotation of the cradles. Thus there is no effect between the amount of back twist and the final cable lay as long as the components making up the cable cannot change their length of lay and the components cannot flatten by virtue of cradle tension.

In your case however, you are cabling stranded wire and the degree of back twist is also applied to the length of lay of the stranded wires. This in itself could change the diameter of the strands slightly and is the basic reason why telephone cables were always rigidly stranded and cabled left, left, left (Units, Groups, Cables.) Thus the individual components of the telephone cable were always tightening. In those products however, the small change in the diameter of the components and the change in the length of lay was not even noticed in terms of the final and slightly deformable cable diameter.

Again, in your case, the precise nature of your cabling process may show a small but measurable change in the final cable diameter as explained previously. If you are using a single or dual wheel capstan, and length of lay could change and the line speed could change slightly. This then would show as a small change in the application angle. (Wrap angle.) Perhaps this is what you are seeing.

While you can relate the cable diameter, the cable lay, the direction of cable lay, the strand diameter, the direction strand lay and the percentage of back twist together, the amount the strand deforms (Flattens.) when applied to the cable due to the backtwist effect is a true unknown. The tension in each strand also has an effect here.

Thus it would be best to develop a simple, empirically determined table for this cabling process.

Kindest regards,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com