Calculating Lay Lengths

Can anyone provide a general formula for calculating the wire lay length for an 8 wire strander. The wires will be laid around a core. I'm also looking for the formula to convert the results of the lay length to a speed ratio between the inches per minute of the core and the revolution per minute of the strander. Sorry if this does not sound technical. I'm a mechanic trying to understand a process so that I can help to maintain it.

The length of lay is the precise length measured along the axis of a wire or cable required for a single strand of bare wire (In reference to a stranded conductor.) or bare or insulated conductor (In reference to a cable as is your case.) to make one complete turn (Precisely 360 degrees.) about the axis of the wire or cable. For a twisted pair it is the exact distance required for the two wires to completely twist around each other. (Again exactly 360 degrees.) In a cable the lay is usually measured or calculated in inches with at least two decimal places.

To a mechanic, this is exactly the same as the pitch of a screw (Threads per Inch) and not surprisingly the wire and cable lengths of lay come in both left and right lay directions.

The length of lay can be calculated and thus predicted by determining (Calculating) how much the capstan (The device which axially advances the stranded cable.) moves the cable forward for one exact turn of the cradle which is the rotating part of the strander holding the eight conductors. (There are many different designs of cable stranders so I hope the concept of a rotating cradle holding the eight conductors is not confusing to you.) Verify the length of lay by actual measurement of same downstream of the capstan while the cable is still under tension so that you know that your gear train length of lay equation is precise.

Often, a number of successive lengths of lay are measured so that the length of lay can be more accurately determined by averaging.


The Lay Length (Inches) X the Strander RPM = The Line Speed (Inches per Minute) (For double twist machines multiply the Line Speed by 2)

Line Speed (Inches per Minute) X Time/12 = Cable Manufactured (Feet)


Sincerely,

Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com