Helical Drum Cable Payout

Notes & Use

Calculates the payout (in feet), and total number of wraps for a grooved helical drum.

  • The drum must be of constant pitch diameter
  • This calculator will calculate for both bitter-end rigging and operating-loop rigging.

Figure

Terms

$l$ = Groove lead, inches

$D_{pitch}$ = Pitch diameter of drum, inches

$w$ = Width of drum’s grooved surface, inches

$n_{lines}$ = number of lines:

  • count each operating cable loop as 1 line.
  • count each bitter-end line as 1 line.

$g_{safe}$ = safety wraps (per line)

  • use total number of safety wraps for a single loop (E.g. three safety wraps at ends of cable is a total of 6 safety wraps for 1 operating loop.)
  • Bitter-end lines such as dead hauls are a little more straightforward

$g_{total}$ = number of total grooves on drum

$g_{usable}$ = number of usable grooves, does not include:

  • safety wraps per line
  • wasted wraps per line (usually a half-wrap per line)
  • unusable half-groove at both ends of drum

$L$ = Payout, length of cable that will fit onto the drum, per line, feet

Equations

Calculate the total number of wraps: $$ g_{total} = \frac {w} {l} $$

Calculate the usuable wraps: $$ g_{usable} = g_{total} - g_{safe} n_{lines} - \frac {n_{lines}}{2} - 1 $$

Return the payout… $$ L = \frac {\pi D_{pitch} g_{usable}} {n_{lines}} \frac {1}{12} $$

Sources