OCIMF_Mooring_Equipment_Guidelines_(Second_Edition)

sensitive to moisture aDd corrosion. The relatively simple AC pole changing motor is inferior to

other drives in pull/speed characteristk (speed variable in steps only, high starting torque, on/off

control only). The. AC/DC drive has excellent drive characteristicoS, but is expensive and more

sensitive to moisture,. corrosion and overheating than any other drive type.

7.5 WINCH BRAKES

The brake is the heart of t.he mooring ystem. since the brake secures the drum and thus the mooring
line at the shipboard end. A further important function of the brake is to act as a safety device in
case [he line load becomes exce sive, by rendering and allowing the line to shed its load before it
breaks.

Ideally, a brake sl10uld hold and render wi!.hin a very small range. and once it renders. shoul.d shed

only eoough load to bring the line tcnsion back to a safe level. Unfortunately, the widely used band

brake with screw application is only marginally satisfactory in fulfilling these requirements and its

operation requires special care.

7.5.1 Layers oj Mooring Line On Drum

Split-drum Winch

The rated brake bolding capacity is only achieved with one layer of wire on the tension drum.

Operation with additional layers will decrease the bra.ke bolding capacity.

As an e..xample, lhe. theoretical reduction in holding load for more than one layer on a 24 H diameTer

drum is as follows and aS5ume~ a rated brake holding capacity of 55 tonne.~:

Layer of wires

Undivided Drum Winch

1

2

3

4

5

Theorelical

holding capacity

(tollnes) (kN)

SS 539

49 481

45 441

41 402

38 373

0/, Rated

holding capacity

100

89

82

75

69

Brake holding capacity of the undividc{! dwm winch, as with lhe split-drum winCh, is affected by

the number of layers on tbe drum. It is therefore, essential that the operator of an undivided drum

winch knows the number of layers of wires on the drum that the manufacturer states will develop

the design brake capacity. More layers on the drum wiU increase the moment applied to the drum

because of tbe increased moment arm. This means t.hat a smaller mooring line load will provide the

moment which win cause the brake to slip. Conversely with less layers on the drum. mooring line

loads must be larger to cause the brake 10 slip_ It is very possible that mooring line loads could

become excessive tbereby increasing the possibility of mooring line breakage. Thus. tbe operators, as

io tbe case of split-drums. should be advised and trained in tbe use of the und.ivided drum winch

[0 ensure integrity of mooring and safety of vessel.

Wben applying the requirement for winch brakes to hold to a mlnllllUm holding load of 600]0 of

MBL 10 undivided winch drums, due consideration should be given 10 the number of layers of wire

which will be found on a drum during normal operations. For this purpose, it should be assumed

that 30-50 rn of wire minimum will be outboard of the fairlead.

7_5.2 Band Brakes

Band brakes follow t.he same principle as w-rapping a rope around a bilt or warping head to hold

a line's force. Relatively little force is required to hold a high load. This principle provides for easy