OCIMF_Mooring_Equipment_Guidelines_(Second_Edition)

application since it can only be wholly effeclive on wincbes with a single drum, which are not
common on VLCCs. On double or lriple drum winches. it is nOI normally possible 10 engage all

drum shafts and al the same time maintaiD equal tension on the mooring lines. If only one drum is

engaged, t.h.i.c; would result in the lines with lower brake bolding capacity Slipping and transferring
loads to the drum whose brake is augmented by steam. This could result in the parling of the line.
Thus, {he particular circumstances should be fully assessed before USiJlg this emergency procedure.

7.5.2.4 Friction Coefficient

A smaU change in the friction coefficient will cause a large change in the holding capacity. As a

rule of thumb for typical moori:ng winch brakes, Ihe cllangc in holding capacity is Iwice the change

in the friC1ion coefficient. For example. a 100J0 change in friction will cause a 20(lja chang'e in holding

capacity. Tbe friction change can be due to wear of Ihe rim or brake lining, or 10 changes in weather

conditions. among others. This drawback can be partially offset by stainless steel liuing of brake

rims and by frequent tesring and recalibralion.

7.5.2.5 Load Dependency of Holding CapadlY

Once a tine load is applied to tbe drum, t.be brake band will stretch. redUCing the load on the brake

controls. For this reason, a conventional screw brake tan be easily re-tightened when the mooring

line is under hlgh load, even if it was set hard originally. This means thal there is 00 way 10

deterr.nine the proper handwheel torque required once the winch is subjected to a high line load. The

danger exists that under worsening environmental c:onditions Ille brakes can be re-lightened 10 the

pOinl where the line may pan before the brake slips. The problem can be solved by using spring

applied brakes. The spnng automatically compensates for the elongation of the brake band, thus

assuring a constant bolding capacity of tbe brake. It also prevents the brake from being over-lightened.

HVDRAUUC PUMFI

iFo' 8 ke ~I_f

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PI1ESSUll[ SIDE

Hf\NOWHEEL

(Fol AdJustmenl 0/ Spring

. nd EtMrqency Ael .... )

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--+.-

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FIGURE 7.4: SPRING-APPLIED BRAKE WITH HV'ORAULlC RELEASE

A schematic of a spring-applied band brake is shown in Fig. 7.4. In the case shown, the brake is

released by a hydraulic hand pump. To apply lhe brake. a valve is opened to release the hydraulic

pressure. Winches with hydraulic drives can u.lilise t.he main hydraulic pressure [0 release the brake.

The handwheel is nOI used for routine operation: it serves 10 adjusI the spring compression during

calibration, and to release tbe brake in case of a hydraulic malfunction. The, bandwheel should be

secured witb a seal after each calibralion to prevent tampering. Another ad,'antage of tbe spring

applied brake is ils fail-safe feature.