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Mooring winch - Load test




WHY LOAD TEST ?
To check performance and condition of the mooring winches.




Gear backlash measurement

The gap (backlash) between the two gear tooth in contact (gear wheel and pinion) is measured in this test. For this purpose a solder wire is placed on a tooth on the pinion wheel (connected to the hydraulic motor) and the motor is turned. After one round, when the tooth of the pinion contacts that of the gear wheel the solder is flattened to a thickness corresponding to the gap between the two tooth. This thickness is measured using a vernier caliper. It is compared to the design specification.



Load checking

The winch is rated for a particular load according to design. This load is simulated using a weights attached to a wire rope and lifted vertically. The arrangement is shown in the figure. While considering the total load on the winch, we must also take the efficiency of the sheave, η. Considering there are three sheaves, load on the winch (F) and the total weight (W) is related by: 
F = W / η³.





Speed checking

The winch is rated for a particular speed of heaving as per design. This is checked with load of 16 T (16042 Kg) on the winch. Two markings are made on the wire rope at a distance of 2 m. A vertical rod is placed near the rope and the time between when the first mark crosses and the second mark crossed the rod is measured.

From the time (T) the speed in meters per minute is calculated by 120 / T.

Winch rpm checking

The winch is rated for a particular rpm according to design. The speed is checked for loaded condition and no loaded condition. Certain ships have winches with two speeds. In such cases the following speed tests are conducted: heaving (lower speed, with load), heaving (lower speed, no load), pay out (lower speed, no load), heaving (high speed, no load), and pay out (high speed, no load).

The speed is checked up using an rpm gauge. A magnet with a slot is placed on the warping drum end and the probe is inserted in the slot

Brake holding test

The brake is the heart of the mooring system as it secures the drum and thereby the mooring line at the shipboard end. It also act as a safety device in case the load in the line becomes excessive, by rendering and allowing the line to shed its load before it breaks (at MBL or Maximum Breaking Load). For normal operation the recommended (OCIMF) setting on the brake is 60 % of the MBL of the rope. For new ships this is tested for 80 % of the line's MBL since brakes may deteriorate in service.

Every ship is supplied with a brake testing kit (in this case bracket type brake test kit) to test the brake at least once in a year. The break testing kit simulates the load on the line by means of hydraulic pressure producing a torque on the winch drum as shown in the figure below. For a particular load, the pressure can be calculated as shown in an example below.

In this example:
Winch capacity : 16 T x 15 m/min
Mooring line MBL : 51 T
Brake holding force : Po = 40.8 T at first layer (80 % of MBL)
Drum diameter : Dd = 48 cm
Rope diameter : Dr = 2.8 cm
Effective area of hydraulic jack : A = 33.2 cm²
Test force moment arm : L = 132.5 cm
Rope PCD : Rpcd = Dd + Dr = 50.8 cm
Brake torque : Tb = Po x Rpcd / 2 = 1036.3 T-cm
Force on hydraulic jack : F = Tb / L = 7.8 T = 7800 Kg
Therefore required pressure : P = F / A = 234.9 kg/cm² (at 80 % MBL) = 230.5 bar

Now the setting is made as per the drawing with the bracket connected on the drum flanges (setup above). The turn handle is used to tighten the brake as the brake band screw tighten with respect to the brake nut. The pressure is increased in the hydraulic jack using a manual pump. After the pressure reaches the calculated value (here 230.5 bar), a marking is made on the brake band. It is held for a minute and the marking is checked to see if there is any slip.


Clutch operation

The clutch is used to engage the drums with the main shaft. There is slot in the drum where the clutch slides and locks in. Once engaged, the drum turns with the main shaft. The main shaft is connected to the two drums (in this case) and one end connected to the warping drum and other to the gear wheel. This gear wheel is driven by another gear that is connected to the hydraulic motor.

During this test it is checked whether the clutch operates smoothly and that the locking pins to secure the clutch in position is also proper.


Turn handle (brake handle) operation

The brake handle operation is checked for its smooth operation.


Dry running

The mooring winch is subjected to dry running (without load) for five minutes. During this time it is checked for abnormal noise coming from the gear case. Also the bearings are checked for any temperature rise (overheating).

The hydraulic pressure on the test supply lines and the voltage across the electric motor (driving the shore hydraulic pump) is checked.
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