Slewing bearing rocking test

Rocking Test is a test which is carried out to find, wear down of the slewing bearing of the deck crane on ships.

This measures the play (or relative movement) between the inner and outer bearing race, to give an indication of the wear taking place.

(Wear down of the slewing bearing can be found by analyzing the grease sample. The metal content of the sample may give indication of wear down.)

The Rocking Test need to be carried out according to manufacturer recommendation.

Methods and tools for rocking test:

Measurements are typically taken in four positions on 
the slew bearing, with the jib pointing:

• forward to the ship
• starboard
• aft 
• port side.

Neither a load nor any cargo handling equipment should be attached to the hook.It is important for the same positions to be marked as a datum reference. 

Crane manufacturers generally recommend that ‘rocking tests’ are carried out every six months in accordance with their instructions, and that the results are recorded and monitored in order to ensure the wear tolerances remain within the limits permitted by the designers. 

Cranes which are particularly at risk are those with single row ball slewing bearing. Cranes fitted with this type of slew bearing must have rocking tests carried out, in accordance with manufacturer’s instructions, to determine that the amount of wear in the bearing races is within permitted limits.If no rocking test has been carried out in the previous six months, the crane in question will not be certified for further use.Over the years the slewing bearing will have some wear, and to be able to know if there is a need for changing the slewing bearing it is needed to keep a record of the wear.

Rocking test procedure:

On the cranes bottom plate two reference point need to be marked. The reference points have to be always used for the future measuring. The marked surfaces should be on the front and rear bottom plate of the crane.

It is important to use about the same position for all further measurements to be able to compare the measurements.

The dial indicator should be positioned between the rotating and non-rotating bearing races and crane jib must be raised maximum up and maximum down.

Measurements shall be taken on four positions of the slewing bearing. With the jib pointing: forward, starboard, aft, port side.

Set the indicator at zero then raise the crane jib at maximum up and record the value. The indicator of dial indicator should return to zero when the crane jib is lowered back to its original position.

Above I presented one method that I am familiar with, but out there are others method of slewing gear ring measuring like: rotation method or tilt method.

The reason of slewing bearing rocking:

The main reasons for the slewing bearing rock are the axial clearance, radial clearance, the gear machining accuracy and the gear jump.

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