What is shaft alignment condition?

“A static condition observed at the bearings supporting the propulsion shafts.”

The basic procedure for determining vertical alignment of shafting and gears is to calculate, for the cold condition at time of aligning, a shape of shaft line that will produce in the hot operating condition essentially equal loads on the main gear bearings and reasonably equal loads on line bearing.

Two methods used for establishing the desired alignment are:

• the gap and sag method
• the bearing load measurement method. 

An overview of Shafting system response at static/dynamic condition:

1st-Bearing Reaction & The Influence of Matrix Table

The influence coefficient matrix tabulates the relationship among relative bearing reactions due to the unit offset change at each particular bearing/support.

As you can see that the engine shaft line is very sensitive to hull deflections (see un-stable area) . Most of the time, it is not because of the engine itself, but rather because of the discrepancy in stiffness between the engine structure and the double bottom structure below the shafting. This may consequently result in the two aft-most main engine bearings (#4 and #6) being very sensitive to change in intermediate shaft bearing (#3) offset deviation.

Key note:

Satisfactory bearing reactions are one of the primary criteria for alignment acceptance. It is difficult to establish an acceptability margin as the factors influencing reaction load are very difficult to predict accurately. Essentially, alignment is acceptable as long as the bearing reactions are always positive (+) (under all operating/loading conditions) and no bearing is unloaded. For practical reasons, at least 10% of the allowable load would be desired on the bearing in order to prevent over-loading on the other bearings.

2nd-Bearing Reaction Measurement

Bearing reactions are measured directly and indirectly. The most commonly applied methods that measure the alignment condition are:

• Sag and Gap –  indirect methods to correlate those measurements to the bearing reactions.

It is recommended not to commence with the Sag and Gap procedure before the following is completed:
*Engine and reduction gear are installed.
*Temporary supports are installed.
*Shafts are placed inside the vessel and propeller is mounted.
*Propeller shaft is in contact with a bottom shell at the foremost stern tube. (Jack down)

• Jack-up –  direct reaction measurement where a hydraulic jack is used to lift the shaft and measure the load at the particular bearing.

Jack-up method is a direct way to check bearing reactions. Due to its simplicity, it is the most widely applied method in the industry. Measurements are conducted by hydraulic jacks which are placed in close proximity to the bearing which reaction is to be measured.

Although it directly records the load, jack-up method does not measure bearing reaction directly, as the jack is lifting next to the bearing location. This requires correction factors to be applied which introduce some error as well –> That’s the final approval from Engine Manufacture to be submitted base on the Jack-up report been carried out by yard and witnessed by Class/Client.

The evaluation of jack-up test report is made with the plotted jack-up test results which indicate the progression of jack force versus shaft lift.

Explanation the curve:

(slope 1) – Initially all the static load is in the bearing and none is in the jack. When the jack starts to lift the shaft, then the static load of the jack is progressively increased by the static load transferred from the bearing to it until the bearing is unloaded .

(slope 2) – As soon as the jack has unloaded the bearing next to it, relatively less lift force increase occurs in relation to the further lift. The reduced force-lift progression after the unloading of the bearing next to the jack is caused by the unloading of another bearing which has a larger distance (lever) to the jack. The section of the plotted curve which is recorded right after unloading of the relevant bearing (slope 2) but before unloading of an additional bearing (slope 3,4) is used to determine the jack load corresponds to the original shaft bending line – virtually supported by the fully loaded jack and virtually without a bearing.

The force-lift progression is also recorded when the shaft is lowered and the loads are transferred back to the bearing again.

The jack-up curves indicate a higher jack load than the jack-down curves. This hysteresis is caused by friction. It is compensated by taking the average value of the jack loads obtained from the lifting and the lowering curve.

A correction factor is applied to allow for the longitudinal distance between jack and bearing when determining the bearing load. It is provided by the alignment calculation.

As noticed before, no Bearing is unloaded as practical satisfactory.

PROBLEMS DURING SHAFT ALIGNMENT:

The Sag and Gap procedure is commonly applied as an alignment verification method prior to the shafting assembly. The Sag and Gap should not be regarded as an acceptable method of confirming the final alignment condition, but rather as a cursory check of the pre-assembly condition of the shafting.

Question:

What is the procedure if Sag and Gap, as measured, do not comply with the calculated values?
Should the I.S bearing offset be adjusted to obtain better agreement with the analytical data, or should it just be recorded and the eventual adjustment left for after the bearing reaction measurement? ( Note: This method is always used for alignment because of the less correction of Sighting through & Pre-sighting works)

The Answer:

It is strongly suggested that the bearing offset, and engine position, or gearbox position not be amended based solely on the Sag and Gap measurements. As mentioned above, the accuracy of the method is not sufficient to ensure that the alignment is being improved.

It may well be that the alignment will worsen by readjusting the bearing offsets in order to obtain the Sag and Gap values which are neither accurately measured or not calculated for the particular vessel condition.

DO NOT MIS-UNDERSTAND ABOUT THE MENTIONED ASPECTS WITH THE “NEGATIVE OFFSET CONDITION”

Negative Offset Alignment: is characterized by the shafting design where most of the bearings in the system are located below the after stern tube bearing position. The bearing offset should be selected so to satisfy the alignment criteria and the contact area is relatively large, also relative misalignment slope between the shaft and the bearing is estimated to be around 0.3 mrad.