The shaft alignment problem

Proper operation of marine propulsion machinery requires the shafting to be correctly aligned in all ship operating conditions. Modern propulsion technology has raised the demands made on analysis of the ship?s shaft alignment. The various bearings are displaced by carefully controlled amounts (offsets) to achieve a favourable balancing of bearing loads and shaft properties. The purpose is to avoid any mechanical behaviour of the shaft that causes over- or underloaded bearings. It is of primary importance to protect the gear and engine bearings as well as the sterntube bearings.

Continued growth in the size of tankers has resulted in more flexible aft hull structures in combination with very rigid shaft systems. Much of the difficulty experienced with these vessels can be attributed to alignment problems brought about by hull deflections. It is clear that shaft alignment has become a major problem.

Propulsion machinery damage is commonly related to misalignment. However, misalignment can quite often not be strictly proved. This type of

damage occurs mostly when the ship is at sea. The surveyors come on board when the ship is in port with a restored or a different shaftline. In the engine room there are no systematic follow-up facilities for alignment changes due to hull deflections etc. Usually the

engineer still puts his hand on a bearing to feel the heat and to check if it is necessary to reduce the engine speed.

Alignment technique for marine propulsion machinery has been something of a protected enclave in the

technical development. Instead, the difficulties in establishing misalignment should influence the shipyards to encourage the modernization of the alignment technique. Although the calculation algorithms have been computerized, this has been of little consequence for misalignment risks. The computerization has only applied the same linear theory used for the manual calculations in the old days.

Linear theory

Linear theory

is used in all alignment software throughout the shipbuilding industry. Using this theory, you are

generally able to balance the total shaft weight between the bearings. However, to balance the distribution of pressure within the bearing is not possible with this theory, since the length and the clearance are not taken into account. The bearing model using linear theory is illustrated in Figure 1. With this model you may obtain a seemingly satisfactory bearing load even if there are two counteracting reactions concentrated at each bearing edge, as illustrated in Figure 2. Nevertheless, in alignment calculations the calculated bearing load is simply divided by the supporting area. An evenly distributed pressure is obtained in spite of the fact that there is a risk for weardown at the bearing edges. Unfortunately, simplicity does not necessarily result in reliability. You must be aware of certain risks when implementing the results from alignment calculations.

Misalignment

The bearing offset accuracy required in the shaft alignment is less than a tenth of a millimetre. During ship operation, the relative bearing displacements may amount to a couple of centimetres. Thus, to expect a satisfactory shaftline at sea is patently absurd when the shaft is left to its own fate. Ships? propulsion machinery is practically always operating under misaligned conditions. This results in gradual or catastrophic bearing failures. Loss of hire and other costs arising from misalignment are in the aggregate a very high figure, borne mainly by ship owners and insurance companies. What can be done to reduce this cost burden?

Total elimination

The shipbuilding industry should take a responsibility for the alignment even after launching. The shipowning industry should demand a total elimination of misalignment during ship operation and stop regarding misalignment as a disease for which there is no medicine. Ensuring a satisfactory operating alignment should be a natural part of the “after sales service”.

Neutralization of hull

There is a need for an automatic control system to neutralize the hull

deflection effects on bearing pressures. A proposal for such a system is shown in Figure 3. The computer-controlled hydraulic pistons are intended to adjust the shaft position both vertically and horizontally. Optimum alignment can be maintained in all ship operation conditions despite the hull deflections. The system requirements can easily be achieved by modern hydraulic technology. The pistons

are not intended to supplement the bearing support.

Jacking

The computer-controlled hydraulic pistons can do all the jacking and adjustments needed in the shaft installation. The traditional and troublesome gap-and-sag procedure is not needed. The basic alignment work can be done before the superstructure is mounted on the hull and the required time is considerably reduced.

Whirling

Shaft whirling problems associated with bearing lubrication instability can be avoided by the hydraulic pistons. This requires that the control software includes routines for whirling vibration.

Non-linear theory

The hydraulic pistons must be

controlled by software using non-linear theory. Using linear theory may do more harm than good. The only necessary recurrent input data are the hydraulic fluid pressures and the shaft stresses at appropriate points. Of paramount importance is the determination of a realistic effective reaction point within each bearing, in particular in the sterntube bearings. The required non-linear equations are derived using the “multipoint” bearing model shown in Figure 4.

Software calculations

The shaftline equations are solved using Math Library routines. The calculation procedure is divided into 4 steps. In steps 1 and 2, linear theory is used to provide realistic initial guesses, which are essential for steps 3 and 4 using non-linear theory.

1. Appropriate piston offsets in the vertical are estimated by a linear least squares optimisation routine balancing the loads between the bearings. The bearing clearance is neglected.

2. An effective reaction point within each bearing is determined separately for the vertical and the horizontal positions. The bearing clearance is taken into account. The solutions are obtained using an algorithm similar to the revised simplex algorithm for solving linear programming problems.

3. The values obtained from step 2 are the initial guesses for the non-linear problem of determining a combined solution for the position of each

effective reaction point.

4. Tuning of the piston offsets with respect to the clearance using

a non?linear least squares optimisation routine. The objective function

balances the bearing loads and

pressures. n