Waterjets are becoming more and more popular as a propulsor at ships. ShaftDesigner software allows calculating alignment of waterjet shafting as well as whirling, torsional and axial vibrations.
Waterjet has a specific shafting configuration and corresponding alignment procedure. This article illustrates waterjet alignment calculation performed by the IMT company in September 2019. The project consists of the main shaft with impeller and plain bearing, CENTALINK shaft and the gearbox.
The waterjet shafting model was made by Drag and Drop technique taking into account specific features of the construction. CENTALINK shaft was modelled as an equivalent shaft and flexible couplings. The gearbox shafting has a gear wheel with specified parameters and two bearings.
To find the optimal alignment design solution, the ShaftDesigner’s alignment application was used. For better accuracy, the aftmost bearing was modelled as a multi-support bearing. Alignment application allows considering several shafting states. The traditional states are
- Cold Static
- Hot Static
Besides, we calculated conditions with both cylindrical and conical wearing out of the aftmost bearing. Direct contact between water stream and a rotating shaft causes additional Magnus forces, that were calculated using CFD and input as external loads applied to the main shaft. The optimal alignment design means bearing offsets combination providing the most reliable shafting operation. Acceptance criteria are formulated by Class and equipment manufacturers for each of the shafting states.
The first and the simplest alignment design is all bearings in-line. We checked it with the direct alignment calculation, and the report shown several unsatisfied criteria.
In this project, only gearbox bearings can be adjusted so their acceptable offsets are to be found. ShaftDesigner has a special tool, Offset explorer, searching for the optimal bearings offsets with maximum tolerances. The calculated values are the optimal gearbox bearings offsets. It is vertical bearing adjustments that are to be implemented onboard.
After the new bearings offsets were applied, the direct calculation was performed again. After that, the alignment design meets all requirements. The generated reports contain comprehensive information about the initial data, bearing’s loads and shafting bending parameters. The generated report was exported to MS Word to make the final document and submit it to the Class.
The optimal shaft alignment in static and operational conditions was developed with ShaftDesigner software using:
- extended bearing model
- bush wearing out
- hydrodynamic impeller loads
- Magnus forces
- bearing thermal growth
- gear wheel loads.