shafting
Larger vessels pose the greatest shafting challenges
Careful measurements are required to determine the best positioning of bearings to limit accelerated wear and early failure which usually results in the vessel becoming immobilised
by Keith Henderson
ince the first use of ship propellers, the correct alignment of the shaft to the engine has been critical. Over the years, methods have evolved to carry out this critical task. Towards the close of the last century, however, the number of shaft failures increased. In the majority of cases, this was caused by alignment problems. The increasing failure rate was most noticeable in very large crude carriers (VLCC), bulk carriers and large container ships. What these ships have in common is their large size (length), higher engine powers and larger, heavier propellers running at slower speeds. The higher power, lower speed combination produces higher shaft torques requiring increased shaft diameters which are stiffer and less flexible. Shafting installation is usually performed in the launched condition with light draught (ship unladen) and at ambient temperature – that is, the engine is cold. Complex calculations show the best position for the intermediate bearing(s) and exact engine position. As the ship’s cargo increases, it lies deeper in the water. With the increasing hydrostatic forces, the hull becomes distorted (hogging) whereby bow and stern drop relative to midships. To compensate, the engine is usually inclined at an angle, the intermediate bearing is lowered below the centre line and the sterntube is bored at a sloping angle. In the case of ships with a reduction gear, allowances must be made for the gearbox and offset as required. After trials, an optimisation measurement can be made and final offset adjustments applied. Unfortunately, for larger vessels it is not that simple because there is a variance between the conditions at installation and typical service conditions. As the engine gets up to working temperature, the expansion of the upper part of the engine is greater than the lower part and engine bed. This creates a hogging distortion of the engine bed that must be taken into consideration. Hogging of the hull grows with
S Fretting of a propeller shaft flange due to failure of the flange bolts
increasing draught. In addition, the dynamic conditions of ship speed and weather with increasing wave heights also plays its part in making small, but nevertheless significant changes in the ship’s dimensions. These small variations can contribute to bearing failures caused by shafting misalignment. The final bearing
offsets in a shafting
installation should be determined so that the alignment can meet all relevant requirements, including static and dynamic conditions. Although this can be achieved during the initial installation followed by bearing load measurement and readjustment, if a major alteration of the shafting system is required at a later stage (for example, relocating the intermediate bearing after sea trials), the cost will be considerably greater.
Predicting that the shaft bearings are correctly positioned along the shaft and offset by the correct amount is made easier today using computer software such as that produced by Propulsion Software AB, Sweden – a leading developer of ship shaft alignment and vibration analysis software.
The Swedish software incorporates a new multi support point bearing model for predicting shaft bending and load distribution in shaftline bearings. Using this method in conjunction with tilted bearings instead of the traditional single
50 I Marine Propulsion I February/March 2012
support point model, the company claims that shaft alignment and bearing lubrication will be optimised under all operating conditions. Accurate bearing offset determination requires the use of a model that considers the length and clearance of the bearing. The multi support point bearing model is treated as being divided into several sub-bearings, with the length and clearance of each sub-bearing being considered in the mathematical model. Each sub-bearing is then represented as a support point and all these are considered separately in the calculation. This computational modelling is used to optimise the offsets and reduce misalignment to a minimum within well-defined constraints thereby controlling the lubrication oil film pressure at the support points. Using tilted bearings, forward and aft, offset variables can be introduced, thereby further reducing eccentricity between the shaft and bearing centreline. As the eccentricities due to misalignment are minimised, the bearing oil stiffness is easier to calculate for a defined operation and these initial values are then used to obtain oil stiffness measurements for other operating conditions such as changes in load, thermal expansion and hull deflection.
Classification societies also have their own software packages to determine the most
www.mpropulsion.com
Polish Ship Register
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