Understanding Shaft alingment And its methods

Shaft alignment is the process of aligning two or more shafts with each other to within a tolerated margin. Shaft system would ideally be installed with straight alignment and remain in that state during ship operation.But many factors affect and alter alignment during building and throughout the life of the ship example hull flexure due to  different conditions of loading (cargo, ballast, bunker, FW etc).

The hull of a moderately sized ship can flex 150 mm in heavy weather. High deck and low sea temperatures in the tropics cause differential expansion and hogging. Intermediate shaft is also subjected to variations of torque due to propeller racing as well as compressive stress due to end thrust from the propeller. Misaligned shaft imposes severe bending stresses on the shaft line. Also it will result in overheating of the bearings. Bending of intermediate shaft imposes severe stresses on coupling bolts, subjects them to additional shear stresses, leading to failures. Good shaft alignment guarantees correct loading of bearings and ensures that the shaft is not unduly stressed.

Difficulties in checking shaft alignment :

Shaft alignment for ships is very different from shore based engines because during alignment of shore based engines , conditions prevailing at sea are not present. At sea, number of external forces are continuously acting on ship structure which can cause limited amount of misalignment continuously. As long as this misalignment is with-in certain limits, shafting system works properly.

External factors affecting Shaft alignment readings:

• List and trim of the ship
• Mooring of the ship
• SW temp & atmospheric temperature.
• Temperature of liquids in D.B. tanks below bearings.
• Un- availability of accurate precision instrument.
• Bearing supports on tank top which may hog or sag.
• Intermediate shaft is sufficiently heavy to sog.
• Stern tube through which tail end shaft passes is slightly sloped 1:80 to avoid edge loading.
  Weight of propeller causes bending of the shaft.
• Crankshaft & engine structures are rigid, only possible flexible length is intermediate shafting.
• Shaft passes through many bulkheads. These bulkheads causes hindrance & difficulty while checking alignment.
• A very bright light makes it difficult to get accurate alignment. Generally alignment is carried out in a dim light and in early hours of the morning.

Shaft Alignment methods:

(a) Pilgrim wire method
(b) Sighting by light method (optical telescope)
 

(a) Pilgrim wire method: Produces fairly accurate results. Used where we can’t use sophisticated methods like method of Flexible wire having uniform weight over its length (piano wire) is used as a Reference line. Wire is tensioned to a standard amount by fixing at one end and having a weight over pulley at the other end.

Items required : a length of piano wire, Ball bearing pulley, anchoring or support clamp & weights.
Distance from wire to bearing are then measured either electronically or using micrometer. Allowance being made for sagging of the wire. Maximum sag will occur at the centre of the span if the pulley and support are horizontal. Flexible wire having uniform weight over its length, when it ends are supported ----- curved formed is a catenary which has certain mathematical property.