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Engine Operation

Crankshaft Deflections

 

 

 INTRODUCTION

Crankshaft deflections are taken to check the alignment of the crankshaft in the bedplate and its alignment with the intermediate shaft and propeller shaft. During installation of the engine and shafting, the alignment is adjusted by jacking the engine bedplate to the correct height and checking this by measuring the distance from the bearing pockets or the crankshaft main journals to a wire stretched at a known height above the true line that the crankshaft should follow. (Modern methods use a laser beam projected at the correct height, but the principle is the same.) When the engine bedplate is correctly aligned, chocks are fitted or poured and it is bolted down. (see "How is the engine fixed into the ship")

In case you were wondering, the wire does droop due to its weight. It is tensioned to a known value, and  the droop of the wire at any point can be calculated.

The hull of a ship is not a stiff unyielding block. It flexes in the water and will hog or sag depending on it's condition of loading. It maybe distorted permanently by collision or grounding. If the hull can flex then it is fairly obvious that the engine bedplate bolted into the hull will flex as well and this can alter the alignment of the crankshaft.

Even if the hull retains it's original shape, the crankshaft can still move out of alignment. If the chocks on which the engine is mounted wear (a condition known as fretting) and the holding down bolts are retightened, the bedplate will be pulled out of alignment. The most common reason for misalignment though, is because the individual main bearings are excessively or unevenly worn.

WHAT ARE DEFLECTIONS?

 

If a plain shaft sitting in two bearings of equal height is considered, when the shaft is loaded as illustrated by the arrow, then the shaft will bend. The inside of the bend will be in compression (and get shorter) and the outside of the shaft will be in tension (and get longer)

 

If we now transfer the same thinking to a crankpin which is at Top Dead Centre, and where the load on the crankpin is the mass of the con rod, piston rod and piston, it can be seen that as the pin deforms as previously described, then the webs will tend to move apart

 

When the crankpin is at Bottom Dead Centre, the crankwebs will tend to close in, again due to the mass of the running gear acting downwards and tending to bend the crankpin.

 

If a dial indicator is placed between the webs as shown, then the amount the webs open and close as the crankshaft is rotated can be recorded. These measurements are known as the crankshaft deflections.

When the engine has been installed in the vessel and the ship is afloat, a set of deflections, which measures how far the crankwebs open and close, will be taken and recorded. These will be used as the benchmark against which future deflections will be compared.

 

If, for example, because of excessive weardown or bedplate misalignment, two adjacent main bearings at the centre of the engine are lower than the rest, then at this point the crankshaft centreline will be lowered to form an arc. This will cause the intermediate crank throw to deflect in such a way that it ‘‘closes’’ when turned into bottom position and ‘‘opens’’ in top position. Since the magnitude of such axial lengthening and shortening increases in proportion to the difference in the height of the bearings, it can be used as a measure of the bearing alignment.

WHEN SHOULD DEFLECTIONS BE TAKEN?

After any maintenance is carried out on the crankshaft bearings, when in doubt about the alignment of the crankshaft (for instance after an accidental grounding of the vessel), or at the regular maintenance intervals laid down by the engine builder.

WHAT PRECAUTIONS SHOULD BE TAKEN BEFORE TAKING DEFLECTIONS?

Make sure that the indicator cocks on the engine cylinders are open. A build up of pressure within the cylinder will increase the downward load on the crankpin, increasing the deflection readings.

Make sure that permission is obtained to turn the engine.

Ensure no-one is working anywhere else on the engine, shafting or propeller.

If taking deflections after carrying out work, ensure all tools and chain blocks are removed from the engine.

The person taking the deflections should be the only person able to turn the engine.

ARE THERE ANY TIMES WHEN DEFLECTIONS SHOULDN'T BE TAKEN?

Deflections shouldn't be taken when the vessel is loading or discharging cargo, as the shape of the hull, and therefore the deflections will be changing.

Deflections should not be taken in dry dock as when the vessel is on the keel blocks, the hull takes up a different shape than that when the ship is afloat.

 

HOW ARE DEFLECTIONS TAKEN?

As stated earlier, a dial indicator is placed between the webs and the amount the crankwebs open and close, or deflect, as the engine is turned is recorded. To ensure that the dial indicator is always placed at the same position between the crankwebs, the indicator is located in two pop marks directly opposite each other in the webs.

 

If a little thought is applied it should be evident that when the crankthrow is at Bottom Dead Centre the connecting rod will be in the way of where the dial indicator should fit. For this reason a deflection cannot be taken at BDC. Instead the crankshaft is turned until it is just past BDC and the dial indicator fitted. The pointer is adjusted to read zero. The engine is then turned so that the crank throw is horizontal (90º before TDC) and the reading on the dial recorded. This is repeated again at Top Dead Centre and at 90º after TDC. The engine is further turned until the crank is just before BDC and a fifth reading taken. The bottom two readings either side of BDC are then averaged to give the reading at Bottom Dead Centre.

When taking deflection readings for the three aftmost cylinders, the turning gear should, at each stoppage, be turned a little backwards to ease off the tangential pressure on the turning wheel teeth. This pressure may otherwise falsify the readings.

Opening of the crankwebs is a positive deflection, closing of the crankwebs is a negative deflection.

WHAT DO WE DO WITH THE READINGS?

The readings are recorded on a table and the vertical and horizontal deflections calculated.

 

Cylinder Number 1 2 3 4 5 6
Crank Position A 0          
Crank Position B -0.1          
Crank Position C 0.45          
Crank Position D -0.05          
Crank Position E -0.2          
Average of A and E (F) -0.1          
Vertical deflection (C - F) 0.55          
Horizontal deflection (B - D) -0.05          



Once the deflections have been calculated, it is normal to compare them with the those obtained from the original readings and the last set of readings taken. The engine makers manual will give a guide to the maximum allowable deflections. The table below gives an example for a 900mm bore engine:

 

New or recently overhauled

mm

Realignment Recommended

mm

Absolute Maximum Permissible

mm

1 2 1 2 1 2
0.3 0.6 0.81 0.92 1.22 1.22

1. Normal for all crank throws.
2. Permissible for the foremost crank throw, when the crankshaft fore end is provided with a torsional vibration damper, tuning wheel or directly coupled to a generator rotor and for the aftmost crank throw, when the crankshaft aft end is provided with a flexible coupling.

This is because the overhanging mass of coupling or damper will give a greater deflection on the adjacent crankthrow.

WHAT IS THE CAUSE OF EXCESSIVE DEFLECTIONS?
Deviation from earlier measurements may be due to:

  • Human error: - check the readings!

  • Incorrectly loaded cargo: - causing the ship to hog or sag more than normal.

  • Journal eccentricity: - this should not exceed 25% of bearing clearance.

  • Floating journals: - The journal must be sitting down in the main bearing; check with feeler gauges. If not, the causes could be excessive wear down on the bearing, or misalignment of the bedplate.

  • Wear of main bearing: - change the bearing.

  • Displacement of bedplate (fretted chocks etc): - realignment may be necessary,.

  • Displacement of engine/shaft alignment: - normally manifests itself as a large deviation in the aftermost throw deflection.

Other more obscure reasons like a cold sea and a hot day have been known to cause excessive deflections. ( the deck and upper part of the hull expands, while the hull in the water contracts). A cold engine and hot lubricating oil in the sump have also been found to give excessive readings).

If the deflections are outside the limits set by the manufacturer then the cause must be investigated and corrective action taken. Operating the engine with excessive deflections could result in a broken crankshaft because of increased bending stresses.

By looking at a set of crankshaft deflections it is difficult to tell whether the crankshaft is hogging or sagging. This can be achieved by plotting a graph of the readings.

HOW IS THE ALIGNMENT OF THE CRANKSHAFT CHECKED?

The old way of establishing a datum is to stretch a wire between two fixed points above the crankshaft/intermediate shafting. The wire must be passed through the crankcase of the engine vertically above the centre line of the main bearings. Blank flanges are provided at the end of the crankcase for this purpose. On a six cylinder two stroke engine the engine can be turned to a point where the con rods do not interfere with the line of the wire. Above six cylinders and some of the con rods must be disconnected from the bottom ends and swung out of the way. The height between the two is then measured using a micrometer (a low voltage electrical circuit which lights a light bulb can often used to obtain an accurate reading). The disadvantage with this method is that the wire will sag in the form of a curve , known as a catenary, even when stretched between two points, although this sag can be calculated and adjustments made to the measurements.

Modern method of crankshaft alignment is similar to the piano wire method, but instead optical means are used to measure the height of a laser line above the crankshaft journals. Alternatively, a laser is used to align the edges of the bedplate.

MAN B&W check the alignment of the crankshaft by stretching a wire along each side of the bedplate, again loaded to a known horizontal force. At the centreline  of each cross girder the distance is measured between the wire and the machined face of the top of the bedplate.

The shafting alignment can be checked by measuring the bearing load at:

  • The aftermost main bearing.

  • The intermediate shaft bearings (plummer blocks).

  • The stern tube bearing.
     

Taking these measurements normally requires specialist assistance.

 

 

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