Simple Explanation Of Crankshaft Deflections, Measurement Procedure, Readings,Curves

why crankshaft deflections are taken?
>Consider one section for better understanding i.e. two crank webs, a crank pin and two journals.

>If a straight length of shaft is supported at either end and is subjected to a central load
>The effect in shaft is upper material in compression and the lower in tension.

>This effect is applicable to the section of crankshaft
described above with the bearings supporting the assembly at the journals and the point loading being effect by the weight of the piston and conrod assembly ( ignoring other loads found operational conditions such as combustion and centrifugal ).

Effect on Crankshaft

>It can be seen that the effect is to increase the distance between the webs at top dead centre (TDC) and reduce the distance at bottom dead centre (BDC). 
>This deflection is normally found in all crankshafts although for smaller engines with very rigid cranks this may be very small.

>A set of measurements taken from an engine will reveal this deflection which should be constant through each crank/piston unit. 
>The caveat to this is that increase deflection is seen at the fly wheel and cam chain gear wheel sections due to the increased loading.

Finding faults

>After initial installation and alignment a set of deflections are taken. These then form the datum line to which all other recordings are measured against.

·         It should be noted that changes in circumstances will effect the deflections are not indicative of faults. These include;

·         Ambient temperature

·         Engine temperature

·         vessel hull loading (hogging, sagging etc)

·         vessel afloat, dry docked ( again vessel hull loading can cause effects even in drydock due to movement of blocks, which tanks are full etc)

·         these effects are well known and an experienced engineer will take into account these factors when looking at a set of recordings

>If a situation now occurs where a bearing becomes more worn than an adjacent one the effects will be shown as a change in the pattern of delections. 
>When the cranks is turned from BDC to TDC the weight of the running gear causes the crank webs and crackpins to bend in such a manner that the distance between the webs decreases, and continues to decrease until the bearing is no longer in contact with the journal
>The deflection when the crankshaft is approaching TDC will then go from its normal positive reading to zero and then to negative readings at which point the assembly is supporting the weight without the assistance from the lowered main bearing.

Thus, any changes from natural deflections can be related to main bearing misalignement and is proportional to the differences in height of the bearings

Crankshaft Deflection Measurement Procedure

>These are generally taken using a spring loaded dial gauge. And fit as shown in fig.

>The crank webs are punch marked to ensure that the readings are taken in the same place each time.
> Five measuring points are taken- TDC, 90' either side of TDC and 30' either side of BDC. The latter two measurements are required as it is not possible to measure at BDC due to the Con rod.

>The measurements are always taken starting at the same starting point. 
>In this case we will say Port side near BDC. The gauge is fitted and zeroed.
>Rotate the engine continuusly and the readings read off during rotation. 
>After the final reading the egine is rotated back to the start point. If the reading is not zero then it indicates that the gauge is moved and the readings are re-taken.

Crankshaft Deflection Readings Example

These readings were taken from a B&W 6K76EF ( It's the one with the rocker arms and the self adjusting tappets that make you crap yourself when they fail)

Crank Position	No1 Cyl	No2 Cyl	No3 Cyl	No4 Cyl	No5 Cyl	No6 Cyl
Port near BDC X	0	0	0	0	0	0
Port Horizontal P	6	1	7	-9	-4	4
TDC T	12	3	13	-16	-12	5
Stbd Horizontal S	6	3	6	-7	-8	3
Stbd near BDC Y	-1	2	-2	2	1	-4
corrected BDC [(X+Y)/2=B]	0	1	-1	1	0	-2

Vertical Alignement

Crank Position	No1 Cyl	No2 Cyl	No3 Cyl	No4 Cyl	No5 Cyl	No6 Cyl
Vertical alignement [T-B=V]	12	2	14	-17	-12	7

                                    

                                 Crankshaft Deflection Curve

These figures may now be used to draw a misaligement curve similar to the one below and may be analysed to see which bearings are in need of adjustment. he assistance from the lowered main bearing.

Horizontal Alignement

Crank Position	No1 Cyl	No2 Cyl	No3 Cyl	No4 Cyl	No5 Cyl	No6 Cyl
Horizontal alignement [P-S=H]	0	-2	1	-2	4	1

Gauge reading Check

Crank Position	No1 Cyl	No2 Cyl	No3 Cyl	No4 Cyl	No5 Cyl	No6 Cyl
T+B=C	12	4	12	-15	-12	3
P+S=D	12	4	13	-16	-12	7

C & D should be practically the same, hence the readings from No6 Cyl may be suspect

Image Credit:brighthubengineering.com,

About the Author 

Mr.Vineeth Moozhikkara He is a marine engineer sailing with British Petroleum.He has keen interest on compiling marine exam notes.Catch him on Facebook.