Question

(a) Describe the procedure for measuring crankshaft deflection in a marine engine. Include the steps for preparing the engine, positioning the deflection gauge, and recording measurements at different crank angles. (8)

(b) Explain how to interpret the deflection data to assess the condition of the crankshaft. What do variations in deflection readings indicate about alignment, bearing wear, or potential engine stresses. (8)

Answer 1

Part (a)

PROCEDURE FOR MEASURING CRANKSHAFT DEFLECTION

Preparation: Stop the engine, secure the turning gear, and ensure the engine is at a safe condition. Obtain the maker's deflection gauge (a dial test indicator with a magnetic/ spring base) and the crankshaft deflection record sheet.
Position the gauge: The deflection gauge is mounted between the crank webs (the two webs of a crank throw), at a fixed distance from the crankpin (usually at the web face, at a marked position). The gauge measures the change in the distance between the webs as the crank rotates.
Zero/ reference: Set the gauge to zero at a reference crank position (e.g. at bottom dead centre, BDC, or at the position specified by the maker).
Record measurements at different crank angles: Bar the engine over and record the deflection reading at each of the standard positions - typically at BDC (bottom), and at 90-degree intervals (e.g. BDC, 90 deg, TDC, 270 deg, back to BDC). The readings are recorded on the deflection sheet for each crank throw.
Repeat for all crank throws: Measure the deflection of every crank throw in the engine.
Record: Record the readings (in mm/ 0.01 mm) on the deflection record sheet, noting the crank angle and the direction (the sign convention - whether the webs close or open).

Part (b)

INTERPRETING THE DEFLECTION DATA

The deflection readings indicate the change in the distance between the crank webs as the crank rotates, which reflects the bending/ distortion of the crankshaft and the alignment of the main bearings.
A "positive" deflection (webs closing at TDC) indicates the crank throw is being compressed/ the shaft is sagging between bearings (the crank is "hogging"), suggesting the main bearings are low/ the shaft is unsupported in the middle.
A "negative" deflection (webs opening at TDC) indicates the crank throw is being stretched/ the shaft is "sagging" (the crank is "sagging"), suggesting the main bearings are high/ the shaft is supported too high.
Variations in deflection readings between crank throws indicate misalignment of the main bearings, bearing wear, or a bent/ distorted crankshaft.
The deflection is compared with the maker's/ Class allowable limits. A large deflection (beyond the limit) indicates serious misalignment or bearing wear that must be corrected.
The deflection readings are trended over time: an increasing deflection indicates progressive bearing wear/ misalignment, and the engine should be re-aligned (bearings re-chocked/ re-metalled) before the deflection exceeds the limit.
Deflection also indicates potential engine stresses: excessive deflection causes high bending stress in the crankshaft, bearing overload, and risk of crankshaft failure.

✓ Verified Model Answer (Text Solution)

PROCEDURE FOR MEASURING CRANKSHAFT DEFLECTION

INTERPRETING THE DEFLECTION DATA