Marine Engine-Question and answer

1.(a)Explain why bottom end bolts are prone to failure under normal conditions.
(b)Describe how bottom end bolt failure is either aggravated or inhibited
during maintenance.
(c)Describe with the aid of sketch, the features incorporated into bolt design
to inhibit failure.

(a)
On a 4 stroke engine, the bolts and their nuts are subject to tensile stress when tightened and additional varying tensile stress during operation. The total stress level is high and varies with time, giving rise to the risk of fatigue. The connecting rod is in compression during the compression and power strokes, but due to the inertia forces in the running gear when the piston changes direction between the exhaust and inlet strokes, the connecting rod is put into tension. This increases the tension in the bottom end bolts, leading to cyclic stressing.

(b)
Over-tensioning of bolts should be avoided as this increases the stress level. If bolts are damaged during maintenance, stress raisers will result, thereby increasing the risk of fatigue.

When inspecting the bolt check for:

Corrosion – acid attack from LO.
Elongation.
Mechanical damage – dents, burrs, etc.
Fretting on landing faces
Thread condition
Cracks – NDT, dye penetrants.
Only tighten to manufacturers recommendations; hydraulic jacks, stretch measurement, or using jig and feeler gauges

In order to minimise the risk of fatigue failure, bottom end bolts should be replaced if the bolt elongation has reached a maximum (go – no go gauge) or after about 15,000 running hours.

(c)