1. Loose tie bolts
2. Loose chocks
3. shatter chocks
4. Inadequate tightness of holding down bolts
5. Bedplate or foundation plate deformation
6. Unbalanced or overloading of engine
7. Excessive vibration
8. Ageing
9. Collision or grounding
10. Fire

Actions to be Taken for Slack Holding Down Bolts

When the chocks and their mating surfaces on the bed plate and tank top have fretted, the chocks cannot properly support the engine. If the holding down bolts are tightened, the crankshaft alignment may be seriously affected, with lesser effects being felt on cross head guide and cylinder alignment.

The seriousness of the situation will be depend on the amount of fretting that has occurred. Before any tightening of the holding down bolts is carried out, the alignment of the crankshaft should be checked, by taking deflections with a dial gauge. If the crankshaft alignment is satisfactory, the slack chocks can be removed and smoothed on the mating surfaces and replaced. The bolts can then be tightened to harden the chock. After all the walls and chocks have been tightened, the crankshaft alignment must be rechecked.

 why HDB are free through the bedplate, chocks & tank top:

1. To avoid stress concentration on the bolts.
2. To avoid the chances of fretting or notch effect this may cause fatigue failure.
3. To transfer the tensile stress through the bolt from bedplate to tank top without concentration any particular parts.
4. The bolt should be free to elongate otherwise they will lead to fatigue failure.

why HDB are long and made of high UTS steel

Holding down bolts are made long

1. to increase fatigue strength modern engine use long elastic bolts.
2. Because of greater length the bolts have greater elasticity hence less prone to crack

Bolts are made of high UTS steel because: TTFD

1) To increase tensile strength.

2) Have higher fatigue strength.

3) Have toughness property to sustain variable load without failure.

4) Have good ductile properties.

5) High UTS steel bolts can be tightened to a higher torque to reduce stresses.

6) Steel is resistant to corrosion.