Answer MEP Question 37

Q. If soon after joining a motor ship, you found a number of holding down bolts slack and fretting to have occurred in the area of slack bolts describe how you would handle the situation?

The Classification societies requirement is that Holding-down bolts be checked by a Surveyor, within each survey cycle. This interval of time may be too long and the bolts should preferably be checked at 6-monthly intervals unless there is a case history of the bolts going slack more frequently.

In new vessels, the bolts should be checked within one month of the commencement of the maiden voyage, or earlier if possible. The interval may then be gradually increased if all is found in order. After a vessel has been through bad weather, the bolts should be checked as soon as possible.

A rough method of checking Holding-down bolts is the hammer test. Hold the tip of the thumb on one side of the nut face and strike the nut on the opposite side. If the nut is slack, the nut and stud spring against the thumb and then retract. The movement can be felt against the thumb.

If a holding-down bolt is of the fitted type, this test cannot be used, and a hydraulic jack must be used. Due to the presence of bilge water on the tank top at various times, the holding-down bolt nuts may rust and seize on the studs.

In this case, the seized condition makes it seem as if the nut is tight. The hammer testing method, however, can be used in finding slack nuts, even when they are seized on a stud.

Action to be taken if a number of holding-down bolts are found to be slack. When chocks and their mating surfaces on the bedplate 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 crosshead 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 then replaced. The bolts can then be tightened, to harden the chock.

After all the bolts and chocks have been tightened, the crankshaft alignment must be rechecked.

The main factors, which Result in fatigue failure of holding down bolts, are:

a) Under tightened bolts: If the bolts are subjected to less, stress or less pretension during initial tightening, the stress fluctuation will increase; which will lead to fatigue failure of holding down bolts.

b) Slack studs: The studs may get slack due to over loading of the engine, which will increase the fluctuation of stress.

c) Damaged studs: Any scratch, pop marks or surface flaw of the naked section -of the stud can lead to the localizing of stress that can cause fatigue failure.

d) Slack chocks: Slack chocks will also cause the bolts to be slack. Slackness of bolts increases stress fluctuation.

e) Fretting of mating surfaces: Severe fretting on mating surfaces of bedplate, chocks end foundation plate will cause the bolts to run loose, which will lead to fatigue failure.

Reasons for persistent slackening of holding down bolts of a main engine

a) Holding down bolts often run loose due to vibration. Vibration is due to torque fluctuation and shock loading.

b) When the ship is subjected to severe load the deformation occurs. When that load is removed the ship tends to go into its original position. This action continues and the ship is called vibrating.

Due to this vibration the bedplate will be-under severe stress and fretting will occur between the mating surfaces of bedplate and chocks and in long run bolts will get loose. Main causes are Vibration, Overloading of engine, Slack chocks and Hull deflection. Worn chocks producing an uneven foundation. Incorrect tension applied to the holding down bolt during tensioning. Holding down bolts not square within the fastening device. Excess loads applied to the bolting arrangement, either from the unbalanced engine or the propeller thrust

The holding down bolts passes through the holes in the bedplate, chocks and foundation plate. They are subjected to fluctuation of stress. So they may often run loose and the consequences of running engine with slack bolts are disastrous.

The effects are:

(a) Vibration: Excessive vibration and abnormal movement of the upper part of the engine occur if the bolts run loose.

(b) Fretting: Continuous operations with slack bolts allow severe fretting on the matting surfaces of bedplate, chocks and foundation plates.