Answer EKG Question 44

Q44. State with reasons, how cracking in EACH of the following location is caused, rectified and avoided 

a. “A”-frame. 

b. Bedplate longitudinal girder. 

c. Bed plate transverse girder.

Answer: a. A frame

• A frames can be susceptible to cracking, so regular inspection is necessary. Causes can be a stress concentration caused by a sharp change in section, incorrect plate edge preparation, welding root faults such as lack of penetration.
• Another main reason for the cracking of the A frames is that the terminations of the horizontal stringers within the engine are sometimes too hard.
• The A frames are erected over each transverse member. They support the entablature and with the side plating form the space for the running gear (the crankcase). They carry the crosshead guides in which the crosshead slippers reciprocate.
• Fabricated from low carbon steel (max 0.23%) they can be of single plate construction with stiffening webs and stringers to support the crosshead guides or of a double plate box construction.
• Sharp changes of section must be avoided to prevent stress concentrations and cracking.
• The guides can be integral with the A frame or manufactured separately from cast iron and bolted on.
• After manufacture, crack detecting and stress relieving shall be carried out the landing faces are machined and the frames are then erected on the bedplate aligned and located using fitted bolts.

b. Bedplate longitudinal girder.

Gives longitudinal strength, maintains alignment by giving sufficient rigidity to withstand hogging and sagging of ships hull.

Cracks may occur at the following places: 

• weld around the bearing pocket 
• junction welds 
• radially at tie bolt and frame bolt holes 
• around lightening holes 
• at the base of main bearing

c. Bedplate transverse girder.

• The most obvious place for cracking to occur in the transverse girder is under the bearing pocket. These cracks can be radial or circumferential.
• Cracking can also occur at the joining welds between transverse and longitudinal girders and at any change sharp change in section e.g. at the root of any stiffening webs.
• The cracking in the transverse girder of a bedplate will be due to fatigue; i.e. the action of a varying tensile stress.
• Circumferential cracking can be caused by overloading of the units either side of the bearing pockets, causing flexing of the transverse girder.
• This can also happen if the plate thickness of the girder is not substantial
• Radial cracking can be caused by bending moments set up due to slack or broken tie bolts. Cracks can start from where the transverse girder is welded to the longitudinal girders or where there is a change in section at the root of any stiffening webs.

Cause of defects:

1. Poor level of manufacture, and inspection. This will affect the quality of the welds primary but can also account for stresses locked into the structure from new, although post manufacturing neat treatment should reduce this.

2. Incorrect operation of engine. Imbalance of engine firing loads, on rotating masses (due to piston removal) will affect stress levels.

3. Incorrect tension and maintenance of fastenings such as holding down bolts, tie bolts and top bracing.

4. Incorrect operation of vibration dampening units. 

Vibration occurs due to three main sources: 

Hull structure vibration excited by axial vibration of the crankshaft. 

Hull structure vibration excited by varying propeller thrust induced by varying tension and 

Transverse vibration due crosshead guide forces. 

Most detuners on axial dampeners are fined units with little adjustment. However they can internally wear and their efficiency should be checked by vibration measurements on regular internal inspection

Inspection and repair of engine structure cracks:

• In order to determine if cracks are present, inspections should be concentrated on the areas where crack/dejects are most common.
• the initial search will probably be carried out visually with cracks appearing as paint dejects. The defects should be recorded as to: position, length of orientation.
• If possible clean the surrounding area and use NDT to improve inspection
• Dye penetrant is easy to use and interpret, but magnetic particle inspection (MPU) will show those cracks which are beneath the surface.
• Note that fatigue cracks occur with very little plastic deformation and the absence of any deformation makes detection more difficult.

Actions after detecting crack:

The following course of action could be taken

1. Most cracks are dormant on will only grow slowly and not pose any problems. Always check tension of surrounding bolts. a crack only grow  when the stress levels imposed on it are higher than the strain energy which it can dissipate through the parent material.

2. Should the crack be found to be growing, especially if accelerating then action should be taken

3. If shore facilities are not available, then try to bridge the crack and place the crack affected area in compression will include drilling and tapping out, drilling the end of the crack can be beneficial but finding the crack tip will be difficult

• If shore facilities are intensive, then the crack should be removed and filled by metal similar and superior to the parent. This will include gouging by girding on, then welding with possible pre and post heat treatment. Obtain approval of class surveyor before and after such work and closely monitor the following work.
• Try and obtain the welding procedure approval certificates to be used, so that checks can be made that welder is following laid down procedure. 
• Poorly carried out work may worsen the defect rather than improve it, as 'Delayed cold cracking may occur. This type of failure is the most common in higher tensile steel, heavier steel structure, and joints involving canting's