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INTERNAL INTERNAL COMBUSTION ENGINES BEDPLATES & HOLDING DOWN ARRANGEMENTS AIM
Understand the functional purpose of bedplates and holding down arrangements Understand the basic design features of modern m ain propulsion engines bedplates and holding down arrangements Understand problems associated with bedplates and holding down bolts Understand maintenance procedures of the components
LEARNING OUTCOMES (Specific Learning Objectives) On completion of this lesson, students should be able to State the functional purpose of bedplates of bedplates and holding down arrangements Describe the basic design features of modern main m ain engine bedplates with particular reference to longitudinal and longitudinal and transverse girders Identify the areas of bedplates prone to failures Describe proper maintenance proper maintenance procedures of bedplates Describe the basic design features of modern main m ain engine holding down arrangement Identify the areas of holding down arrangements prone to failure Describe proper maintenance proper maintenance procedure of holding down arrangement
OVERVIEW The lesson will provide students with the understanding of functional purpose, design features and problems associated with modern highly rated main propulsion engine bedplates and holding down arrangements. Bedplates are the backbones of engine struct ure which are subjected to severe stresses and often neglected because these are situated below the f loor plate along with the holding down bolts. Consequences of bedplate failure of running engines with slack holding down bolts could be disastrous. Key questions have been set at every stage and at the end of the lesson for better understanding and it is essential for you to answer these questions for successful completion of this module. Answer to these questions must be submitted with assignments.
BEDPLATE Introduction Bedplate provides the rigid seating for the crankshaft which is absolutely essential for satisfactory engine operation. Bedplate takes up the static load from the running gear and the other constructional components. It also takes up the dynamic load coming down the piston and connecting rod during operation. The central portion of the bedplate cross girder which is often referred to as `bearing saddle’ is the most heavily loaded part of the engine structure and must be checked thoroughly during crank case inspection. Singapore Polytechnic
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Basic Design Features The bedplate is constructed of deep longitudinal beams (running along the length of the engine) and transverse girders (across the engine). Two most important components of bedplates are the transverse or cross girder and longitudinal girder . The central portion of the transverse girder is called the `bearing saddle’ which forms the main beating housing. The transverse girders and particularly the bearing saddles are the most heavily loaded part of the engine structure. The static load of the running gear and dynamic load down the piston and connecting are transmitted through the crank journal to the bearing saddle of the cross girder. It has also to be noted at this stage the cross girder of the bedplate has no support at the centre. The support is only at the two sides with the aid of chocks. So, the cross girder is like a beam with load at the centre and support at either ends. The bending moment is maximum at the centre. Support at the centre of the cross girder will result in misalignment of bedplate and eventually the crankshaft will be out of alignment. The longitudinal girder plays the most important role in maintaining the longitudinal rigidity and strength of a bedplate. This in turn keeps the crankshaft aligned in longitudinal direction.
Each main bearing of main engine is fitted at the centre of the transverse girder of the bedplate. Identify the number of transverse girders fitted to the main engine bedplate. Is there any support at the centre of the transverse girder? It is essential for you to pay maximum attention to transverse girder of the bedplate and the junction of transverse and longitudinal girders during crankcase inspection.
Fig 1 Bedplate & Engine Frame (Source: Low Speed Marine Diesel by John B Woodward)
Fig 2 Transverse section through: (a) Older type slow-speed engine bedplate; (b) Modern slow-speed engine bedplate (Source: Low Speed Marine Diesel by John B Woodward)
Materials of Bedplate Parts of the bedplate of main propulsion engines are constructed separately and the joined together by welding in most of the cases. The bearing saddle and in some of the cases the whole transverse girder is of cast construction and made of the cast steel. The longitudinal girders are of fabricated construction and made of mild steel. Transverse and longitudinal girders are joined together by welding. What are the materials used for the main engine bedplate on board your ship? How is the transverse girder connected to the longitudinal girder? It is extremely important for you to pay close attention to the junction welds between transverse and longitudinal girder during crankcase inspection. Singapore Polytechnic
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Forces on Main Engine Bedplate Fluctuating gas pressure from the engine cylinder and through the connecting rod, crank shaft to the bedplate Inertia forces from the reciprocating masses Static weight of all engine parts Torque reaction from propeller Hull deflections Vibration forces due to torque fluctuations, shock loading Thermal stresses Forces due to ship's movement in heavy seas.
Bedplate is the most heavily loaded part of the main engine structure, particularly the transverse girder and junction welds. In modern engines, there is extensive welding on the longitudinal girder of the bedplate. Pay close attention during crankcase inspection.
Faults in Bedplates Bedplates are constructed under stringent classification society and engine builders requirements. So, faults should not occur in the bedplates of well maintained engines. Consequences of the faults are disastrous. Bedplate faults manifest themselves as follows:
Cracks Loose Chocks Oil Leakages
Likely Areas of Cracks During the crank case inspection of main propulsion engine, check for cracks particularly at following areas: Central portion of transverse girder (main bearing saddle) and around the saddle Junction welds between transverse and longitudinal girder and all the welded points Tie bolt holes and around the holes (top of the transverse girder) Lightening holes (if any) Base of main bearing keeps Can you identify these areas on bedplate of your main engine?
Reasons for Failures & Cracks Excessive vibration Slack tie bolts Overloading of main bearings due to wear Poor welding Loose chocks
HOLDING DOWN ARRANGEMENT The engine bedplate is supported on a series of chocks fitted around the underside of the periphery of the base of the bedplate. The chocks sit on the inner bottom plating (in case of main engine) of the hull structure which forms part of the engine seating. Holding down studs pass through the bedplate, chock and tank top plating. The studs in most of the modern engines are free through the bedplate, chocks and tank top plating. Singapore Polytechnic
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Holding down bolts often run loose and consequences of running an engine with slack bolts is disastrous. This will be discussed in detail later.
Design Features The holding down bolts pass through holes in the bedplates, chocks and foundation plate or engine seating (inner bottom plating in case of main engine). The bolts are free through the components mentioned above. Some of the modern engines use long bolts with spacers. The chocks are fitted after the engine is aligned with the intermediate shafting. The chocks are fitted more closely to the cross girder as the girder has no support at the centre. The top and bottom of the chocks have to be given a high degree of surface finish. The bottom of the bedplate and tank top in way of the chocks have to be given a high degree of surface finish. Sometimes pads are fitted on the tank top plating in way of chocks.
Fig 3 Typical Holding Down Arrangement (Source: Lamb’s Question & Answer on Marine Diesel Engines by Stanley G Christensen)
Most of the modern engines are fitted with Side and End chocks in addition to main supporting chocks. This reduces shear stress in holding down bolts. The holding down bolts are subjected to fatigue, because of fluctuation of stresses. So, the engine builders aim is to reduce stress fluctuation. In modern holding down arrangements, the bolts are subjected to high stresses but this eventually results in less stress fluctuation. Fig 4 MAN B&W Large Engine Holding Down Arrangement (Source: MAN B&W Engine Instruction Manual)
Materials The chock material in most of the cases is Cast Steel . Chock can be also made of cast iron in smaller engines. Number of highly rated modern engines are using Epoxy Resin. The bolt materials is high UTS (Ultimate Tensile Strength) steel. Some modern engines eg MAN B&W have mostly adopted long thin elastic bolts to increase fatigue strength.
Faults in Holding Down Arrangements Loose bolts and chocks (This is probably the most common problem ) Cracked chocks Pay maximum attention to loose bolts and chocks during inspection. Never tighten when engine is in operation.
Classification Societies (eg LRS, DNV, ABS) require the bolts to be checked as part of CSM cycle ie once every 4 year period. In practice this is too long a period. Bolts should be checked every 3000 RHrs (approx 6 months) unless there is inherent problem of slackening. Check engine manual on board for inspection interval of holding down bolts. Is this interval satisfactory? (Check with Chief Engineer or Second Engineer). Singapore Polytechnic
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Consequences of Running Engine With Slack Holding Down Bolts Severe fretting (wearing under heavy load) can occur on the mating surfaces of bedplates, chocks and foundation plate (inner bottom plating). If fretting occurs in number of adjacent chocks, severe crankshaft damage can take place through misalignment. Crankshaft damage can go unnoticed for considerable period time with serious effect on the engine and safety of the vessel. Excessive vibration, abnormal movements in the upper part of the engine are some of the indications of slack holding bolt. Sometimes there may be no visible indications. Inspect Holding Down Bolts at regular intervals.
FINAL CHECK Now that you have worked through this chapter of the notes, it is the right time to check if you have understood some of the basic concepts, maintenance procedures and problems related to the components which were covered in this chapter. If you are unable to answer the following questions, you must go back to the notes and review your work and activities related to this chapter.
Self Evaluation 1 2 3 4 5 6 7
What are the main parts of a bedplate of large main propulsion engines? Why is the transverse girder most heavily loaded part of the bedplate? Is there any support at the centre of the transverse girder? Where is the main bearing placed in the transverse girder? What are the most likely places for faults in a bedplate? Why is it essential to inspect the holding down bolts at regular interval? Are the holding bolts on your engine hydraulically tightened?
If you can answer these questions confidently, please move on to the next chapter. Congratulations!
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