Main Engine Holding Down Bolts - Report Example

Name Tutor Subject Date Маin Engine Ноlding Down Bolts Main engine holding down bolts are paramount to a ship. That is because the engine must be held inflexibly in place so that alignment can be maintained between the propeller shaft and crank shaft. Pitching, rolling and vibration will make the engine shift position. The resulting engine thrust is transmitted to the bedplate from the crankshaft by the bearing sited at the rear end of the engine. Chocks are important as they are used to mount the bedplate securely to foundation plate. Hull has this foundation plate as part of its structure. A holding down bolt that has been fitted into place is shown below. During the mounting of the engine, the holdings down bolts are used to fasten it the iron chocks. Currently, epoxy chocks are used and have become the standard in an effort to reduce the cost of installation (Woodyard 327). The traditional chocks made from cast iron are used as an option to the epoxy chocks. The holding down bolts together with epoxy chocks have the tightening force significantly reduced compared to when cast iron chocks are used. The reason for this is the greater friction coefficient. In that context, epoxy chock allows for the use of thinner bolts that with time have become standard. However, many ship owners prefer the long bolts because they considered efficient and long lasting. They are also said to be flexible. RTA engines have a modest amount of holding down bolts. Thrust brackets and end stoppers are not needed because the thrust sleeves or fitted bolt provide the transmission of the thrust. The thrust sleeves and fitted bolt are applied to some holding down bolts (Woodward 419). Figure 1: Source: marineengineering.com The holes that can be seen in the top tank are screw cut. They can also be made by using frame cutting. Also, the studs are fasted down to a point where the conical face at the bottom of the stud sits on the top of the tank. The stud must form a watertight joint together with the grommet as shown in the figure above. The holding down bolts that are used currently on primary propulsion slow speed engines have long sleeves in addition to being hydraulically tensioned. The main purpose of the long sleeves is for strength and greater elasticity. The long sleeve holding down bolts are also less vulnerable to cracking, unlike earlier bolts. Also, the hydraulic jack bolts that were traditionally found in the main bearings have been replaced by a set of holding down bolts whose primary purpose is to fasten the bearing cap to the bedplate directly (Woodward 398). Mirrlees Blackstone models from the UK have cylinder heads with that has six holding down bolts. They are grouped in three pairs. Arrangement of Holding down Bolts The entire MC kinds of Engine models have the same principles, and their primary objective is reliability and simplicity. The large bore engines have the rigid bedplate constructed with longitudinal side girders. The cross beams are welded with cast steel bearing props. On the other hand, smaller bore engine types have a bedplate made of cast iron. It is specially designed to incorporate holding down bolts that are long and elastic. The holding down bolts are aligned in one row and fastened with hydraulic tools. The need to make sure that engine length is maintained at a minimum, a good bearing design is essential. The main housing of the bearing is mainly designed to incorporate a bearing shell with a thin shell. Four long and elastic holding down nuts are employed for each primary bearing cap. The two pairs of these bolts are believed to provide maximum distribution of the load to be held down in diesel engines (Woodward 403). The use of longer and stronger bolts is quite common around the globe. That is because the increased length of the bolt means there will be increased extension in case of tension. That means the cracking of the bedplate that would usually occur when short sleeved bolts would not happen. In that context, the long bolt will retain much of the tension should fretting happen. Together with the spherical washers, the long bolts are highly unlikely to become slack or fail. Additionally, the studs or nuts allow for the close location of tie rods to the bearing. That ensures the pressure loads are efficiently transferred. For the cylinder covers, eight long and elastic holding down studs are used in four pairs enhancing compactness and, in the long run, contributing to a shorter length of the engine. The manufacture of the whole engine is, therefore, simplified in modern ship models. In any case, the holding down bolts should always be fastened as near as is possible to the longitudinal girders and the floor. That is necessary to ensure that the engine is firmly held to the surface. The floor surface is more stable for an engine than when it was on top of an object. Also, the holding down bolts are sufficient on stable surfaces. The primary objective of holding down bolts is to make sure that a satisfactory load is evenly distributed on the transmission shaft, on all the bearings and the stern tube. Also, both the chocks and the holding down bolts will provide the necessary clamping force so that there would be no grinding between the chock and bedplate when the ship’s body resists the propulsion thrust. When the holding down bolt is not adequately fastened, an increase in vibration can happen. The vibration may be as a result of a change in alignment. Maintenance and the Installation Chocks are used to maintain the arrangement of the aligned bedplate that must be as firm as possible. That is to make sure that the crankshaft is straight and rigid. It is imperative that the engine and the propeller shaft are in line so that bending stresses can be avoided. The ship’s engine must be fastened to a flat surface using the holding down bolts. In cases where the surface is irregular, and the bolts are secured, there would be a distortion of the bedplate. The result of that would also lead to the distortion of the crankshaft that would cause unacceptable stresses during the running of the engine. In such a situation, cracking of the bedplate can occur leading to greater losses. Before the engine can be mounted, jacks are used so that alignment with the tail shaft bearing is ensured. To make sure this happens, a laser is used these days instead of the jack. The traditional methods used a wire that was stretched above the crankshaft and tail shaft. The distance between the midpoints of the crankshaft bearing from the wire would then be measured. The jacking bolts are adjusted based on the alignment of the propeller shaft. After perfect alignment of the bedplate is achieved, chock made from cast iron are hand fitted between the bedplate and the foundation plate. However, it must be ensured that the engine is in the correct position and the deflections from crankshafts show that there is no misalignment. The chocks are manufactured after space between the seating and bedplate is measured. It must ensure that the cast iron chocks bear most of the load to at least 85% of the area (Russell, Morton, and Jackson 44). Then it should be made sure that there are no bending stresses that may be harmful, the top plate underside that will be in contact with the holding down bolt and the surface of the nuts must be machined parallel. The task requires skill and care because the objective is to attain 80% contact. The higher the contact, the more rigid the engine is. The holding down bolts go through the chocks. As mentioned, the task of installing the engine requires skills and great care so that distortion of the bedplate would not happen that may lead to crankshaft misalignment. The jacking bolts are only removed after both the holding down bolts and chocks have been installed. The bolts should only tolerate tensile stresses. Any shear stresses should be avoided at all costs (Russell, Morton, and Jackson 44). The importance of the side and end choking is that they maintain the traverse and lateral locations. The number of side chocks will be based on the engine’s length. The side chocks ensure that the engine does not move sideways. The taper is hand fitted and held in place to ensure maximum contact. Where epoxy chocks are used, a resin material is poured around the sleeves. The main purpose of chocks is to prevent movement of the engine caused by thrust from moving components or the movement of the ship. Bolts are not used on chocks, but rather, they are welded to the foundation plate. Welding of the chocks ensures maximum rigidity. Figure 2: Source: Marine diesel Once the chocks support the engine, the jacks are then taken out. A hydraulic jack is employed to fasten and stretch the holding down bolts. For maintenance, it is imperative that the holding down bolts are checked on a regular basis for tightness. If looseness is allowed to happen, fretting can occur. Fretting in this context is the wearing down of mating surfaces due to friction. If fretting does no stop, the holding down bolts become more tightened down. The main bearings and the bedplate are then pulled out of alignment. To ensure that the resilience of the holding down bolts is maintained, the unthreaded part should have a small diameter, and the whole length of the bolt should be increased. The chocks should also be inspected to ensure that there is no evidence of looseness or fretting. There are several circumstances that can make a particular holding down bolt to look as if it has tightened while, in actual sense, it has stretched. Nut binding or corrosion on the threads may mean that the holding down nut is held tightly which may make it appear as if it is firmly in place. One way that the holding down bolts can be measured is by putting a finger or a thumb by the side of the nut so that any movement can be detected (Russell, Morton, and Jackson 45). The opposite side of the nut is then tapped with a hammer, and any movement that the finger detects will indicate that stretching of the bolt has occurred and is, therefore, not contributing to the holding down of the engine. In such a case, it is imperative that the bolt is replaced. The top nuts must be hydraulically tested to ensure that they are adequately fastened. To test that the chocks are well in place, a hammer is used to test at regular intervals (Wharton 80). The hammer test is also used after damage, or heavy weather is experienced. During maintenance, the main bearing is pulled to pieces, and any wear down checked and inspected. To check the alignment, the shaft is turned by hand. All the connections that require lubrication should have the pipes secured, and they also must be clear. The shaft coupling bolts together with the holding down bolts must be regularly examined for tightness. After all the parts are boxed up, water can be applied to cool them up (Wharton 108). All lubricating oil, water, and exhaust valves should also be checked to ensure the efficiency of the engine. It would not be important to concentrate only on the bolts and overlook other important things relating to the engine. It is important that holding down bolts are regularly checked because if they would loosen and come off, possible cracks to the bedplate can occur. The hold down bolts and cupped washers are some of the most overlooked places in case a movement in the engine is felt. Some of the main areas where cracks can happen include under the main bearing pockets and also near the welded joints in the space between the longitudinal girders and the transverse (Chief Engineer Officer And Second Engineer Officer 35). In that context, the tightness of bolts or studs should always be checked to ensure that such situations do not happen. However, cracks in the bed plate can be caused by sudden loads or faulty materials. Good quality materials should always be sourced from reliable manufacturers. If slacking bolts are detected, the holding down bolts should be tightened. However, the crankshaft’s alignment should first be checked. That is done by using a dial gauge to check deflections. If the alignment is found to be satisfactory, the removal of the slack shocks can occur and the mating surface smoothened and replaced. Consequently, the bolts can be tightened so that the chocks are hardened. After all the bolts and chocks have been fastened, the alignment of the crankshaft must be checked again to ensure that that the problem has been taken care. The quality and condition of the washers should always be considered. Most of the times, maintenance involves checking the holding down bolts and not checking the washers. Washers can also affect the alignment of the shaft. Therefore, washers should be verified as part of the ship maintenance process. Figure 3: Source: marine diesels Works Cited Chief Engineer Officer And Second Engineer Officer. London: London IMO, 1999. Print. Russell, Paul A, Thomas D. Morton, and Leslie Jackson. Motor Engineering Knowledge for Marine Engineers. London: Thomas Reed, 2012. Print. Wharton, A. J. Diesel Engines. Jordan Hill, Oxford: BH Newnes, 1991. Print. Woodyard, D F. Pounder's Marine Diesel Engines and Gas Turbines. Amsterdam: Elsevier/Butterworth-Heinemann, 2009. 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