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We learnt about the phenomenon of journal bearing lubrication in marine diesel engines in our previous article. In this article we will learn about another type of bearing which falls under the general category of a journal bearing but the situation is quite different from that of a typical shaft type rotary motion.
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The type of motion of the crosshead of a marine diesel engine is oscillatory in nature wherein it oscillates to a few degrees on either side of the center of oscillation. Apart from that the speed of oscillation is not very significant which in turn means that it will not generate a pumping action for lubrication unlike the case of the fast rotating shaft in a bearing which we saw in the previous article.
Due to the nature of the two stroke cycle both in the normal 2 stroke marine diesel engines as well as supercharged 2 stroke marine diesel engines, the force acting on the crosshead bearing is always acting in the downward direction throughout the cycle without any such time period for which this load might be relieved. This certainly presents a challenging situation for effective lubrication to take place.
To overcome these challenges it is important not only to find a suitable lubrication technique but also to improvise on the design of the bearings so that lesser lubrication effect is required. Some of the steps which can be taken in this regard are as follows.
- The connecting rod design can be modified in a manner which provides better support for the crosshead pin.
- Apart from that the pin is also made thicker that what is required from calculations of load so that the large surface area of the thicker pin reduces load per unit area.
- The pin is also finished to a high surface finish and made more wear resistant so that the oiliness of the oil keeps it sticking to the surface
- Bearing shells are lined with a special kind of antifriction white metal that helps it to attain higher strength.
The hand drawn sketch below shows a typical crosshead just in case you haven’t got the idea about the arrangement of the crosshead bearing.
Apart from taking the above mentioned measures the lubricating oil is pumped at high pressure to the loaded region using high pressure pumps. This is necessary since the slow oscillatory motion of the pin is not sufficient to produce oil pressure that would keep the surfaces separate from each other.
Yet another interesting and equally challenging aspect is the lubrication insider the cylinder liner where the piston rings move continuously up and down against the liner. We will learn about this problem in the next article.