The “cam effect”, or “ridge wear”, as it is sometimes referred to as well, negatively affects crankpins on four-stroke, trunk-piston engines. It is characterized by uneven wear as a result of the lubricating oil groove in the lower bearing shell and is further accelerated by abrasive particles in the lubricating oil. While the propensity varies according to the design of the crankpin bearing, no trunk-piston engine is completely free from this problem.
The cam effect develops over time. Generally speaking, the higher the number of running hours that the engine has accumulated and the larger the amount of abrasive impurities in the lubricating oil, the more pronounced the problem is. Engines that have a comparatively heavy piston or that frequently operate at low load are more susceptible too. Efficient lubricating oil treatment (filtration, separation, oil changes) are essential to keep the development of the cam effect in check as much as possible. Equally important are careful checks during engine overhauls, ideally with Prussian blue (engineer’s blue) and a dummy shell or straight edge.
If present, then the cam effect will manifest itself in two ways:
- Through uneven wear in horizontal direction, with nearly no wear at the centre of the pin and at the edges, but with easily noticeable wear to the left and right of the oil bore.
- The pin is not affected evenly throughout its circumference. The cam effect is usually most pronounced at about 30 – 45 degrees after Bottom Dead Centre (BDC). For this reason it is called “cam effect” – the pin is not perfectly circular anymore.
Because the problem is very common and has led to many engine failures, some of the major manufacturers of four-stroke engines have recently issued Service Letters to create awareness. QuantiServ appeals to owners and operators of medium-speed four-stroke engines to sensitize the crew about the cam effect. We highly recommends that the pins are carefully checked whenever an engine overhaul or bearing replacement is carried out. If any uneven wear patterns are detected, then the pin must be machine-polished to restore its proper geometry before any new bearings are installed and the engine is restarted.
Because they have experienced crankpin failures in the past, several of our customers, many of them cruise ship and power plant operators, are now asking us to routinely polish the crankpins every 60,000 running hours. Eliminating the cam effect has become part of their maintenance planning, with so far excellent results.