Operational Information

The Medium Speed 4 Stroke Trunk Piston Engine

The Camshaft

 

 

There are several different methods of manufacturing camshafts for medium speed 4 stroke marine diesel engines. On the smaller engines, the camshaft may be a single forging complete with cams.

Alternatively the camshaft can be built up in single cylinder elements, each element made up of the fuel, inlet, and exhaust cam on a section of the camshaft with a flange on each end. So that the element can be used on any unit in the engine, the number of holes for fitted bolts in the flanges must be sufficient to allow the cam to be timed for any unit on the engine. For example, on a six cylinder engine, the flanges must have 6 equi spaced holes or a multiple thereof. The cams must be hard enough to resist the wear and abrasion due to impurities in the lub. oil, yet they must be tough enough to resist shattering due to shock loading. The cams are therefore surface hardened using the nitriding process.

On the larger engines it is usual to manufacture the camshaft and cams separately. The nitrided alloy steel cams are then shrunk on to the steel shaft using heat or hydraulic means. Because the cams are fitted progressively onto the shaft, if the bores in the cams were all the same diameter, it would be very difficult, if not impossible, to fit the first cams all the way along the length of the shaft  to the correct position. To overcome this problem the camshaft is stepped, with the largest diameters at the end which has the cams fitted first. The larger bored cams fit easily over the small diameter steps till they reach the correct position on the camshaft.

Keys are not generally used to locate the cams as they would act as stress raisers.

Most medium speed engines are unidirectional (i.e they only rotate one way). This is because they either are driving an alternator, or because if they are used as direct main propulsion they tend to be driving a controllable pitch propeller.  In the case where the engine is reversing, then the camshaft has two sets of cams, one for ahead operation, and one for astern.

To reverse the direction of the engine, pressure oil is led to one side of a hydraulic piston which is coupled to the camshaft. The whole camshaft is moved axially and the cam followers slide up or down ramps which connect the ahead and astern cams.

The camshaft is either chain or gear driven from the crankshaft. Because the engine is a four stroke, the camshaft will rotate at half the speed of the crankshaft. (the valves and fuel pump will only operate once for every two revolutions of the crankshaft).

In a case where the cams are shrunk on the camshaft, if a cam becomes damaged and has to be replaced, then it can be cut off using a cutter grinder. Care must be exercised not to damage the camshaft or adjacent cams during the operation. The replacement cam is fitted in two halves which is then bolted on the camshaft in the correct position and the timing rechecked.

This is a damaged fuel cam from a ZA40 engine which has been cut off using a grinding wheel. Note the grooves which are used to distribute the hydraulic oil when expanding the cam on the camshaft.

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