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Crankshaft Oil Drillings

crankshaftsWe should all be very familiar with the primary functions of a crankshaft, namely as part of the mechanism which converts reciprocating motion into rotary motion, and to transmit torque to the outside world, where it might drive a gearbox, a generator or other piece of equipment. What a great many crankshaft designs also do is to provide lubrication channels which allow the passage of oil to the big end bearings, and possibly thereafter to the small end of the connecting rod.

Many years ago, crankshafts were designed with external oil gallery circuits made of thin metal tubes fastened to the crankshaft and rotating with it. These are only practical on large slow-speed engines, and would not be a realistic proposition on a modern racing engine. However, the large racing engines and piston aircraft engine of yesteryear have provided some ingenious methods for transporting oil to the connecting rods, and some of these methods are still in favour today. More of this a little later. First we will look at what has been general practice for many and, in a large number of cases, continues to be.

Common practice among road and racing crankshaft manufacturers has been to feed the main bearings with oil from a pressurised gallery. In order to feed the connecting rod, the pressurised oil on entering the main bearing, feeds into a central circumferential groove that commonly covers all 360 degrees of the bearing. From this groove containing pressurised oil, a drilling, or series of drillings takes the oil to the crankpins. This can often be done with a single compound angle drilling between the crankpin and the main bearing for each oil feed. However, some engine designers and some crankshaft manufacturers prefer to have axial drillings along the crankshaft, with drillings perpendicular to the crankshaft axis taking oil to these axial drillings from the main bearings and from the axial drillings to the crankpins.

Anyone who has studied crankshaft design, even briefly will know the importance of the oil holes to the fatigue life of a crankshaft. Along with main bearing and crankpin fillet radii, the oil holes are the prime causes of fatigue failure of crankshafts. Torsional fatigue failures of crankshafts often show cracks that have initiated at the oil hole exits, the condition of which is critical. It is absolutely essential to provide proper instructions about how these oil holes are to be treated, as left in the as-drilled condition, failure is likely to follow. At the very least, these should always be provided with a small radius to break any sharp corners, and this should be done before any surface treatment such as nitriding. After surface treatment, these radii should be polished to remove any imperfections or, in the case of nitrided parts, any ‘white layer’. It is important to radius the edges before nitriding so that the all-important residual compressive stresses are maximised in this critical area. If we were to radius the edges after nitriding, we would remove not only the material containing these stresses, but we would also have a more difficult task of producing these radii.

Written by Wayne Ward.

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