Those Vital Bearings

Press, Volume CIV, Issue 30715, 2 April 1965, Page 11

Those Vital Bearings

QNE engine component often taken for granted, mainly because it gives so little trouble, is the bearings. There are about 23 types of bearing in the average engine, but it is the biggest type, the connecting rod and main crankshaft bearings, which do the hardest job. The bearings, usually in two halves or “shells,” fit around a rotation shaft or journal. The gudgeon or wrist pin bearing is usually in one piece, known as the full-round type or bushing. The typical bearing shell is made of a steel or bronze backing, with a liner of bearing material. This material is comparatively soft, and is a descendant of the white metal, or babbit metal used as a bearing material in older cars, which often have this material cast into the actual connecting rod halves.

The bearing material is made soft so that if wear takes place, it will be the comparatively cheap bearing that wears, and not the more expensive shaft.

Ideally, the shaft must never come in actual contact with the bearing. Instead, it must ride on a very thin cushion of oil. This is why lubrication with a good oil is so vital, and why it is important to have the correct bearing clearances when reassembling an engine. If the clearance is too great, oil will be lost and oil pressure will fall. If the clearance is too little, there may be contact between the shaft and the bearing, resulting in overheating, rapid wear, and possibly seizure. Some bearings have an oil groove in them to permit the oil to circulate.

As the oil moves across the not only lubricates, but also cools them and flushes away dirt. An average bearing clearance would be .0015 in. As wear increases, more oil flows—with an .003 clearance the oil flow might increase by five times, and by 25 times with a .006 clearance.

Wear As bearings wear, more and more oil is thrown on to the cylinder walls. Eventually the piston rings cannot control it, and it burns. Too tight a clearance can result in too little oil reaching the cylinder walls. The bearing shells usually used in modern engines are so precisely made they can be installed without any machining, as opposed to the poured type of bearing,

where machining was always needed.

Blit bearings must do other things besides carry the load, which is considerable. Bearing loads have nearly doubled in recent years. Bearings must resist metal fatigue from constant stressing, and they must also have “imbeddability.” This means that when dirt enters the engine and is worked on to the bearing, the latter protects- itself by letting such particles sink in. If this did not happen the dirt wouid lie on the surface- and scratch the shaft, and also the bearing.

"Flow" A bearing must also have “conformability,” or the ability to “flow” slightly and make up for minor irregularities in the shaft. This gives more even bearing loading. Also, when particles of grit imbed in the bearing, the metal is displaced to compensate for the added mass. The material must also be resistant to corrosion, and sufficiently hard to wear reasonably slowly, while still having imbeddability and conformability. So it can be seen a bearing is very much more than just a simple piece of curved metal. Today, the bearing surface is usually an alloy of several metals, such as cadmium, silver, antimony, mercury, lead, tin, and copper. A common mixture for highperformance engine bearings is lead indium. This is the material also used in the bearings of racing engines.