Just like any other bearing in any automotive engine, it needs a constant source of oil in order to keep spinning. It does this by utilizing oil galleries which are drilled throughout the block. These are fed oil from the oil pump, which draws oil (in most cases) from the oil pan reservoir. Since I know Small Block Chevrolet (GenI) engines, I'll use them as an example to show you what it looks like:
This is a drawing of the oil flow through the GenI engine. I have color coded the flow so I can explain where it's going and what is getting oiled. Let me step you through it:
- The oil starts in the pan at the bottom, where it is picked up by the oil pump pickup tube and brought into the oil pump.
- The oil is pushed from there to the oil filter (yellow lines)
- The oil flows from the filter up to the main oil gallery (green lines)
- In the oil gallery, it is fed down to the camshaft bearings (med blue lines)
- It flows around and through the cam shaft bearings and down to the main bearings (purple lines)
- It then flows through holes which are drilled through the crankshaft out to the rod bearings (thin light blue lines).
- Back up near where the oil enters the main oil gallery at the back of the block (opposite the fan), it also flows into side galleries which supplies oil to the lifters (red lines)
- After the oil flows through the lifters, it flows up through the push rods to the rocker arms to supply oil to the rockers and valve tips (bright purple lines).
In order for the oil to flow out to provide lubrication to the cam shaft, the cam bearings have holes in them which when installed align with holes in the block. The bearings also have grooves in the back of them which allows oil to pass around the bearing and down to the another hole which provides the oil to the main bearings. Here is a picture of a cam bearings so you understand:
Note the holes pointing towards you (which would point up in the block), and the groves on the outside of them.
As the oil is pushed out onto the cam bearings, it sort of looks like this (please note this is an exaggeration - the space when supplied with oil is much more even than what is shown here):
This is how the cam bearings in the SBC stay lubed as long as the oil pump is running and there is a supply of oil. Over time, the bearings do get worn. As this happens, oil is allowed to squeeze through easier and oil pressure in the engine starts to decline. This allows the bearing to wear faster and the oil pressure to decline more. Once this starts happening, it is a vicious cycle which can only end in disaster. Please note, in a SBC engine, this takes a long while, unless the owner does not keep up proper maintenance. The main and rod bearings will usually go bad before the cam bearing goes bad.
As many types of engines there are out there, there are just as many ways to oil an engine and the cam bearings. The cam bearing, almost regardless of engine, are round as described above. They all get their oil from a gallery, which has to come from the oil pump some how. If oiling was attempted with just a splash on method, it would be pure devastation for the cam and for the bearing. The cam has to endure a lot of load as the lobes come around and push the valves (through whatever method - direct or indirect). The oil is there to cushion, support, lubricate, cool, and clean. Without the direct pressure coming from the oil pump, it would not last very long at all.