Camshaft drive mechanism

     Each cam must revolve at exactly half the speed of the crankshaft.  This isaccomplished with a 2，1 gear ratio. A gear connected to the camshaft has twice thenumber of teeth as a gear connected to the crankshaft. The gears are linked in one of threeways：

    ．Belt drive：a cog-type belt can be used. Such belts are made of synthetic rubberand reinforced with internal steel or fiberglass strands. The belts have teeth，orslotted spaces to engage and drive teeth on gear wheels. A belt typically is used onengines with overhead cam valve train.

    ．Chain drive：on someengine，a metal chain is used to connect the crankshaft andcamshaft gears. Most push-rod engines and some OHC engines have chains.

    ．Gear drive: the camshaft and crankshaft gears can be connected directly, ormeshed. This type of operating linkage commonly is used on older six-cylinder,inline engine.

    Camshaft driven by a chain of belt turns in the same directions as the crankshaft. Buta camshaft driven directly by the crankshaft gear turns in the opposite direction. Timingbelts are used because they cost less than chains and operate more quietly. A typical timingbelt is made of synthetic rubber reinforced with fiberglass.