Engine camshaft

A camshaft made of cast iron or forged steel with one cam per valve is used to open and close the
Valves . the cam surfaces are hard made to obtain enough life under hard working conditions.

The camshafts are driven from the crankshaft
by camshaft drivers that may be gear , belt
,or chain.

Combustion gases in four-stroke engines are
controlled by the valve mechanism,a complex
structure, often referred to as a Valve train ,
of which the camshaft is an integral part .The
valve train can determines the overall engine
performance.

The valve train consists of a valve operating
mechanism and a camshaft drive mechanism.
The valve operating mechanism transforms
rotation of the crankshaft into reciprocating
motion in the valves.The valves front enter into
the Combustion chamber and are pushed back
by the reactive force of the Valve spring .

Several types of valve trains have been developed. The overhead camshaft is the most popular mechanism used in high-speed engines. There are two types, the Double overhead camshaft
(DOHC) and Single overhead camshaft (SOHC) .

SOHC type valve system mechanism :

                            SOHC type valve system installing twin rocker arms

In this mechanism " The SOHC mechanism " , it uses one camshaft, which drives a pair of inlet and exhaust valves via the rocker arms.The face that is in contact with the cam lobe needs to have good
wear resistance .

The position of the camshaft depends on the design of the engine ,it could be in the engine block cross to the crankshaft these called the pushrod or overhead valve system .

With pushrod actuation, four-valve heads become complex. However, the increase in complexity is less significant when applied with the valve actuation gear on a modern twin overhead camshaft engine than with that of a pushrod actuated unit.

the main jobs for camshaft is driving valves and Distributer and sometimes the Oil pump and Fuel pump .

To span the distance between the camshaft and the exhaust valves , the rockers have to be long.
Consequently forged En8 steel , instead of the usual cast iron , rockers are used . Because of the relatively poor bearing characteristics of steel , however , oil from a radial hole in the rocker bearing
is fed down a groove on top of the rocker arm to the cam follower pad .To obviate any possibility of
oil dripping down and getting into the exhaust Valve guide , where it could form carbon deposits,
a heat-resistant flexible seal is fitted over the upper end of the guide.

DOHC type valve system :

                                                 DOHC type valve system

This mechanism uses two camshafts, one camshaft drives the inlet valves and the other drives exhaust valves through the valve lifters.

In the DOHC mechanism, the Cam lobe makes contact with the head of the valve lifter directly or via
a thin round plate (pad or shim) , which is positioned on the valve lifter head. The high contact pressure means a much harder material is needed for the shims.