The rocker arm of an internal combustion engine changes radial movement into linear movement; this kind of device is properly called a reciprocating lever. It takes the spinning motion of the overhead camshaft and turns it into the up-and-down movement that opens and closes the valves. Because they are typically made of steel, rocker arms have a great deal of strength for their weight, and can therefore exert a great deal of leverage.
The camshaft is not a true spherical rod; it has bulges on it called lobes. The lobes project outward from one side of the shaft, while the back of the shaft -- opposite the lobe -- does not have a projection. As the shaft rotates, the side with the lobe lifts the outside end of the rocker arm, and then the side without the projection allows it to fall back. The cam’s movement is transferred to the rocker arm by a device called a cam follower, of which there are different types that vary by manufacturer and by vehicle age. As the cam shaft spins, the part of the rocker arm acted upon by the cam follower is lifted and lowered, lifted and lowered.
Just as the outside end -- the “cam end” -- of the rocker arm moves up and down, so does its opposite side. As the cam lobe comes round it lifts the outside of the rocker arm, which rocks, and the inner end pushes down on the valve stem, opening it against its spring. As the lobe rotates away from the rocker arm the outside drops, the inner end lifts and the valve’s spring closes it. In this way, the opening and closing of the valve is kept in sync with the rotation of the cam shaft, and thus with the movement of the pistons. Collectively, the cam lobe, follower, rocker arm, valve and valve spring are known as the “valve train.”
Rocker arms have “ratios” -- the ratio is a measure of the arm’s leverage, which in turn determines how powerful the force is that it can transfer to the valve stem. The ratio is determined by the length of the arm, both between its center of rotation and the point it contacts the cam lobe, and between its center of rotation and the top of the valve stem. A typical automotive rocker arm has a 1:1.5 ratio, meaning that it moves the valve one and a half times further than the peak of the cam lobe projects up from the camshaft.
A tapping noise from the top end of the engine can indicate that insufficient lubrication oil is being delivered to the rocker arm. Most of the oil delivered to the top end of the engine is there to keep the camshaft area lubricated; much less is used for the rocker arms. Top end tapping is typically an early warning that an issue is developing with either the mechanical parts or with the lubrication system; prompt service is strongly recommended.
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