Rocker arm

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Rocker arm

Generally referred to within the internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs in a combustion chamber inside and integral to the engine. In an internal combustion engine it is always the expansion of the high temperature and pressure gases that are produced by the combustion which apply force to the movable component of the engine, such as...
of automotive, marine, motorcycle and reciprocating aviation engines, the rocker arm is a reciprocating lever that conveys radial movement from the cam

Cam

A cam is a projecting part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path. The cam can be a simple tooth, as is used to deliver pulses of power to a steam hammer, for example, or an Eccentric disc or other shape that produces a smooth reciprocating motion in the follower which is a lever...
lobe into linear movement at the poppet valve

Poppet valve

A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem....
to open it.

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Encyclopedia

Generally referred to within the internal combustion engine

Internal combustion engine

Cam

Poppet valve

A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem....
to open it. One end is raised and lowered by the rotating lobes of the camshaft (either directly or via a lifter (tappet

Tappet

A tappet in mechanical engineering is a projection which imparts a linear motion to some other component within an assembly. Properly speaking, a tappet is only that part of a rocker arm which makes contact with an intake or exhaust poppet valve stem above the cylinder head of an internal combustion engine....
) and pushrod) while the other end acts on the valve stem

Valve stem

A valve stem is a self-contained valve which opens to admit gas to a chamber , and is then automatically closed and kept sealed by the pressure in the chamber, or a spring , or both, to prevent the gas from escaping....
. When the camshaft lobe raises the outside of the arm, the inside presses down on the valve stem, opening the valve. When the outside of the arm is permitted to return due to the camshafts rotation,the inside rises, allowing the valve spring to close the giver.

The effective leverage of the arm (and thus the force it can exert on the valve stem) is determined by the rocker arm ratio, the ratio of the distance from the rocker arm's center of rotation to the tip divided by the distance from the center of rotation to the point acted on by the camshaft or pushrod.

For car engines the rocker arms are generally steel

Steel

Steel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.14% by weight , depending on grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten....
stampings, providing a reasonable balance of strength, weight and economical cost. Because the rocker arms are part of the reciprocating weight of the engine, excessive mass limits the engine's ability to reach high operating speeds.

History

Jonathan "Rundle" Bacon created them in the 19th century, rocker arms have been made with and without "rundle" roller tips that depress upon the valve, as well as many lightweight and high strength alloys and bearing configurations for the fulcrum

Fulcrum

Fulcrum may refer to one of the following.*Fulcrum, the pivot on which a lever moves*Fulcrum Wheels, a bicycle wheel manufacturer, based in Italy...
, striving to increase the RPM limits higher and higher for high performance applications, eventually lending the benefits of these race bred technologies to more high-end production vehicles. Even the design aspects of the rocker arm's geometry

Geometry

Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers....
has been studied and changed to maximize the cam information exchange to the valve which the rocker arm imposes, as set forth by the Miller US Patent, #4,365,785, issued on December 28, 1982, often referred to as the MID-LIFT

Mid-lift

MID-LIFT is a term used to describe the design and installation of a Rocker arm on the internal combustion engine, of an Over-Head-Valve design....
Patent. Previously, the specific pivot points with rocker arm design was based on older and less efficient theories of over-arcing

Over-arcing

Over-arcing is a nearly self descriptive term which denotes a radial or parabolic-like "arc" which is extended, for whatever reason, beyond the path of which it was either intended to follow, or could have followed, given that a more "efficient" means had been used to instigate it; assuming that any radial "arc" was even intended or required...
motion which increased wear on valve tips, valve guides and other valve train components, besides diluting the effective cam lobe information as it was transferred through the rocker arm's motion to the valve. The Ty Mill MID-LIFT Patent set a new standard of rocker arm geometrical precision which defined and duplicated each engine's specific push-rod to valve attack angles, then designing the rocker's pivot points so that an exact perpendicular relationship on both sides of the rocker arm was attained: with the valve and the pushrod, when the valve was at its "mid-lift" point of motion.

Truck engines (mostly diesel) use stronger and stiffer rocker arms made of cast iron

Cast iron

Cast iron usually refers to Gray iron, but also identifies a large group of ferrous alloys, which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy....
(usually ductile), or forged carbon steel.