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Rocker arm

A rocker arm (in the context of an internal combustion engine of automotive, marine, motorcycle and reciprocating aviation types) is an oscillating lever that conveys radial movement from the cam lobe into linear movement at the poppet valve to open it. One end is raised and lowered by a rotating lobe of the camshaft (either directly or via a tappet (lifter) and pushrod) while the other end acts on the valve stem. 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 valve.

Overview[edit]

The drive cam is driven by the camshaft. This pushes the rocker arm up and down about the trunnion pin or rocker shaft. Friction may be reduced at the point of contact with the valve stem by a roller cam follower. A similar arrangement transfers the motion via another roller cam follower to a second rocker arm. This rotates about the rocker shaft, and transfers the motion via a tappet to the poppet valve. In this case this opens the intake valve to the cylinder head.

Leverage[edit]

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. Current automotive design favors rocker arm ratios of about 1.5:1 to 1.8:1. However, in the past smaller positive ratios (the valve lift is greater than the cam lift) and even negative ratios (valve lift smaller than the cam lift) have been used. Many pre-World War II engines use a 1:1 (neutral) ratio.

Materials[edit]

For car engines the rocker arms are generally steel stampings, providing a reasonable balance of strength, weight and economical cost. Because the rocker arms are, in part, reciprocating weight, excessive mass especially at the lever ends limits the engine's ability to reach high operating speeds. Truck engines (mostly diesel) use stronger and stiffer rocker arms made of cast iron (usually ductile), or forged carbon steel.

History[edit]

Jonathan "Rundle" Bacon created them in the 19th century. Rocker arms have been made with and without "rundle" roller tips that press upon the valve.

Use of alloys[edit]

Many lightweight and high strength alloys, and bearing configurations for the fulcrum, have been used in an effort 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.

Geometry[edit]

Even the design aspects of the rocker arm's geometry 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 to James Miller on December 28, 1982, often referred to as the MID-LIFT Patent. Previously, the specific pivot points with rocker arm design was based on older and less efficient theories of over-arcing 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. Jim Miller's 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.

See also[edit]

References[edit]


http://www.youtube.com/watch?v=gG0go345My8


Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Rocker_arm — Please support Wikipedia.
This page uses Creative Commons Licensed content from Wikipedia. A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia.

19 news items

Shooting Times Magazine

Shooting Times Magazine
Tue, 15 Jul 2014 12:42:11 -0700

A handgun attaches securely to the base of the Ransom Rest by an appendage known as the rocker arm. The rocker arm moves freely and pivots upward when the gun fires (Figure 1). Its movement is restricted by spring tension and steel and fiber friction ...
 
Motorsport.com
Wed, 23 Jul 2014 13:24:02 -0700

NASCAR's cars and engines are antiques, fifty years out of date, but tube frame chassis and traditional gasoline-burning, rocker-arm V8 engines are at the heart of America's stock car racing industry from the Sprint Cup series down to local short tracks.
 
PR Web (press release)
Tue, 22 Jul 2014 02:00:00 -0700

Moroso Fabricated Aluminum Valve Covers are extra-tall to clear virtually every rocker/arm combination. They're also baffled to control oil movement. The covers are designed to fit the 2003-08 Chrysler 5.7L and 6.1L engine. Installation on 2003-05 ...

Newsday

Newsday
Fri, 11 Jul 2014 14:54:26 -0700

Stiles' car had a rocker arm-stud issue and, because of the holiday, he could not find anyone to help him with it. Basically, the arm studs are like bolts and need to be screwed into the engine tightly. Without the studs, "it would feel like a ...

autoevolution

autoevolution
Tue, 22 Jul 2014 06:07:05 -0700

Even more, the whole engine received a matte black coat for engine and rocker arm covers, cylinders, air filter box and which extends even to the fork shrouds and headlight casing. It would have been glorious to have the same color on the wire-spoke ...

Azom.com

Azom.com
Fri, 04 Jul 2014 03:57:53 -0700

In turbochargers, silicon nitride helps in reducing engine lag and emissions; in petrol engines, it is utilized for rocker arm pads to reduce wear; and in exhaust gas control valves, the material allows for increased acceleration. It has been estimated ...
 
Overdrive
Tue, 01 Jul 2014 21:01:48 -0700

Also part of the 'Blue Core' ethos is an improved combustion chamber and air intake, optimised crank inertia, lighter moving parts, advanced electric charging system, roller rocker arm and a closed loop FI system. But a mere three cubic inches shouldn ...

Overdrive

Overdrive
Fri, 04 Jul 2014 07:38:54 -0700

Elsewhere, Yamaha mentioned the roller rocker arm, lighter moving parts, optimised crankshaft inertia, an advanced electric charging system (which consumes much less power when the headlight is off, for instance) and the offset crank (the con-rod is ...
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