Rocker Arm Design - CHP How It Works

With The Help Of Comp Cams And T&D, We Break Down The Intricacies Of Rocker Arm Design

Stephen Kim Jun 22, 2010 0 Comment(s)
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Roller Tips
Brad Rounds: "For overhead valvetrains, sliding tips do not work very well at all. At valve lifts above 0.550- inch, a plain tip wants to bend the valve stem because the side loads become very high from the sliding motion. Also, with the high spring loads used in racing applications, slider tips don't last long and cause excessive wear on the end of the valve. Even in applications with high valve lifts combined with 5/16-inch or less valve stem diameters, our regular roller tip has a tendency to push the valve stem around, so our needle roller tip is highly recommended. T&D offers a roller tip on a shaft as a standard feature, and a roller tip with needle bearings on a shaft as an option. Both styles of tips benefit from T&D's oiling system, where the rocker pivot is fed directly from the rocker body. T&D also offers a tip axle of a harder material as an option.

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"To fit needle bearings into a roller tip with a given outside diameter means that the inside diameter of the tip must increase. This thinner wall thickness is thought by some to lead to failures, but that hasn't been the case in our experience. Tip failure is usually the product of excessive valve lash and a valvetrain that has gotten out of control. In those cases, the tip would have been destroyed with either a standard roller or needle bearing tips. Certainly the psi loading increases with a needle bearing tip, because at any one time, only two individual needles are taking the load. However, the decrease in friction offsets that. Today, with increased camshaft lift and valvespring rates, as well as decreased lubricating agents and lessened viscosities in race oil, needle bearing tips have become more the standard."

Alignment
Chris Mays: "The best way to check for proper alignment between the rocker arm tip and the valve stem is by finding zero lash. Shortening or lengthening of the pushrod to achieve centering of the roller tip over the valve stem is normally necessary. There are many problems associated with not checking the rocker geometry, the worst being the tip rolling off of the valve stem. This geometry is vital for proper engine dynamics to be achieved. We recommend running 0 to 0.004-inch lash on hydraulic race cams and 0.012- to 0.016-inch lash on solid cams for street applications. Finding zero lash is sometimes tough with a hydraulic lifters, but always check for it on the base circle and tighten the rocker until you feel slight drag when rotating the pushrod in your fingers."

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Design Priorities
Brad Rounds: "Proper valvetrain geometry is always the most important design parameter. We always work toward making the rocker assembly fit the head the best without modifications. This may sound very low tech, but usually it is the greatest challenge. As engines have grown in every dimension, it has become increasingly difficult to get all the pieces together with the proper clearances and the proper geometry. That said, each application sets its own parameters. Some take a fairly standard approach, where a rocker design T&D has already been using can be changed slightly to work just fine in a different application. Others need a clean sheet of paper due to the needs for offsets, ratios, valve angles, and a dozen other variables. We have recently completed the task of developing rocker sets for nearly all the different types of race heads on the market. There are roughly 25 cylinder head manufacturers that we deal with daily. That is, everyone from Dart, Brodix, and Edelbrock to a guy that buys rough castings and builds a few heads for himself. To keep up with demand, those manufacturers take a great deal of T&D's engineering time. Nevertheless, all T&D rocker systems receive the same tireless focus in engineering."

Sources

Comp Cams
Memphis, TN 38118
800-999-0853
http://www.compcams.com
T&D Machine Products
Carson City, NV
775-884-2292
http://www.tdmach.com

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