J.W.Performance Transmissions

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FLYWHEEL SELECTION FOR HIGH COMPRESSION ENGINE APPLICATIONS AND RING GEAR FAILURE. During recent years, the use of engines having compression ratios upward of 18:1 has pushed the envelopes of many materials. In the case of flywheels, in some instances, stresses on ring gears have exceeded material strength. A worst-case example is a high compression engine that uses a small diameter, 130-tooth flywheel. Although ring gear failure may occur at any time, the cause of the failure most often is generated during startup. Consider the forces to which the ring gear teeth are exposed: During startup, inertia and high compression must be overcome to bring the rotating assembly up to cranking speed. Ignition timing then enters the equation. Advanced timing is very likely to cause engine “kick-backs” thereby introducing substantial additional pressure on the ring gear. Although the use of ignition retard devices is helpful, it is wise to consider the fact that during startup, the engine must rotate a few degrees before the timing is actually retarded. During that time, the ignition is at its advanced setting. Should ignition occur during those few degrees of rotation, the force on the ring gear is dramatically increased. During the period when ring gear force is maximized, tooth failure may occur. In addition, very minor cracks may be created in the area between the teeth. Some are detectable under close visual inspection while others may only be seen under a microscope. Cracking to any degree flywheel replacement prior to further operation. Once cracking begins, it is only a matter of time until the crack travels completely through the ring gear (i.e. Crack Propagation). At that time, centrifugal force begins to separate the ring gear from the flywheel. As the ring gear separation increases, so doe’s centrifugal force. Usually, the ring gear separates from the point of the crack to the next weld area. At times, the force is sufficient to cause the entire ring gear to separate from the flywheel. Close inspection of failed flywheels revealed the fact that much of the ring gear remained attached to the weld, clearly indicating the fact that the forces had exceeded the strength of the ring gear material. As a note of interest, larger diameter flywheels are exposed to a higher centrifugal force, yet their failures are substantially less frequent. The reason? Larger diameter flywheels provide more leverage and are therefore subject to less stress during cranking. To reduce flywheel stress, many racers use ignition switches in addition to retard devices. Until the engine reaches cranking speed, the ignition switch is turned off. When the switch is turned on, the engine benefits from the same inertia force that was previously a deterrent and “kickbacks” are less likely to occur. JW highly recommends that ignition timing management be used in all applications at and above 12:1 compression ratios. While this is not a “cure-all”, it contributes to flywheel stress reduction. Most every manufacturer of ring gears use 1045, 1050 or comparable materials.

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