July '16

Issue link:

Contents of this Issue


Page 112 of 119

OCTOBER 2015 PRECISION ENGINE 9 PRECISION ENGINE For example, our 7.0-liter/427- ci V-8 has a primary order of 4. At 6,500 rpm torsional vibra- tion is occurring at a frequency of 433 hertz ((4 x 6,500) / 60). That means the crank is cycling through twisting and rebounding 433 times per second! Amplitude Torsional vibration has ampli- tude. Amplitude is the amount of the crankshaft deflection in degrees. It is measured two ways. One is from center to peak twist. Another is the total amount of deflection from peak twist, through center, to peak rebound. Crankshaft length in relation to the position of the firing cyl- inder and the amount of mean effective cylinder pressure applied determines amplitude. Amplitude increases with higher pressure and/or a longer crankshaft and/or taller connecting rods. From our LS engine example above, as frequency is occurring 433 times per second at 6,500 rpm, the crankshaft may be twisting and rebounding peak-to-peak through an amplitude range of 0.7 degrees. On 2.559-inch-diameter mains that equates to 0.015 inches of movement ((.7xπ)/180) x (2.559/2)) Resonance The rotating assembly emits its own natural frequency. Vibration is determined by the spring rate and inertia of the components as they rotate. Spring rate is material stiffness. Inertia is the mass and its speed of deflection. Amplitude spikes occur when torsional vibration frequency aligns with natural frequency. At this critical moment amplitude can double or triple in size. This is where parts are most susceptible. Continuing with our example, and for simplicity, let's assume the natural frequency of the rotating assembly is also 433 hertz. When rpms reach 6,500, torsional vibration frequency aligns with natural frequency. Peak-to-peak amplitude doubles to 1.4 degrees. Now crankshaft fluctuation is 0.03 inches. Vibration this significant robs power, reduces timing efficiency, accelerates bearing wear and causes crankshaft fatigue. It's enough to risk snapping the crank. The trend in engine building is to lighten the mass of the rotating assembly. Lighter mass shifts the natural frequency upward and often out of range for what the stock tuned elastomer harmonic balancer is designed for. In addition to the crankshaft, vibration will also pass through metal-to-metal contact and affect everything driven off it. Oil pump components, timing gears and the valve train become a con- cern. High-rpm engines often experience torsional vibration wear and damage that involve components other than the crankshaft. PROVE IT Crankshaft torsional vibration movement is small and fast. You cannot visually see it. That doesn't mean it's science fiction. SAE technical papers date back over 100 years on the topic. Support companies and entire departments at the OEM level, plus top racing engine builders are dedicated to calculating and resolving vibration issues. Professionals can model it with computer-aided engineering and finite element analysis software during design. On an oper- ating engine, you can measure it with sophisticated equipment by recording crankshaft fluctuations thousands of times per second. All the orders and amplitudes through the rpm range, plus where natural frequency resonance occurs, are revealed by this method. Every engine design is different, but they lay it all out and com- pensate for it. Finite Element Analysis modeling example that highlights stress along the crankshaft during a single internal combustion event.

Articles in this issue

Links on this page

view archives of THE SHOP - July '16