THE SHOP

Performance & Hotrod Business - July '14

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I n the automotive performance world, two things will get you every time: heat and vibration. By preventing or reduc- ing these two from reaching the breaking point of our components, we can create the foundation for a highly durable, highly efficient engine. The alternative is to keep replacing bro- ken parts. With today's savvy consumers increasingly mindful of their budgets and actively searching for higher-quality com- ponents, there is an undeniable require- ment for jobbers to build it right the first time. Heat can be dealt with, but vibration is inherently much harder. Enter the job of the harmonic balancer. This critical build- ing block of engine performance is there to provide greater durability and improve efficiency. Before we discuss which harmonic bal- ancer to recommend, let's review a brief introduction as to why we need them, so you can provide some educational context to help your customers decide. Torsional Vibration 101 When discussing vibration or engine harmonics, it is essential to know that there's more than just the orbital kind we commonly think of when balancing tires or a rotating assembly. There's also back-and-forth axial vibration controlled by main clamps and thrust washers. Then there's torsional vibration (see Figure 1). Torsional vibration occurs naturally and is the slight end-to-end twisting and rebounding of the crankshaft that results from internal combustion. During each combustion cycle, the change in mean effective pressure combined with the driv- ing mass of the rod and piston is enough to deflect the area around the crank web ahead of its natural rotation. When pressure is relieved, it snaps back beyond its center axis. This creates the stage for torsional vibration, a science that has been documented in SAE papers for over 100 years. The amount of torque applied increases the vibration's amplitude and rpm determines the frequency. Secondary, tertiary and more torsional vibration orders set up as it transverses A Question of Balance By Brian LeBarron Figure 1: illustration depicts deflection of crankshaft web during combustion cycle. This motion sets up torsional vibration. 22 n PRECISION ENGINE n July 2014 PRECISION ENGINE Getting a handle on vibrations—the harmonic balancer is critical to engine performance, durability and efficiency. PEJUL.indd 22 6/2/14 2:35 PM

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