July '16

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8 PRECISION ENGINE JULY 2016 PRECISION ENGINE The harmonic balancer takes the twist out of the crankshaft for durability and performance. It is that simple. Recommend a harmonic balancer upgrade to your customer, however, and things get complicated, fast. Have you heard, "crankshafts don't twist" or "my engine is balanced, I don't need a harmonic balancer"? These are legitimate customer perceptions. Perception is reality. Given the terminology and complexity of the topic, it is easy to draw misconceptions. This article will help you and your staff to explain the science in a way that the purpose of a harmonic balancer becomes obvious. Simple guidance can create a life- long customer and one beast of a perfor- mance engine. MECHANICAL VIBRATION Vibration accelerates wear, breaks parts and robs power. There are three types of vibra- tion that engine builders are concerned with. Each type has its own method to control it. They are: Unbalanced. This is what we commonly think of. It occurs once per revolution and is associated with weight imbalance. Crank- shaft counterweights work to control this. When you balance the rotating assembly you are minimizing unbalanced vibration. Only if a harmonic balancer contains an external counterweight does it contribute to balancing the rotating assembly. Axial. Axial vibration is forward and backward movement of the crankshaft. The main bearing support plate and thrust bearings work to control this movement. Torsional. Torsional vibration is the end- to-end twisting of the crankshaft by force. The harmonic balancer reduces the twist. Three unique movements of vibration. Three completely different ways to control each. WHATS TORSIONAL VIBRATION? Internal combustion is the force that sets off torsional vibration. There's no getting around it. A 7.0-liter/427-ci LS engine producing 450 foot-pounds of torque can generate 1,088 psi on the crank in a given stroke. Strike a shaft with that kind of force and it is going to flex and rebound. The degree to which it matters depends on the design of the crankshaft and how torsional vibra- tion sets up. Frequency & Orders Torsional vibration has a frequency mea- sured in hertz or cycles per second. Fre- quency calculation is rpm times order, divided by 60 (cycles per second). An order is how often a vibration event occurs during one revolution of the crankshaft. In a four-stroke engine, the primary order is half the number of cylinders. This is because only half the cylinders fire during one revolution of the crankshaft. Other orders that set up are deviations from vibration oscillating through the crankshaft. As you move up through the rpm range, the frequency of each order increases. Keeping an engine's internal movements balanced and in line. UNDERSTANDING UNDERSTANDING VIBRATION ISSUES By Brian LeBarron Comparative torsional vibration analysis is conducted on an engine dyno using a high-resolution optical sensor capturing speed fluctuations of the crankshaft thousands of times per second.

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