Solutions Current Issues > Oct.Nov.Dec_2008 >ON THE SURFACE
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VIBRATION CONTROL
Most systems (and people) have a limited tolerance for vibration.
LEADING THE TECH TEAM: Ray
Podesta heads up the Engineering
and Material departments at RL
Hudson.
by Ray Podesta
What do music, the roar of a jet engine, and an earthquake have in common? That’s right, they are all examples of vibration. We are surrounded by vibration, and much of it is good, but sometimes vibration must be controlled to avoid negative consequences.
WHAT IS VIBRATION? Vibration can be defined as “mechanical oscillation about an equilibrium point. ”Think of the motion of a pendulum in a grandfather clock (periodic vibration), or the motion of your car’s tires over railroad tracks (random vibration).
Within these two basic types, vibration may also be “free” or “forced” (also called “induced”). In free vibration, the object — a classic example is a tuning fork — is physically impacted, then continues to vibrate at a constant, characteristic frequency until the vibration eventually dies out. In forced vibration, the object is subjected to an alternating mechanical force that causes it to vibrate at the same frequency.
Free vibration is based on the principle of conservation of energy. A vibrating object may be thought of as a mass and a spring (see Figure 1).
Figure 1
Once it is set in motion, the mass moves, compressing or extending the spring until the force exerted by the spring causes the movement to stop. When the mass has stopped moving, the kinetic energy of motion has been transformed into potential energy. The spring then releases this stored potential energy, causing the mass to move back to the equilibrium point, where all the energy is transformed back into kinetic energy. The mass then continues to move until the spring’s force again causes it to stop, and the cycle is repeated. This oscillation about the equilibrium point is what we call vibration.
In the absence of friction, and given a perfectly elastic spring, this oscillation would continue indefinitely at a frequency proportional to the square root of the stiffness divided by the mass:
Consequently, reducing the mass and/or increasing the stiffness will increase the frequency. This frequency is called the undamped natural frequency and is key to the management of vibration because the amplitude is at a maximum when an object vibrates at this frequency, referred to as the resonant frequency. Figure 2 illustrates the effect of excitation frequency on the amplitude of vibration.
In a system of components, whether it is a device or a person, vibration can be transmitted because it is a form of mechanical energy. A vibrating gasoline engine causes the mower deck to vibrate, and as it does, it causes the handle mounts to vibrate. The vibration of the handle mounts causes the handle to vibrate, which in turn causes the operator’s hands to vibrate. Because of resonance, the vibration in the handle may be at a different frequency and magnitude (amplitude) than the original vibration of the engine.The ratio of the vibration output amplitude to the vibration input amplitude is called transmissibility, as shown in Figure 2.
Figure 2
THE ROLE OF DAMPING There is one more critically important piece to the vibration puzzle: damping. In general, damping reduces the amplitude of vibration within the region of resonance, with the minor penalty of having slightly higher vibration amplitudes than an undamped system at frequencies above resonance. If resonance cannot be avoided, damping must be used to minimize the vibration amplitude. Figure 2 illustrates how the vibration transmissibility is affected by low, medium and high levels of damping.
WHEN VIBRATION BECOMES A PROBLEM What makes some types of vibration acceptable and other types unacceptable or even damaging? The duration, frequency and amplitude define the exposure limit, whether a person or an object is being affected. Most mechanical and electronic systems have a limited tolerance for vibration. If not addressed, vibration can loosen fasteners, cause wear, damage seals and bearings, and lead to reduced service life.The magnitude, direction, frequency and duration of vibration exposure that does not result in reduced service life are collectively called the overall system fragility. While components and devices can be tested under controlled conditions, there is no simple formula for human tolerance because people respond both physically and emotionally to vibration and noise.
In spite of the wide range of tolerance among humans, much is known about the physical effects of vibration on people. As it relates to the human body, the frequency range from 0.5 to 30 Hz is of primary interest. Certain frequencies can cause resonance within the organs and structures of the body, potentially causing fatigue, blurred vision, or insomnia in severe cases.
The cumulative effects on the body include white finger disease (Raynaud's Syndrome), hand-arm vibration syndrome, carpal tunnel syndrome, tendonitis, raised blood pressure, impaired cardiac rhythm, and increased energy dissipation which can contribute to exhaustion. Any of these effects could accompany the use of machinery, recreational equipment, and outdoor power tools. Because duration cannot be controlled, it is up to the designer to minimize amplitude and, where possible, move operating frequencies into less harmful ranges.
ADDRESSING THE PROBLEM The simplest and most effective way to prevent vibration is to eliminate the source, but many times, that isn’t cost-effective or even possible. Instead, we must determine the component or system’s response to vibration, design a means of isolating the component or system to prevent resonant responses, and provide damping within the resonant frequency range.
Most vibration and shock isolators consist of one or more attachment points and an elastomeric body. Loading may be in compression, tension, torsion, or shear, or often a combination of these — a characteristic that opens up many opportunities for the designer. An additional benefit is that a single design can often be used in multiple applications, simply by changing the compound.
WHY ARE ELASTOMERS SO EFFECTIVE AS VIBRATION AND SHOCK ISOLATORS?
- Elastomeric isolators can be designed with a wide range of stiffness and damping characteristics.
- Material and isolator design can be tailored for the application, with an almost unlimited range of possibilities. Shape, dimensions, hardness, and the nature of the material itself can all be manipulated.
- Elastomers are unaffected by dirt, grit, and corrosion, and can be compounded to be resistant to chemicals, oil, and solvents.
- Elastomeric isolators are less expensive than other types because they require no moving parts, seals, fluid, or mechanical linkages.
What can RL Hudson do to help you solve vibration problems? This rather basic introduction to vibration is just “the tip of the iceberg.” Vibration is a highly complex phenomenon whose management requires specialized knowledge and careful engineering. At RL Hudson, we can deliver cost-effective solutions to vibration problems because we have material and part design know-how,we can analyze vibration problems, and we can build and test components. Our engineering design team, modeling capabilities, and in-house compounding and testing facilities, combined with a can-do attitude, all work together to improve customer satisfaction, enhance system reliability, and shorten your time-to-market.
