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Solutions 1Q07

In Focus

First Articles

Benny & Bruce

From the Top

Cover Story

Tech Session

Solutions Current Issues > January/February/March 2007 > TECH SESSION

Vibration Control

Careful design can keep you from getting rattled.

by Shenghong Yao, Ph.D.

I’ve been a Product Engineer here at RL Hudson since 2002. During the past five years, I have worked on many different types of applications. Some of the most interesting projects have involved the need to control vibration. I’ve found that this can usually be accomplished through good design.

Vibration itself is a pretty universal phenomenon, especially with motorized machinery. In most cases, vibration is detrimental, and for a number of reasons. First, vibration wastes energy. Second, vibration can cause premature fatigue of a part or assembly through unnecessary movement. Third, vibration is typically damaging to things (and people) within its surroundings; this damage might take the form of cracked floors or ringing ears. Fourth, vibration can affect a machine’s ability to function properly. Think about the CD players now common to so many vehicles; without good vibration control, they would never be able to get past the first few notes of any song without skipping. Vibration control seeks to eliminate or at least minimize these detrimental effects. But in order to successfully control vibration, first we must understand exactly what it is.

ANATOMY OF VIBRATION Webster’s dictionary defines vibration as “a periodic motion of the particles of an elastic body or medium in alternately opposite directions from the position of equilibrium when that equilibrium has been disturbed.” Try reciting that the next time you’re at a company party! More simply, vibration is an oscillatory (back and forth) mechanical motion. In mechanical applications, vibration typically takes the form of a quivering or trembling. An exaggerated example of vibration is a pendulum swinging.

Vibratory movements behave like waves in that they have amplitude, frequency, and traveling speed. Amplitude refers to the extent of the vibratory movement (again, think of the pendulum swinging higher or lower) as measured from the middle (or mean) position to the extreme. Frequency refers to the number of times the vibratory movement is completed in a given period (for example, every second). Traveling speed refers to how quickly the center or mean position of the vibration moves. A wave with a center position staying in one place is called a standing wave as opposed to a traveling wave.

Any object that vibrates tends to do so at a specific frequency (or frequencies). The characteristic frequencies – known as the object’s natural vibration frequencies – are the most important attribute to consider when designing products to control vibration. An object’s natural vibration frequency or frequencies – the way in which it responds to its environment – depends chiefly on the composition of the object, both in terms of molecular structure and mechanical properties. Vibration frequency also varies according to the object’s size, structure, weight (material density), and shape, as well as with how the object is constrained.

An object’s natural vibration frequency can be heightened (accentuated) or lowered (attenuated) by connecting to other objects through certain connectors. An object’s reaction to outside or internal vibration excitations is usually referred to as frequency response. When the frequency of the excitation is the same as (or close to) the natural frequency of the object, it results in resonance, a state in which vibration amplitude tends to grow very large (magnified). Conversely, when the excitation frequencies are very different from the object’s natural frequencies, the object’s reaction to the excitation tends to be much less; its vibration amplitude tends to be reduced. In fact, the further apart the frequencies are, the less vibration remains. This is one of the principles making vibration control possible.

Depending on the specific application in question, a variety of objects can act to control vibration. These include dampeners, isolators, and bumpers / shock absorbers. I’d now like to offer a bit more information on each of these types of products.

DAMPENERS Vibration dampening reduces the energy of unwanted vibration in a structure or material by reducing the resonant amplitude of the vibration. A dampener dissipates vibration energy and converts it to a different form of energy, typically heat.

For mechanical systems, vibration energy is dissipated by intentionally introducing friction into the system. This friction converts the vibration kinetic energy into heat. For this to happen, it is critical for a dampener to have the right dynamic response to the vibration it is meant to reduce. The designer must therefore know the frequency and amplitude range of the vibration sources, as well as the natural frequency (or frequencies) of the whole system (including the dampener).

A dampener is generally a part or system with some viscoelastic mechanism, which will slow down the acceleration of the oscillatory motion in the vibration media. There are a number of options in this area. We at RL Hudson can design a damping system with rubber, or rubber with metal insert, or rubber with plastic insert, or foams, or foams with plastic or metal insert.

Also, a dampener can be produced in many different shapes. It can be a mount for an engine to its frame. It can be part of a foot for a heavy table or machinery. It can be a pad of rubber. It can be a metal spring with a rubber or foam tube. It can also be a closed rubber or foam structure to act as an air spring with damping. The possibilities are nearly endless.

ISOLATORS Vibration isolators are devices used to limit the amount of unwanted vibration transmitted to certain controlled areas. The function of an isolator is to decouple the unwanted vibration sources from their recipients, such as machinery supports or facility floors.

Because an isolator typically reduces vibration transmissibility by lowering the natural frequency (or frequencies) of the controlled system with respect to the excitation vibration frequencies of the sources, the key to designing an isolator is to know the vibration frequencies to be isolated and to find an isolator that will decrease the natural frequencies. For a metal spring or an air spring (which can be simplified as a linear system), it is not too difficult to find the natural frequencies of the system. But rubber or composite materials are nonlinear; their response to vibration can be related to the amplitude and frequency of the sources. Therefore, finite element analysis (FEA) or physical testing may be needed for a detailed design. The nonlinear materials, in many cases, can also function as dampeners; that is, they dissipate the vibration energy and reduce transmissibility at the same time.

Isolators can take numerous shapes; in many cases, they are similar to dampeners. An isolator can be a mount for an engine to its frame. It can be part of a foot for a heavy table or machinery. It can be a rubber pad. It can also be a metal spring.

BUMPERS / SHOCK ABSORBERS A bumper is a protective device for absorbing shocks or impeding contact. It can be made of metal, plastics, composite, rubber, or any combinations of these materials, usually attached to either end of a moving object, such as a truck or car, to reduce the forces of impact at contact.

The working mechanism of bumpers is to increase the impact time and/or to absorb impacting energy during contact to reduce impact forces. This is accomplished by using an object that can deform or collapse in a certain way under impact load. Therefore, it is critical to choose the material with the right mechanical properties and to design the right geometry for a bumper.

Bumpers can be categorized by their shape: cylindrical, hemispherical, square, conical, recessed, snap in, or stem, to name a few. Bumpers can be made of all rubber, foam,all plastic, all metal, or rubber plus metal, or rubber plus plastic. A bumper can be a doorstop with metal springs and a rubber cap, or an old tire on the side of a boat. Because of the wide range of possible bumpers, we at RL Hudson work closely with our customers to design the right bumper for each application.

DESIGN ASSISTANCE So in order to design an effectivevibration control product, it helps to know a handful of things, including the vibration source (frequency spectrum,amplitude), loading information, and the vibration or impact loading reduction requirement. As we at RL Hudson design products for vibration control, we carefully consider the frequency response characteristics of a designed system, as well as the thermal, mechanical, and viscoelastic properties of materials. For any vibration control part, material choice (be it rubber, metal, foam, or combinations) is paramount. If you have an application in need of vibration control, please do not hesitate to give us a call at 1-800-722-6766. We’ll be happy to help.