The Basics of Vibration and Shock

Did you shake a wrapped gift as a child?

Sometimes the rattling sound may indicate the hidden contents. A sweater from Grandma would make a muffled sound, while a new Lego set would rustle the many parts.

Your customers may not shake the package to guess its contents, yet your product will experience vibration and shock.

The environments of assembly, transpiration, use, maintenance, etc all include shock and vibration inputs to your product. The operation of your product may include producing vibration and shock loading across the product.

Understanding the basics of vibration and shock will assist you to design your product to operate as expected in the noisy, shaky world.

The rattling about your product experiences is just one of the many environmental and use factors.

What are vibration and shock?

Vibration is the mechanical phenomenon of oscillations which occur about an equilibrium point.

The oscillations may be periodic or random.

Vibration may be part of the function of the device, say a violin string. Or, it may be an indication of imperfect alignment or operation, such as with a motor bearing.

Shaker tables may provide a periodic, often sinusoidal, vibration.

A sinusoidal sweep may reveal a resonant frequency, which means the vibration induced displacements are maximized. It is a good design practice to ensure the resonant frequencies are well above any vibration loads the product is likely to experience.

Random vibration, often induced in HALT chambers, excites all frequencies simultaneously. Both sinusoidal and random vibration occur in your product’s environment.

Shock is an impulse applied to a system. It is a sudden acceleration.

A drop, kick, slam, or explosion are examples of shocks. The impulse is short, thus the change in velocity, the acceleration, may be quite large.

To describe a shock pulse use the peak acceleration, the duration, and the shape of the pulse (half sine, triangular, etc.). The unit g represents multiples of the acceleration of gravity and is the vector.

A shock load is one way to initiate vibration within a system.

There are three types of vibration:

Free vibration occurs when the system is set in motion, like striking a drum, and then allowed to vibrate freely.

The shape and material of the drum will determine the sound created. The different sound is caused by the different drums having different natural frequencies.

The struck surface or structure of your product will tend to vibrate at its natural frequency.

 

Forced vibration is a time-varying load, displacement, or velocity imparted on the system.

The disturbance may be periodic, steady-state, transient or random in nature. An example of forced vibration may be the motion of your phone across the table (and onto the floor) when the vibrator feature alerts you of a phone call.

A poorly tuned vehicle motor may cause a shaking of the entire vehicle when idling (roughly).

 

Damped vibration occurs when the energy of a vibrating system is dissipated by friction or some form of resistance.

A vibration system without damping may oscillate for an extended duration, whereas a damped system may quickly return to its equilibrium position.

 

Causes and damage of vibration and shock

Nearly anything that moves involves overcoming variable friction loads, or uneven surfaces, creates vibration within a system.

The frequency and magnitude of shocks induce in a vehicle traversing a rough road is felt through the vibration imparted to your body via the vehicle’s structure.

A smoother road results in fewer shocks and a smoother ride, i.e. less vibration.

Even standing still though, a vehicle’s idling motor may induce vibration due to poor alignment of moving parts, unbalance elements of the motor, poor bearings, etc.

A moving vehicle, on a smooth road and with the motor off (say coasting down a hill) will pick up loads from the air.

The wind will shake the vehicle, thus inducing vibration. Air moving through a heating duct creates sound, due to the vibration caused by the loading.

Vibration may also occur due to hydraulic or pneumatic pressure fluctuations.

Some product function with vibration- the violin for example.

Outside musical instruments and a few other products, the shock and vibration loading may induce damage to the system.

The types of damage may include:

• Fatigue damage
• Mechanical hardening
• Mechanical wear
• Fastener loosing
• Hydraulic or pneumatic leaks
• Connector separation
• Unwanted noise

The analysis of a system’s vibration may indicate the source of unwanted vibration.

For example, if a peak amplitude occurs at 60 hertz the cause may be an electric motor using 60 Hz power.

Designing, assembling and operating a quiet and smooth (low vibration) system is often one of the goals for machinery.

Your equipment will experience vibration inputs from its environment, thus designing a system to minimize the effects of that input, plus minimizing internally generated vibration.

It only reduces the unwanted noise created by the system it also tends to prolong the operating life of the equipment.

Further reading:

Piersol, A. G. and T. L. Paez. 2009. Harris’ Shock and Vibration Handbook. 6th ed. New York: McGraw-Hill.

Steinberg, Dave S. 2000. Vibration Analysis for Electronic Equipment. New York: John Wiley & Sons.

O’Connor, Patrick D. T. and Andre Kleyner. 2012. Practical Reliability Engineering. Chicester: John Wiley and Sons. Section 8.7.

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Related:

Failure Modes and Mechanisms (article)