There are many different types of mechanical devices in use today that rotate, oscillate, spin, bounce, swing, lift, drill, and dig. Some have electric motors or run on gasoline, roll on wheels, have gears, wash clothes, cut wood, forge steel, and stamp out coins. These all involve some kind of motion which often generates some type of vibration. There are good vibrations and there are bad vibrations. Good vibrations are used in conveyor belts that vibrate back and forth to sort out different sizes in gravel and fruit. Good vibrations would also be adding vibrators to the instrument panels for the first jet aircraft. Engines were so smooth on these new jets that instruments from the old propeller engines would stick and not work properly. The propellers from the old airplanes generated much more vibration so the same instruments worked very well on the old airplanes. Instruments for the new jets had to be completely redesigned to operate with minimum vibrations. Bad vibrations are vibrations that cause failures or malfunctions in equipment exposed to the vibration. Bad vibrations can often be traced back to a structural member that is not stiff enough, so it has a low natural frequency. This generates a higher dynamic displacement that increases the stress levels and reduces the effective fatigue life. Vibration failures in electronic equipment can be very difficult to trace because there are so many different factors to consider. The first item that is examined is usually the failed part, its location in the structure, how is it supported, whether it is a purchased part or a manufactured part. The size and weight of the part, the part material, plastic, metal, ceramic, vibration input level, vibration response level in area of failure, direction of vibration input, duration of vibration, type of sensors used in the failure area, type of test fixture used to hold the test specimen, any history of previous tests, any previous analyses with calculations by hand or computer. Do not overlook the possibility of sabotage by an unhappy employee.
Sinusoidal vibrations are often used for testing various types of electronic equipment because this is often the actual operating environment for many types of systems. This applies to aircraft with piston engines and propellers and also to ships and submarines with screw propellers. Sine vibration tests are not as expensive as many other types of tests and they are easy to set up and run. Sinusoidal vibrations are also used because it can generate a very clear picture of how various types of structures move when their natural frequencies are excited by watching their movements with an oscilloscope. Many previous structural failures could