Impact Hammers.

Impact hammers are used to determine resonant frequencies; resistance to instability and structural damage in machines and other test objects. They are typically used in the Aerospace; Engineering; Civil and Defence Industries.

Impact hammers contain a force transducer in the hammers’ head which enables it to measure the amount of force applied when the hammer strikes the test object [which is usually measured in terms of N or other comparable units]. The force transducer consists of a piezoelectric crystal, when force is applied to the crystal an electrical charge is produced. The amount of the charge is proportional to the amount force applied. This charge is then converted to voltage by a charge amplifier. The impact hammer along with a charge amplifier is usually connected to a data acquisition system or other display and recording systems for the further analysis.

Another important specification is the sensitivity, or the upper frequency for which responses can be tested, this can be adjusted using a variety of tips. Impact hammers provided by Global Sensor Technology are supplied with four tips comprising steel, aluminium, nylon and rubber. The steel tip can produce a force of a high frequency while the rubber tip produces a force at a lower frequency range. The other two tips produce forces between those of the rubber & steel tips.

For successful implementation it is necessary that each strike of the test object be of similar force. Useful measurements are gained by attaching one or more accelerometers to the test object and striking it at multiple locations.

Testing using an impact hammer is suited to small light-weight objects, when the size of an object increases problems can arise as a result of poor signal to noise ratio. This can be overcome with a lager hammer. However, too large a force will cause nonlinear vibrations and overload of the test instrument. If the force applied is too small, some of the vibration modes may not be excited fully. It’s recommended that a number of measurements be made increasing the force to observe changes in vibration and determine an appropriate force amplitude.