Impact of different accelerometers mounting configuration on vibration measurements

09-08-2019


Modern-day accelerometers are designed in different ways in order to suit each and every vibration measurement requirement in wide range of industries. Choosing suitable accelerometers for your applications may depend on different factors such as measuring range, operating temperature, sensitivity level, environment, mounting configuration, etc. Understanding the implications of different accelerometer mounting techniques also holds an utmost importance in achieving optimum results. Using an incorrect mounting technique for any applications may lead to erroneous readings.

Factors to be considered when determining accelerometers mounting configuration

Deciding the suitable mounting configuration for an accelerometer highly depends on the type of accelerometers, the required accuracy of frequency response and the type of surface to be mounted.

The accuracy of frequency response of accelerometers is directly affected by the accelerometers mounting configuration. Stud mounted accelerometer generally allow the widest range of usable frequency ranges for general applications. This is generally the case for IEPE and piezoelectric accelerometer. Adhesive mounting or magnetic mounting techniques adds additional mass to the accelerometer which lowers the resonance frequency of the sensing system. Therefore, using this these mounting configurations may limit the usage and accuracy of the frequency response of the accelerometers. Soft materials, such as that in rubber mounting pads, also affect the frequency response by generating a filtering effect which will dampen the transmissibility of high frequency data.

Accelerometer yields the best results when it is mounted on a smooth and flat-machined surface. This is generally the case when accelerometers are used for measuring high frequency vibration, generally piezoelectric accelerometers. The transmissibility of high frequency measurements can be improved by achieving a higher degree of contact through applying a thin layer of silicon grease between the accelerometer base and the mounting surface.

What are the types of Accelerometer Mountings?

Stud Mounting

In this configuration, the accelerometer and the test surface are fused together by the clamping force of the stud. This mounting configuration makes the two bodies attain the exact duplicate motion at all frequencies, which gives a very close proximity of results. When an accelerometer is stud-mounted, it tends to maximise the range of the usable frequency response of the accelerometer. It is advised to use coupling fluid such as oil, grease, petroleum jelly, etc in the stud mounting process, in order to prevent inadequate mounting torque, surface flatness and surface roughness.

Screw Mounting

Screw mounting is similar to stud mounting. This mounting configuration is ideal for accelerometers manufactured with holes or flanges that is specially dedicated for screw/stud mounting style. An insulated cap screw is used to securely mount the ring-styles accelerometer. It should also be ensured that the screw does not bottom out into the accelerometer base. To achieve a better results and higher contact, a silicone grease layer can be applied between the accelerometer and the mounting interface.

Adhesive mounting

For situations such as mounting the accelerometer on a thin sheet of metal where drilling for stud mounting is not possible, adhesive mounting is considered. The factor that influences the performance of the accelerometer is the thickness of the adhesive used. Some accelerometers can be directly mounted to the test surface whereas some require mounting adapters or accelerometer mounting bases (that are adhesive in nature) for configuring the adhesive mounting setup. The application environment should be carefully examined before selecting an adhesive.
Cyanoacrylate adhesive is recommended to be used as bonding substance as it can instantly set. This material does not form a thick glue line between the test surface and the accelerometer which yields a strong bond that helps in retaining the actual readings. It is to be noted that excessive use of adhesives is not advised as it can cause damping effect to the vibration transmission, particularly at the high frequency range.
Following are the types of commonly used adhesives:

Loctite or Epoxy: This one is of the permanent mounting solutions which is ideal for repeated use. Using debonding agents like acetone is recommended with this configuration.
Wax or Duct Seal Putty: This configuration is not as effective as Loctite or Epoxy adhesives, but it yields a good frequency response. Removing this configuration is very much easy. This configuration is recommended for use in shock and vibration testing environments as it can mechanically filter out high frequency vibration that ends up affecting the internal resonance of the accelerometer.
Adhesive Mounting Pads: This configuration gives a very high frequency response and is ideal to be used with large structures. The mounting pads ensure that the tests can be repeatable and prevent epoxy from damaging the expensive accelerometers. Removing them is quite easy too.

Magnetic mounting

In magnetic mounting configuration, the accelerometer is stud mounted to the ferromagnetic surfaces or magnetic adapters via stud mount and is coupled to the test surface. The magnetic adaptors provide reliable measurement for frequency response under 1 kHz. However, magnetic mounting should be limited to clean, flat and smooth surfaces as contaminants and dirt may heavily affect the results. Magnets with high-pull strength can be considered to obtain the best frequency range. Users may optionally apply a thin layer of silicone grease between the sensor and the magnetic base as well as between the magnetic base and the structure to improve the contact area for better results. The Magnetic mounting is ideal for short term testing application, but should be used with caution so as to avoid damage to the structure or any injuries.