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How to mount accelerometers for reliable vibration measurements

09-08-2019

Modern-day accelerometers are designed in different ways to suit every vibration measurement requirement in a 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 the utmost importance in achieving optimum results. Using an incorrect mounting technique for any applications may lead to erroneous readings.

Why is it crucial to understand the correct mounting configuration?

Deciding the suitable mounting configuration for an accelerometer depends on the type of accelerometer, the required frequency response, measurement accuracy and the type of surface on where the accelerometers are mounted.

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

The accelerometer yields the best results when 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 by applying a thin layer of silicon grease between the accelerometer base and the mounting surface.

Accelerometer mounting techniques

Stud Mounting

In this configuration, the accelerometer and the test surface are fused by the clamping force of the stud. This mounting configuration makes the two bodies attain the exact duplicate motion at all frequencies, which gives 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 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 specially dedicated for screw/stud mounting style. An insulated cap screw is used to securely mount the ring-style accelerometer. It should also be ensured that the screw does not bottom out into the accelerometer base. A silicone grease layer can be applied between the accelerometer and the mounting interface to achieve better results and higher contact.

Adhesive mounting

For situations such as mounting the accelerometer on a thin sheet of metal where drilling for stud mounting is impossible, 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 (adhesive in nature) for configuring the adhesive mounting setup. The application environment should be carefully examined before selecting an adhesive.
Cyanoacrylate adhesive is recommended as a 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 a damping effect to the vibration transmission, particularly in the high-frequency range.
Following are the types of commonly used adhesives:

Loctite or Epoxy: This adhesive is suitable for repeated use and ideal for permanent mounting solutions. Using debonding agents like acetone is recommended with this configuration.
Wax or Duct Seal Putty: Wax adhesive is not as effective as Loctite or Epoxy adhesives, but it yields a good frequency response. Removing this configuration is very much easy. This adhesive is recommended for use in shock and vibration testing environments as it can mechanically filter out high-frequency vibration that affects the internal resonance of the accelerometer.
Adhesive Mounting Pads: The mounting pads yield a high-frequency response and are ideally used for testing of large structures. The mounting pads ensure repeatable results and prevent the epoxy from damaging the expensive accelerometers. They are also easy to install and remove.

Magnetic mounting

In the 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 and between the magnetic base and the structure to improve the contact area for better results. The Magnetic mounting is ideal for short-term testing applications but should be used with caution to avoid damage to the structure or any injuries.

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