Guidelines for Mounting Test Accelerometers

Accuracy of frequency response is one of the most important factors when deciding which mounting technique to use. The accelerometer operating frequency range is determined by a back- to-back calibration method where the accelerometer under test is securely mounted to a reference standard accelerometer. The sensor under test should be stud mounted whenever possible for highest accuracy. Directly stud mounting an accelerometer to a very smooth surface yields the highest mounted resonant frequency allowing the broadest usable frequency range. The addition of any mass to the accelerometer (such as an adhesive or magnetic mounting base) lowers the resonant frequency of the sensing system and may affect the accuracy and limits of the accelerometer usable frequency range. Rubber mounting pads will also affect the sensor frequency response. Any soft material will create a filtering effect that will dampen high frequency transmissibility.

Figure 1: Approximate frequency ranges of mounting techniques

For best measurement results (especially for high frequency measurements) prepare a smooth and flat machined surface for attaching the accelerometer. Inspect the area to ensure that metal burrs and other foreign particles do not interfere with the contacting surfaces. To achieve a higher degree of surface contact, apply a thin layer of silicone grease between the accelerometer base and the mounting surface. This provides the best high-frequency transmissibility.

Stud mounting is recommended if the accelerometer has an integral stud or if it has a tapped hole and is supplied with a removable stud. This mounting method provides a very secure attachment for the accelerometer to the test structure and matches the mounting configuration during factory calibration. Prepare the surface by grinding or machining a smooth, flat area on the test object at least the size of the sensor base, or the size according to the manufacturer's specifications. Then, prepare a tapped hole in accordance with the supplied installation drawing. Make sure that the hole is perpendicular to the mounting surface. Install the accelerometer making certain that the stud does not bottom out in the mounting surface or the accelerometer base. Most PCB® mounting studs have depth-limiting shoulders to ensure that the stud cannot bottom-out into the accelerometer base. Each base incorporates a counterbore so that the accelerometer does not rest on the shoulder. Acceleration is transmitted from the structure surface into the accelerometer base. Any stud bottoming or interference between the accelerometer base and the structure inhibits acceleration transmission and affects measurement accuracy. When tightening, do not apply more than the recommended torque to the accelerometer. A thread-locking compound may be applied to the threads of the mounting stud to safeguard against loosening. Figure 2 is an example of a stud mounted accelerometer.

Figure 1: Approximate frequency ranges of mounting techniques

Use an insulated cap screw to securely mount a ring-style accelerometer. Insert the cap screw into a through-hole in the accelerometer housing and screw it into a tapped hole on the mounting surface (see Figure 3A). The tapped hole should be deep enough so that the screw does not bottom out in the mounting structure. PCB® supplies a cap screw with all ring-style accelerometers.

A cap screw can also be used to mount an accelerometer with a tapped hole onto a thin structure (see Figure 3B). Drill a hole of sufficient diameter through the structure allowing the cap screw to pass through and into the tapped hole. The screw engagement length should always be checked to ensure that the screw does not bottom out into the accelerometer base.

A thin layer of silicone grease at the mounting interface of either configuration ensures the best high-frequency transmissibility.

Figures: 3A and 3B. Screw mounted accelerometers

Adhesive bases provide an alternative mounting method and may be used when mounting by stud or screw is not practical. Using separate adhesive mounting bases is recommended to prevent the adhesive from damaging the accelerometer base or clogging the mounting threads. Most adhesive mounting bases available from PCB® also provide electrical isolation eliminating potential noise pick-up and ground loop problems. The type of adhesive recommended depends on the application. Adhesives available from PCB® include Petro wax for room temperature use and Loctite® 454 quick bonding gel for higher temperature applications up to 248 °F (120 °C). Two-part epoxies offer stiffness, which maintains high-frequency response and a permanent mount. Depending on the application, adhesives such as dental cement, hot glues, instant glues, and duct putty can be viable options.

One adhesive isn't optimal for all applications due to the variety of mounting structures. The application environment should also be considered when choosing an adhesive. Figure 4 shows an example of an accelerometer mounted to a test structure with an adhesive mounting base.

Figure 4: Adhesively mounted accelerometer

To avoid damaging a sensor during the removal process, a debonding agent must be applied to the adhesive. There is no universal debonding agent available due to the variety of adhesives (super glues, dental cement, epoxies, etc.). The debonder for Loctite® 454 adhesive that PCB® recommends is acetone. Allow the debonding agent time (a few minutes) to penetrate the surface to properly react with the adhesive. Wait a few minutes before attempting to remove the sensor. If an adhesive other than Loctite® 454 is used, check with the sensor manufacturer for debonding recommendations.

After the debonding agent has set, an ordinary open-end wrench can be used for removal (for accelerometers with a hex base or square base). The supplied removal tool should be used for teardrop accelerometers (Figure 5). After attaching the wrench or tool, use a gentle shear (or twisting) motion (by hand only) to remove the sensor from the test structure.

Figure 5: Teardrop accelerometer model 352C22 and supplied removal tool model 039A27

Magnetic mounting bases offer a convenient and temporary method for attaching to magnetic surfaces. Magnets with high pull strengths provide the best high-frequency response. Wedged, dual rail magnetic bases are typically used for installations on curved surfaces such as motor and compressor housings and pipes. However, dual-rail magnets can significantly decrease the operational frequency range of an accelerometer. The magnetic base should be attached to a smooth, flat surface for best results. Apply a thin layer of silicone grease between the sensor and magnetic base, as well as between the magnetic base and the structure. Steel pads can be welded or epoxied in place to accept the magnetic base when surfaces are uneven or non-magnetic. Figures 6 through 8 illustrate magnetic bases and a magnetic mounted accelerometer.

Caution: Magnetically mounting an accelerometer has the potential to generate very high and damaging acceleration levels. To prevent such damage, do not allow the magnet to snap on to the test structure. Gently rock or slide the assembly in place. A best practice is to attach the magnetic base to your test structure first, and then screw the accelerometer onto the magnetic base.

Figure 6: Dual rail magnet base Model 080A132 Figure 7: Flat magnet base Model 080A27

Figure 8: Magnet mounted accelerometer

PCB's Easy-mount Accelerometer Clips offer practical and economical installation techniques for accelerometers in multi-channel vibration measurement applications.

The clips can be attached to a test structure via double sided tape or adhesive.

Once the clips are installed, accelerometers are simply snapped into the clips and are ready to take vibration measurements. Anytime a sensor is mounted with a mechanical connection less ridged than a stud, screw or bolt, the effect of the softer connection reduces the upper frequency response of the installation. Typically, the test engineer will have reduction in frequency response down to between 1kHz to 3.5kHz, depending on the clip and whether grease is used. The test engineer should confirm that the resulting frequency response is within the acceptable range of required data. After determining that the resulting frequency response is within bounds, the test engineer is ready to proceed.

More measurement points and orientations can be accommodated with fewer sensors by installing clips at all desired points and populating them with as many sensors as necessary. Sensors are then moved to remaining clip locations until all measurements are completed. Triaxial measurements can be made with single axis, cube-shaped accelerometers by changing axis orientation for successive measurements.

Typical engineering tests may define three axis of measurement in respect to the earth's surface. But, in many cases, the defined sensor locations on the test article may not be oriented correctly. Thus PCB® has designed a sensor clip device to assist in the sensor orientation per the test requirement. Swivel-style clips permit sensors installed on curved or sloped surfaces to be aligned along the desired plane and axis. These clips rotate and pivot to provide full flexibility in alignment.