Four main methods are used for attaching sensors to monitoring locations in predictive maintenance. They are stud mounted, adhesive mounted, magnetically mounted and the use of probe tips, or stingers. Each method effects the high frequency response of the accelerometer. Stud mounting provides the widest frequency response and the most secure and reliable attachment. Figure 5 shows ideal surface preparation for stud mounting sensors.
When choosing a mounting method, both the advantages and disadvantages of each technique should be closely considered. Characteristics such as location, ruggedness, amplitude range, accessibility, temperature and portability may be extremely critical. However, often times the most important and overlooked consideration is the affect the mounting technique will have on the high frequency operating range of the accelerometer.
Shown below are six possible mounting techniques and their affect on the response of a typical piezoelectric accelerometer. (Note that not all of the mounting methods may apply to your particular sensor.) By examining the mounting configurations and corresponding graph, it can be seen that the high frequency response of the accelerometer may be compromised as mass is added to the system and/or the mounting stiffness is reduced.
Note: The low frequency response is unaffected by the mounting technique. This roll-off behavior is typically fixed by the built-in sensor electronics. However, when operating AC coupled signal conditioners with readout devices that have an input impedance of less than 1 megohm, the low frequency range may be affected.
One last point worth noting regarding mounting involves surface preparation. In addition to surfaces being as flat as possible, clean and free of debris, and the mounting holes to be perpendicular, the mounting surfaces should be lightly coated with a lubricant. This coating aids in the transmissibility of the higher frequency vibrations and improves high frequency response of sensors. Silicone vacuum grease, heavy machine oil, or bees wax are commonly used.