Sensor mounting methods

An overview of sensor mounting

Choosing a mounting method, and implementing it correctly, is an important factor in vibration data collection. The goal is to achieve the best possible frequency response from the mounted sensor while minimizing intrusion on the machine and working within any limitations of the application, such as guards or process requirements. There are four basic mounting methods: stud mounting, adhesives, magnets, and probe tips.

Illustrated comparison of sensor mounting methods

 
 
It’s important to consider the frequency range of the application and sensor resonance. The sensor’s resonant frequency is controlled by the internal preloading mass, and this frequency should be at least twice the expected application range. The frequency range of the mounted sensor will be controlled by the stiffness of the measurement technique chosen.
 

Relative sensitivity vs. frequency response for 6 mounting methods

 
Let's examine these methods in more detail and go over best implementation practices.
 

STUD MOUNTING

The best method for reliable data collection is threaded stud mounting, which most closely duplicates the ideal condition of sensor calibration. It allows for the widest dynamic measurement range. Stud mounting is recommended for permanent installations where high frequency (>10 kHz) vibrations are expected, and in harsh environments where handheld data collection isn’t practical. Stud mounting requires drill and tapping a small hole in the machine.

When stud mounting a sensor, be sure to spot face and clean the machine surface before tapping. The hole depth should not exceed the thickness of the mounting surface to prevent damage to the machinery. Check for perpendicularity of the hole to ensure a good surface interface with the bottom of the sensor. It’s also important to apply the appropriate mounting torque. For a typical 1/4-28 stud, 30 in-lbs is generally about right; certain applications can require lower or higher torques depending on severity. Using a coupling fluid such as silicon grease, machine oil or petroleum jelly will increase the mounting stiffness and enhance frequency response.

ADHESIVE MOUNTING

If the machine can’t be drilled for stud mounting, adhesive mounting is generally the best alternative. Cementing pads, when used correctly, approach the high frequency characteristics of stud mounting. They work well for permanent installations where drilling is not an option and for some temporary applications, such as machine investigations.

Adhesive selection is critical for long-term reliability – some common choices are VersiLock 406, Loctite Depend or Liquid Metal, or other ‘hard’ glue (cyanoacrylate). Some adhesives help to electrically isolate the sensor from the mounting surface. Before applying any adhesive, all paint and debris should be removed from the surface of the machine and the surface should be ground reasonably flat. Sensor response depends on how well the surface is prepared.

MAGNETS

Magnet mounts are usually limited to temporary applications like route-based data collection. Magnets used on walkaround data collection routes won’t damage the machine or the sensor. However, due to the poorer contact between sensor and machine, magnetic mounting results in a significantly reduced frequency range. The additional mass of the magnet will result in a new, lower resonant frequency and change in the frequency range. This should be considered when analyzing measurement results.

On flat surfaces or magnet pads, use flat magnets; on irregular or curved surfaces, 2-pole magnets should be used. Flat magnets provide better contact than 2-pole magnets – the former can have a mounted resonance of 5 to 10 kHz, versus 3 to 7 kHz for the latter. With any magnet mount, pay attention to your measurement results, questioning any large spikes in the spectral data.

PROBE TIPS

Handheld probe tips are used on difficult-to-reach areas, aluminum motor frames, or for rapid trial measurement points. Probe tips present the biggest challenge in data collection. They should not be used for measurements less than 10 Hz, as machine displacement makes holding the sensor in close contact with the machine difficult. Vibration data collected with probe tips is the least reliable. Spectral data is often distorted. Additionally, the length of the probe tip will affect the measurement, with longer probes introducing more inaccuracies. Frequency response is very limited compared to other methods.

Tl;dr for those of you who got bored and scrolled to the bottom of the post

Ultimately, the best choice will depend on the application, the dynamic measurement requirements, cost, and operational constraints. Remember these key points:

• In permanent monitoring systems, use stud mounting or cementing pads. These methods provide long-term reliability and the best frequency response of the sensor.
• Good surface preparation is the key to successful stud or adhesive mounting. Following best installation practices is crucial to obtaining reliable vibration data.
• Walkaround monitoring programs can maximize the effectiveness of magnetic mounts by ensuring adequate contact between the sensor and machine and examining data for any anomalies.
• Avoid using probe tips where possible and verify your measurements.

For more detailed information and guidance on sensor mounting, check out the resources below. We’re here to help with application-specific questions – email our technical services team or contact us via our general inquiry form.