Vibration Analysis Basics – Sensor Mounting

Selecting the right sensor mounting methodology

As seen in our previous article, the settings of vibration measurement are crucial if you want to achieve the right detection and diagnostic performances. But, this is not the only thing to consider in order to achieve the desired objectives. For instance, the sensor mounting method to collect vibration data can greatly impact the quality of the time wave form. As a rule of thumb, the closer you put your sensor to the machine, the better it senses the vibrations. This means that the type of mounting accessory used affects the measurement quality, depending on its type and size, and how well vibrations can be propagated through it.

As we know, high frequencies are very low in amplitude (and energy) and are the first type of content to be affected by the mounting method, as there may be loss of energy during transmission to the vibration sensor. 

There are several mounting methods used today, each with pros and cons, so you need to make sure that the mounting technique you are using is in line with your strategy. For example, a probe tip may make it possible to reach a small or narrow area  ̶  to obtain a measurement that nothing else can  ̶  but is not be suitable to monitor high frequency problems, such as bearings, cavitation and gear problems. On the other hand, the best frequency response is achieved through a stud, meaning the machine must be properly drilled, which is often only the case for permanent monitoring applications.

Please see below the illustration of the different options and their frequency response.

At the end of the day, it is often a compromise between how early you want to detect problems  ̶ depending on the machine’s use and failure modes  ̶  and the solution’s “user-friendliness”.

For example, in order to capture high frequencies above 15 KHz, you may need to use an adhesive mounting pad or directly stud-mount the sensor. Practically, you will notice that all high-power rating gearboxes use antifriction bearings and rotate at slow speed. Also, monitoring this type of gearbox is a challenge. A Roller Press or a VRM gearbox in a cement mill, as well as a rolling mill gearbox in a steel plant, are examples of this.

Measurement repeatability is key

Beside the quality of the measurement in terms of frequency content, the mounting accessory can also affect the trending and diagnostic reliability. One of the common issues when running a portable vibration program (where data are collected manually on a periodic basis), is ensuring that the sensors are placed in the same location from time to time, so that measurements can be compared by the vibration analyst. Although magnets are often preferred by users, because of their ease of use, they run some measurement repeatability risks. Hopefully, different techniques or tips can be implemented when using magnet-based accessories, such as marking the machine with a permanent marker or taking pictures of the sensor position on the machine; the OneProd FALCON vibration data collector allows you to both take photos and display them. As a matter of fact, despite the additional time and effort needed, installing cementing studs remains the safest choice to avoid any measurement errors, and guarantees the best possible results for the analyst in charge. Note that to keep the data collection time as fast as possible when using a magnet base, you can install cementing studs compatible with the use of magnets on the machines. Just bear in mind that the metrological performances achieved are impacted by the magnet, as explained in the first part of this article.

In conclusion, keep in mind that the condition monitoring leader has to establish a methodology for monitoring to ensure that:

  • The measurement location (place where the sensor is installed) is a good position, where you have good vibration transmission;
  • Data are being collected at the same location every time, despite potential staff turnover;
  • The frequency content captured matches with both the failure modes you are trying to detect and how early you want to get warned, according to your application.

As illustrated, some types of machines may not leave any room for compromise.

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