How to get trustworthy data from wireless vibration sensors

The reliability of data collected by wireless vibration sensors is directly related to the method of attachment to the monitored component.

A rigid coupling is fundamental to guarantee a true indication of the health of the asset being monitored.

Each mounting method has the ability to interfere with the quality of the results obtained.

Therefore, it is good to know the different forms of attachment and understand their importance and limitations in terms of vibration spectra.

Furthermore, it is also important to choose the correct type of vibration sensor and correctly define the installation location on the machine or component.

Wireless vibration sensors measure vibrations primarily according to the monitoring axis.

Therefore, it is important that the sensor is installed in an ideal position on the machine. Firm anchorage is also essential.

MOUNTING METHOD

Sensors can be installed in different ways. The frequency bandwidth of the sensor is especially sensitive to how it is coupled with the equipment.

The attachment method of the sensor at the point of measurement is one of the most critical factors in obtaining precise results and thus, a counterproductive attachment results in the appearance of irrelevant resonances that can severely limit the useful frequency range of the accelerometer.

Every sensor installation has dynamic characteristics such as rigidity, added mass, and damping.

As a result, this dynamic system contaminates the frequency band of the sensor in the case of inadequate attachment.

Ideally, look for maximum rigidity, focusing on increasing the resonance frequency of the installation.

There are 5 commonly used methods to install sensors and each one has a maximum operational frequency that can be indicated. Figure 1 illustrates the 5 most common attachment methods for measuring the vibration of machines.

Figure 1 – Frequency response of different types of fixation.

Of these five attachment methods, we will focus on the methods most commonly applied to monitoring the condition of machines. These are: (5) screw attachment, (4) adhesive, (2) magnet.

• SCREW MOUNTING VIBRATION SENSOR

Screw mounting is usually used for sensors that are permanently fixed. In general, this is the most efficient method because it essentially turns both parts into one.

Sometimes an adhesive glue can be combined with the screw to prevent loss of attachment due to lateral and torsional vibrations. In addition, for irregular installations with a badly supported or unbalanced sensor, the use of resins or metallic adhesives combined with the screw can help to increase the adhesion of the sensor.

The screw mounting method is not always the most practical, but it is the preferred method for vibration sensors. Because this type of attachment is considered the most reliable, it is expected to produce the best repeatability of all methods.

In these cases of attachment, the sensor (or DynaLogger from the Dynamox Solution) has an M6-1 screw put through its metal slot and joins with the hole in the surface of the measurement point. Figure 2 illustrates the standard attachment to the surface of a machine.

Figure 2 – Standard sensor installation (DynaLogger) of the Dynamox solution.

Among the main guidelines for screw fixing are:

• Use of a spring washer or locknut to guarantee the initial tightening torque and avoid loosening over time. The specified tightening torque is 11Nm or 1.12 kgfm;
• The drilled hole should have a minimum depth of 15mm to guarantee a strong bond between the sensor and the machine surface;
• When drilling new holes for screw installation, care must be taken to guarantee that the hole is drilled perpendicular to the machine surface;
• When the screw installation is made on a curved or irregular surface, the use of metallic adhesive or epoxy is recommended for the sensor to be completely rigid;

The sensor must be tightened with an Allen key as specified in the product guidelines. DO NOT use pliers or other tools to tighten the sensor more than necessary.

Avoid any pitfalls and follow the good practices illustrated in the figure below:

Figure 3 – Good and bad practices of screw installation.

• ADHESIVE ATTACHMENT

Attachment by adhesive should be done as an alternative to the standard screw installation in cases where fixing to a flat surface isn’t possible or when the surface can’t be drilled.

An example would be a thin-walled aluminum engine housing. In this case, the housing could be too thin to accommodate perforations and threading for a screw.

Magnet installation would also be unsatisfactory because it would not be attracted to aluminum.

Another example is the thin walls in bearings, where the surface can’t be perforated.

In addition to these examples, when there is a gap or little area of contact, installation with adhesive might be more appropriate.

A lot of care must be taken to prepare the surface when using an adhesive in order to guarantee a permanent bond, as a defective joint can lead to significant measurement errors.

Alternatively, for portable assemblies, it is possible to mount a support bracket to the machine’s surface.

The type of adhesive must be appropriate for the materials and the environment in which it will be applied. The adhesive must also provide a rigid foundation.

A “soft” adhesive will cause higher frequencies to be absorbed due to its high degree of damping.

When fixing a sensor from the Dynamox Solution, it is recommended to use a bicomponent epoxy paste or metallic glue, with a quantity of approximately 2 cm³ according to the manufacturer’s instructions.

The epoxy paste should be distributed on the outer surface of the DynaLogger, as shown in Figure 4, maintaining a gap of approximately 2 mm from the metal slot in order not to interfere with the temperature measurement.

Press the DynaLogger at the measurement point, orienting the axes as appropriate (shown on the product label). Finally, press any leftover epoxy paste against the walls of the DynaLogger to get the best fixation.

Figure 4 – Application of epoxy adhesive. Note that an excess of epoxy, with a thick layer between the sensor and machine surface, results in loss of rigidity and increased damping at high frequency and should, therefore, be used with caution.

• MAGNETIC MOUNTING VIBRATION SENSORS

Magnetic base attachment is normally used in sensor installation when easy removal is required. In general, when mounted perpendicularly to the surface of the machine, magnetic-based sensors can resist a force of 20-25kgf (196-245N).

Support bases, or pads, exist of all shapes and sizes for magnetic sensors, including those that can adapt to curved surfaces. These pads help to increase repeatability. In any case, there is a high probability of sensor displacement, which affects the reading on the three axes (vertical, horizontal and axial).

Note that the surfaces of adhesion need to be truly magnetic (steel alloys with iron, nickel, cobalt etc.). These surfaces should also be free from irregularities and foreign objects that result in the inclination of the sensor and, therefore, can modify its orientation and the rigidity of the attachment, impairing the measurement.

These recommendations, as well as the illustrations below, should be followed In order to avoid the loss of magnetism and obtain the best results:

Figure 5 – Common situations that lead to the loss of adhesion of the magnetic base: mounting on surfaces with debris or non-magnetic surfaces, mounting on very hot surfaces and dropping onto other surfaces (causing loss of magnetism).

MOUNTING LOCATION

The selection of the location to mount the sensor is important because locations that flex can influence the measurement of the sensor. Flexible attachment to motor fins or guards generally do not communicate reliably with the main sources of vibration.

Note that the installation location doesn’t depend on the type of adhesion. If the sensor is attached to a flexible surface, none of the types of installation informed will be effective.

The vibration sensors should never be used on light structures (with little robustness) because the weight of the sensor can distort the vibrational behavior of the structure. In general, the combined weight of the sensor (with the screw, magnetic base, etc.) should be less than 10% of the weight of the vibrating structure.

To properly measure the source of vibration on a machine, the sensor should always be fixed as close as possible to the most rigid parts of the machine, such as bearings and housings. This is due to:

1. The vibration levels are lower and less noisy, therefore avoiding the collected signal being overloaded and masking vibration components;
2. All of the dynamic forces of the machine are transmitted directly to the supports, so all sources of vibration will be easier to detect. More specifically, it should be fixed as close as possible to the source of vibration of interest in order to avoid capturing distorted signals from origins other than those that really matter.

Examples of inappropriate choices of mounting location and the influence on the frequency response of the accelerometer are shown in Figure 6.

Figure 6 – Typical frequency response of flexible (guards and thin plates) or rigid (bearings and structures) locations.

THE DYNAMOX SOLUTION

To get the best results with the Dynamox Solution, it is fundamental to fix the DynaLoggers correctly at the measurement points.

By following all of the recommendations outlined in this text for each type of attachment and choosing the best monitoring location, all of the vibration sources can be easily detected in three directions with the Dynamox Solution sensors.

In addition, our DynaLogger has a temperature sensor, which can be used in correlation with the increases in vibration sensors. All data is collected wirelessly, being transmitted via Bluetooth to the Mobile App or Gateway and then to the Web Platform, thus enabling the analysis of historic data and defining necessary actions.