In the world of vibration analysis frequency is everything. Frequencies are generated by many sources within a machine.
The running speed of the machine is one example of frequency; this is the velocity of the rotating element which in many cases is a drive shaft connected to an application (like a fan or a pump). Each frequency will have a magnitude associated with it; this is the total amount of energy of that specific frequency. The amount of energy of any given frequency can tell us a lot about what is going on within a machine. When we use a vibration sensor to measure a machine, this is what we are detecting, frequencies and magnitudes of those frequencies.
The maximum frequency that a sensor can measure is called the Fmax. This value lets the user know what type of faults they can detect on a machine. It is typically listed in hertz (Hz). The higher the Fmax value, the greater the number of faults that a sensor will detect, but it will also allow for earlier indication of potential faults, like a bearing failure. Some ISO standards will reference an Fmax of 1kHz – whilst taking a reading up to 1kHz is suitable for most acceptance testing, it will not highlight even the most basic of bearing faults. A minimum of 2.5kHz Fmax is recommended for good coverage.
Recently, a new sensor has entered the market that boasts an Fmax up to 10kHz. This Fmax gives great coverage for a wide variety of faults, and will accurately inform users of issues on complex equipment like gearboxes that have many higher fault frequencies. In addition, a higher frequency will give an earlier indication of bearing faults. Bearing failures will typically start within the subsurface of the metal with a very small amount of energy being emitted to the sensing element, thus having a sensitive measurement device with the right Fmax is critical to understanding the health of your machine early, before failure impacts any processes or causes downtime.
By David Procter