When it comes to analyzing vibration, sensor placement is key to quality of data capture. Motors can be large, surrounded by equipment and difficult to access, so locating an area to place your vibration sensor can be harder than it looks. A poorly placed sensor can, at best, reduce the vibration getting to the sensing element, and at worst, give completely invalid results, wasting time or even missing potential failure frequencies.
Placing a vibration sensor as close to the bearing as possible is the ideal scenario, but anywhere on the solid chassis that houses the bearing is normally sufficient. This is because the high frequencies that a bearing generates during a fault dissipate quickly in metals, reducing the amplitude of the vibration signal. These signals dissipate even more so when crossing an interface boundary, such as a bearing housing to another substructure. As a rule of thumb, try to mount a sensor within 1 foot (25cm) of the bearing to get the best results.
The absolute worst place for a sensor is on partially connected machine component, like the fan housing on a motor, or the connection box. Another location that is common for sensor placement, but is generally a bad idea, is on the motor cooling fins. As these protrude unsupported from the motor, they can resonate at other frequencies, giving further false results.
The interface between the sensor and the mounting surface plays another role in data quality. Magnetic mounts work well for fast mounting but start to attenuate signals above ~5kHz. When using a glue make sure to use a metal-based epoxy that is suitable for vibration signals, be aware of softer epoxy glues with flexible content, as they can drastically impact the signal. Mounting with a stud is always going to provide the best results and will work well, even above 10kHz Fmax.
Acquiring vibration signals should always be completed by an ISO Cat-I vibration analyst or above, as they are trained to do this. When mounting a sensor for continuous monitoring always seek the advice of an expert if unsure.
About the Author
David ProcterSystems Development Manager, Sensoteq
David Procter is an experienced Systems Engineer with a history of working in the automotive, security and industrial markets. His passion is understanding exactly what the customer needs and developing products that solve real problems and create value for those who use them.