Accelerometer Installation Guide
Last Updated: 06/24/2017
Table of Contents:
There are three mounting methods typically used for vibration monitoring applications; stud mounting, adhesive bonding and magnetic mounting. Stud mounting is the preferred method for permanent mounting applications. This method is accomplished by securing the sensor directly to the bearing housing using a mounting stud. This method allows the sensor to measure vibration according to the manufacturers specifications. The mounting location for the accelerometer should be clean and paint free, also the mounting surface should be spot‐faced to a surface smoothness of 32 micro‐inches. The diameter of the spot‐face should be about 10% larger than the sensor diameter. Any irregularities in the mounting surface preparation will translate into improper measurements or damage to the sensor may occur. STI prefers that a mounting pad be placed between the machine and the sensor to provide a smooth surface for the accelerometer to attach to.
Adhesive or glue mounting provides a secure attachment without extensive machining; however this method will reduce the operational frequency range since the adhesive will act like a shock absorber, this is also known as damping. The replacement or removal of the sensor is also more difficult than any other mounting method. The most important issue for using adhesives is surface cleanliness, without a clean surface the adhesive will not fully bond to the machine.
The magnetic mounting method is typically used for temporary measurements with a portable data collector or analyzer. This method is not recommended for permanent monitoring. The sensor may be inadvertently moved and the multiple surfaces and materials of the magnet may interfere with or increase high frequency signals.
Preferred Mounting Locations
Direct Stud Mount:
The instrument wire from the vibration sensor to its transmitter or monitor should be either a twisted pair or triad cable depending on the sensors requirement. These cables should be stranded, twisted, individually insulated, shielded and with an overall jacket. The shields or drain wires must be insulated or isolated from each other and the conduit. The use of multi‐conductor cable with a single shield is not suggested due to its susceptibility to induced noise and line interference.
Signal Interference Sources:
Noise or Line Interference can be induced in a Vibration Monitoring System in a number of ways. However, there must first exist a source for the induced noise. There are numerous noise sources available in an industrial or power generation plant:
AC Power Transients
Noise can be induced in a Vibration Monitoring System through Electrostatic (Capacitive), Electromagnetic (Inductive) or Conductive Coupling (Direct Connection). All noise will be induced in the monitoring system through one or more of its external connections or Field Wiring.
The following table offers a guideline to determine the maximum cable length for different types of sensors. This is based on an ideal installation where the cable is properly grounded and installed in a dedicated conduit system.
A Single Point Grounding scheme must be utilized when installing a vibration monitoring system. All grounds must be connected at one location. It is recommended that the grounding point be at the transmitter/monitor end and not at the machine. On most machines or where other machines are being monitored significant ground differentials can be found between sensor locations and machines.
All Instrument Wire shields must be grounded at one end of the cable, and the other end left floating or not connected. The Instrument Wire should be grounded at the Vibration Monitoring System. If the shield is not grounded, the shield will become an antenna increasing induced noise on the signal path. If the shield is grounded at both ends, it will allow ground current (ground loop noise) to flow through the shield, seriously increasing signal noise.
Sensors with Built In Cables: