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Accelerometer Mounting

 

Most machines involve rotary mechanisms. Motors, pumps, compressors, fans, belt conveyors, and gearboxes all involve rotary mechanisms and are frequently used in machines.

Most rotary mechanisms in turn have bearings that support the weight of rotating parts and bear the forces associated with rotary motion and vibration. In general, large amounts of force are borne by bearings. Bearings are often the place where damage occurs and where symptoms first develop.

Vibration measurements are usually taken at machine bearings, with accelerometers mounted at or near the bearings.

Conclusions regarding machine condition depend on measurement accuracy. Be very careful how measurements are taken. Accelerometer mounting determines the measurement accuracy.

How should accelerometers be mounted to ensure accuracy and how can we do this safely?

Here are some guidelines:

  • Mount as close as possible to the bearing
  • Firmly attach the accelerometer
  • Correctly orient the accelerometer
  • Mount the same accelerometer in the same location
  • Mount the accelerometer on something substantial
  • Take care of the accelerometer
  • Take care of personal safety


Mount as close as possible to bearing

Imagine a doctor who listened to your heart through thick clothing and placed the stethoscope closer to your kidney than your heart. You would likely doubt his diagnosis as he would be basing it on sounds distorted by undue obstruction and noise from other organs.

 

When measuring vibration, always attach the accelerometer as close as possible to the bearing. More specifically, attach it as close as possible to the centerline of the bearing to avoid picking up distorted signals.

Attach accelerometer firmly

For the accelerometer to detect true vibration behavior, it needs to undergo exactly the same vibratory movement as the vibrating component. Attach the accelerometer firmly to the vibrating component so that it does not rock or move independently of the component. A loosely mounted accelerometer produces signals distorted by its own independent movements and gives the wrong message.

Various mounting methods exist, but a magnet mount is the most popular. A magnet offers a balance between measurement reliability and convenience to the user.

To ensure the accelerometer is firmly attached, it must be attached to an even magnetic mounting surface. The magnet must sit securely on the surface, with the accelerometer positioned in the prescribed orientation. 

When pulled away from the mounting surface perpendicularly, the accelerometer's magnet resists with a force of 22 kgf (48.4 lbf).



For the surface to be even, it must be free of debris, rust, and flaking paint.



The mounting surface must be truly magnetic (iron, nickel, or cobalt alloys).



The magnet must not, for example, be attached to an aluminum surface. This is due to iron beneath the aluminum surface.

To avoid loss of magnetism, the magnet must not be dropped or heated. Care must also be taken not to strip the screw thread on the accelerometer and magnet.



Correctly orient accelerometer

Different situations require the accelerometer to be oriented differently. For example, to detect parallel misalignment, the accelerometer is usually mounted in the radial direction of the bearings. To detect angular misalignment, the accelerometer needs to be mounted in the axial direction. The signal produced by the accelerometer depends on the orientation at which the accelerometer is mounted, since the amplitude (amount) of vibration varies in different directions.



Mount the same accelerometer in the same location

Always mount the accelerometer at the same location to minimize measurement inconsistencies that may lead to incorrect conclusions. Where possible, always use the same accelerometer for a particular measurement point.



Mount the accelerometer on something substantial

The accelerometer must never be mounted on a flexible machine part. This is because the spectrum will be distorted by the movement of the flexible part.

Never use an accelerometer on lightweight structures. The weight of the accelerometer and magnetic mounting will distort the structural vibration behavior. In general, the combined weight of the accelerometer and magnet should be less than 10% of the weight of the vibrating structure.



Take care of accelerometer

If the accelerometer is treated roughly, it may produce unreliable signals. Because of the strength of the magnet, take care when attaching the accelerometer to a mounting surface. Approach the mounting surface with the magnet tilted at an angle. When detaching the magnet, do not use the accelerometer as a lever for breaking contact. Instead, the magnet should be gripped tightly, then tilted sideways to break the contact.



The accelerometer cable should never be twisted. Ensure it is anchored in a way that prevents it from damage. Twisted or freely swinging cables can distort the measured spectrum.



Take care of personal safety

Manage hazards at all times. When collecting vibration measurements, three kinds of hazards stand out:

  • Injury by moving parts
  • Electrical shock
  • Magnet-induced damage

Injury by moving parts

When mounting the accelerometer, take care to prevent the cable from becoming entangled with moving machinery. While a quick-release connector minimizes danger, you should not rely on it as a substitute for correct installation.

Other items that could tangle with moving machinery include loose clothing, long hair, data transfer cables, and straps.



Electrical shock

Never attach the accelerometer to any high voltage surface.

Magnet induce damage

Never bring the magnetic mounting near any magnet-sensitive objects such as pacemakers, credit cards, floppy disks, video tapes, cassette tapes, or watches as these items can be damaged by magnetic fields.