Manfred Weber
Metra Mess- und Frequenztechnik in Radebeul e.K.
Since 1954

Accelerometer Mounting

Choosing the optimum mounting arrangement will significantly improve the accuracy.

For best performance, particularly at high frequencies, the accelerometer base and the test object should have clean, flat, smooth, unscratched and burr-free surfaces.

A scratched accelerometer base can be applied to a lapping plate for restoration of flatness. If lapping is not possible, other machining processes such as grinding, spot facing, milling, turning, etc., can produce acceptably flat mounting surfaces.

It is also important to provide a stiff mechanical connection between the sensor and the source of vibration. Sheet metal or plastic parts and other thin and flexible components are unsuited for accelerometer mounting.


Figure 15: Typical reasons of coupling errors

Errors due to unwanted sensor vibrations can be reduced by symmetric mounting. The weight of the sensor including all mounting components should be low compared to the weight of the test object.  As a rule the sensor should not weigh more than 10 % of the test object.
Misalignment of the sensor axis and the measuring directions should be kept as low as possible, particularly if transverse vibration of high magnitude occurs. When using screw mounting, make sure that the screw is not longer than the threaded hole. The must be no gap under the sensor.

The following mounting methods are recommended for accelerometers:

Figure 16: Mounting methods for accelerometers

Figure 17 compares the typical high frequency performance of some mounting methods as a result of added mass and reduced mounting stiffness.


a: probe model 001; b: insulating flange; c: magnetic clamp; d: adhesive; e: stud bolt

Figure 17: Resonance frequencies of different mounting methods

Metra accelerometers may have the mounting thread sizes: M3, M5, M8 and M10.

Many transducers are available with an accessory kit (ordering option "/01") containing all suitable mounting parts.

The following table shows the available mounting accessories from Metra:


Mounting Part
Suitability

Stud bolts
021 (M3)
003 (M5)
043 (M8)
022 (M3 to M5)
044 (M5 to M8)
045 (M5 to 10-32)
046 (M5 to ¼"-28)
  • For best performance.
  • For permanent mounting.Mounting pads
  • Tapped hole in test object required.
  • A thin layer of silicon grease between mating surfaces aids in the fidelity of vibration transmission.
  • Recommended torque: 1 Nm.
  • Make sure that the mounting stud is not too long resulting in a gap between sensor and test object

Insulating Flanges
106 (2 x M3)
006 (2 x M5)
206 (2 x M8)
  • Avoid ground loops.
  • Limited performance at high frequencies.
  • Model 006 not suited for temperatures above 100 °C.

Mounting pads
129 (M3, small)
329 (M3, large
029 (M5)
229 (M8)
  • Adhesive attachment using cyanoacrylate, epoxy glue or dental cement.
  • For applications where drilling is not allowed or possible.
  • Models 029, 129 and 329 provide isolation against ground loops.

Magnetic clamps
108 (M3, small)
308 (M3, large)
408 (M4 tap)
008 (M5)
208 (M8, small)
508 (M8, large)
  • For rapid mounting with limited high frequency performance.
  • Ferromagnetic object with smooth and flat surface required.
  • If not available, weld or epoxy a steel mounting pad to the test surface.
  • Important: Don’t drop the magnet onto the test object to protect the sensor from shock acceleration. Gently slide the sensor with the magnet to the place.Do not use magnets for seismic accelerometers (risk of overload).

Triaxial mounting cubes
130 (M3)
030 (M5)
230 (M8)
330 (M10)
  • For triaxial measurements with three uniaxial accelerometers

Handle adapter
140
  • For the attachment of accelerometers with M3 thread on curved surfaces

Accelerometer probe
001 (M5)
  • For estimating and trending measurements above 5 Hz and below 1000 Hz.
  • Attach the accelerometer via the M5 thread.
  • Press onto the test object perpendicularly.
  • Drilling a countersink will increase repeatability.

Adhesive wax / bee wax
002
  • For quick mounting of light sensors at room temperature and low acceleration.
  • Soften the wax with the fingers. Apply thinly onto the test surface. Press sensor onto it the wax.

Cable clamps
004 (M5)
020 (M3)
  • Avoid introduction of force via the cable into the transducer.
  • To be screwed onto the test object together with the accelerometer.

Proceed to chapter Accelerometer Cabling