In order to measure the appropriate vibrations, a Slam Stick must be mounted securely to the surface the vibrations are to be measured on. There is more than one way to mount a Slam Stick and collect data successfully from a vibrating source. Midé supplies mounting bolts and double-sided tape with any Slam Stick purchase that are recommended for mounting a Slam Stick. While these two methods of mounting are highly recommended and encouraged, other mounting options such as magnetic mounting are also acceptable. An incorrect mounting configuration could result in erroneous data recordings and a user should ensure the Slam Stick is mounted in an appropriate manner before recording begins.
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The importance mounting plays in accurately measuring vibration and shock cannot be understated - it is more important than the sensor quality itself. Visit our blog on accelerometer mounting for more details, but a frequency sensitivity plot is shown for some common mounting methods. Handheld probes are only accurate to about 100 Hz while magnetic mounts can achieve close to 800 Hz. Bolting and adhesives have frequency responses above 1 kHz; but this is heavily influenced by torque and compression/setting of the adhesive. An improper mounting method can significantly amplify the results!
Before mounting, it is important to clean the underside of the Slam Stick and the intended mounting environment. The double-sided tape (3M 950) that is recommended for the Slam Stick is difficult to remove sometimes. We use the 3M Stripe off Wheel to remove the tape and recommend this for applications of repeated mounting and dismounting. Midé provides some plastic razor blades that can also be helpful.
Ensure that any cleaning solvent does not come in contact with the USB receptacle. Liquids, especially cleaning compounds, have the ability to electrically damage the Slam Stick device. Many degreasing and cleaning solvents are flammable and must be kept away from open flames and sparks. Use only under well-ventilated conditions. Do not smoke while using flammable solvents.
Each unit is shipped and packaged with a roll of double-sided tape with each Slam Stick. This mounting method, when the tape is adequately compressed, has been demonstrated to survive severe vibrational environments of over 75 g, vibrating at 1,000 Hz. Frequency sweeps also showed that the tape is as accurate as bolts even up to 3 kHz, but this again requires adequate compression of the tape.
The mounting tape we recommend, and a tape selected after much research and testing, is the 3M 950 adhesive transfer tape. The downside of this particular tape is that it’s quite tacky and can be difficult to remove. 3M offers an adhesive eraser wheel which is effective at removing this tape and other adhesives.
There is no discernible difference in performance between taped and bolted recorders when the tape is adequately compressed.
There are mounting holes which can also be used to bolt down the device with metal 4-40 bolts. The recommended torque for aluminum units is 100 in-oz. The recommended mounting torque is 70 in-oz for plastic cases. For better alignment, use shoulder bolts like the 94035A517 from McMaster-Carr.
Note that it is always good practice to use a washer, especially when mounting a polycarbonate enclosure unit. But over torquing the bolts (in excess of 70 in-oz) can result in damage to the wall of the plastic Slam Stick enclosure.
Visit the article on mechanical specifications to design your mounting assembly, this article includes a CAD model which can be downloaded to assist your design.
Magnets are another acceptable method for mounting a Slam Stick and can be used when it is determined that a taped or bolting configuration is not suitable for the application. Midé recommends using mounting magnets from K & J magnetics. Keep in mind these will have a reduced frequency response typically of around 700 or 800 Hz.
Alternative Mounting Methods
Wax or duct seal putty are other popular adhesives that aren’t as effective as Loctite or epoxy but still offer surprisingly good frequency response. The major benefit of using this type of adhesive is that removal of the accelerometer is much easier. Hi-Test, who does much of the shock and vibration testing for large military systems, recommends using duct seal putty for shock testing because it can mechanically filter out high frequency (and thus low energy) vibration content that could risk exciting an accelerometer’s internal resonance. Petroleum wax would have similar benefits; but note that this can have adverse effects for an application that is interested in higher frequency vibrations. Blanchard wax is a much stiffer adhesive and may be preferred for these applications.