How to Finalize a Measurement Screw With a Bolt Strain Gauge

We know that it is difficult to bond a strain gauge to a bolt while measuring the tightening stress. Normally, this requires the use of heavy machinery. However, by using special electrical strain gauges, a simplified solution can be provided for measuring the axial load on bolt elements. The application of this gauge differs from the conventional procedures of installing a flat strain gauge. The finalization after bonding the gauge in the bolt in order to have a ready-to-use measurement screw is shown below.

This article provides instructions for how to finalize a measurement screw with the HBM strain gauge 1-LB11-3/120ZW that is already integrated in a bolt. This new gauge enables measurement by being embedded into a hole drilled through the top head of the bolt.

Equipment Needed

LB11- Estensimetro per misurazioni interne alle viti

LB11 è un estensimetro cilindrico per misurare sollecitazioni, forza e vibrazioni all’interno di viti, bulloni o altri elementi di costruzione.

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Z70: adesivo per estensimetri

Con un tempo di solidificazione molto veloce, la supercolla rapida Z70 è l’ideale per molti test sperimentali con estensimetri.

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SG250

Il rivestimento protettivo SG250 è una gomma siliconica trasparente applicata dal tubetto. Solidifica a contatto con l’aria a temperatura ambiente.

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Other equipment: Easy and light shielded PFE measurement cable (order no: 1-CABP4/20), LS7 solder pads (order no: 1-LS7), connector for MX1615b/MX1616B modules (order no: 1-CON-S1015)

Wiring of Bolt Strain Gauge

1. We begin with an already-bonded LB11 strain gauge integrated with a bolt. The strain gauge is fixed in the screw and is equipped with 60 mm copper wires. For further information on how to bond this strain gauge in a bolt, refer to the information available on the HBM website.

Step 2: Connect copper wires to solder terminal

2. In the next step, the cooper wires of the screw are to be connected to the solder terminals. In this case, we position the solder terminals on the side of the screw head. The solder terminals can be bonded, for example, with HBM Z70 cold curing adhesive. Then, we use a rubber pen to eliminate any oxidations on the surface of the solder terminal.

 

 

Pre-tin the solder pad with a solder iron

3. Thereafter, we use a soldering iron to pre-tin the solder pad. This makes it easier to solder the copper wire of the gauge later.

Connect copper wires to the solder terminal

4. In the next step, the two copper wires of the strain gauge are connected with the solder terminal.

Tip: We recommend guiding the copper wires close to the screw and minimizing the free length in order to avoid damaging the strain gauge. Use a tweezer to carefully handle the copper wires.

The copper wires are paint-insulated as well as flexible and can be bent around any object with a small radius.

5. Now, the measurement wire needs to be installed between the DAQ System and the gauge. Therefore, we use a 4-wire cable from the HBM portfolio (1-CABP4/20).

6. The screw is connected with a push-in connector, which is suitable for the MX1615B and MX1616B modules.

The wiring of the finalized measurement screw looks as follows. The two-paint insulated copper wires are soldered to the solder terminal. Then, a 4-wire configuration is used to finally prepare the measurement screw.

 

 

7. It’s absolutely recommended to ensure that the measurement screw is robust against mechanical impacts. Therefore, the paint-insulated copper wires should be shortened as much as possible to avoid any damage and minimize the temperature effects caused by the 2-wire cable. Further, the screw head should be protected (we used SG250 silicon). The following pictures illustrate examples of prepared measurement screws.

Example 1
Example 2

Legal Disclaimer: Tech notes are designed to provide a quick overview. They are continuously improved and change frequently as a result. HBM assumes no liability for the correctness and/or completeness of the descriptions. We reserve the right to make changes to the features and/or the descriptions at any time without prior notice.

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