How to Bond a Strain Gauge into a Bolt
Bonding a strain gauge into a bolt that measures the tightening stress is difficult. Using a special strain gauge, a simplified solution can be provided for measuring the axial load on bolt elements.
This article provides instructions on installing and using the 1-LB11-3/120ZW strain gauge from HBM. This strain gauge enables the measurement by embedding it in a hole drilled through the head of the bolt. The installation of this gauge differs from the conventional method of installing a flat strain gauge.
Video - How to Bond a Strain Gauge into a Bolt
Watch this video and learn how to built up a measurement screw by integrating the cylindric strain gauge LB11 and how to wire it to a connector ready-to-use- with a bridge amplifier module.
Other equipment: Drilling machine, solder iron and equipment, bridge amplifier supporting strain gauge ¼ bridges
1. In the first step, drill in the center of the screw with a diameter of 2 mm and an appropriate depth*. The following sketch shows how this shall look like.
2. Clean the drilling by spraying RMS 1 (Part number: 1-RMS1 or 1-RMS1-SPRAY) and air dry it afterwards (compressed air can be used as well). This is to be performed to remove all dirt particles, metal chips and dust from the inner walls. Ensure that the drilling is dry after the cleaning process.
*Note: The required drilling depth and therefore the positioning of the gauge can vary depending on the screw. It needs to be assured that the strain gauge is set in an area where the maximum strain is expected. Additionally, the stiffness of the screw needs to be considered. Drilling reduces the cross section. Therefore, the durability of the screw might be reduced in some cases since the axial force needs to be distributed on this smaller cross section
3. In the next step, take out one of the strain gauges from its package.
4. Position the strain gauge and fix it in a position so that the gauge surface is not in contact with another material.
Tip: Bend the copper wires and use another block to hold the gauge in the air so that the whole surface can be wetted with adhesive later.
5. A bench vise is helpful to fix the screw during the bonding process of the gauge.
6. Fill a syringe with EP150 (Order number: 1-EP150). EP 150 is an easy-to-handle, epoxy-resin, hot-curing adhesive that is suitable for experimental stress analysis applications and for transducer manufacturing as well. During all installation steps, the occurrence of air bubbles needs to be avoided. The use of low-viscous adhesives and vacuum could improve the quality of the adhesive joint.
7. Use the integrated brush in the cap of the EP150 to put the adhesive first on the surface of the strain gauge. Ensure that the whole surface of the cylinder is wetted.
8. In the next step, fill up the predrilled bolt with the adhesive in the syringe. To reduce air bubbles put the syringe tip to the bottom of the drilling before the insertion of the adhesive. When the hole is filled up, pull out the syringe slowly by refilling in parallel adhesive.
9. Clean the surface on the bolt with cleaning pads from residual adhesive.
10. Insert the previously wetted gauge in the hole of the screw. Take care not to compress the gauge during insertion. The gauge shall not be clamped when it is set in its final position.
11. Leave the strain gauge in the drilling hole for about one hour before proceeding with any further steps.
12. Cure the adhesive for around a specified time in an oven.* The heat-up rate should be 2-10 K/min to avoid air bubbles. The curing times of the adhesive are also specified in the instruction manual
13. The bolt strain gauge is now ready for further installation!
|Temperature [°C/°F]||Curing Time|
Note: EP150 contains a solvent that might evaporate if the bolt is not cured under an appropriate time window after its use in the oven. Furthermore, the use of EP150 is recommended in combination with the use of the strain gauge. Other adhesives might work properly as well.
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