How to Bond a Strain Gauge into a Bolt

Bonding a strain gauge into a bolt that measures the tightening stress differs a lot from the conventional method of installing flat strain gauges. This article provides instructions on installing and using the cylindric strain gauges from HBK.

Cylindric strain gauges enable the measurement by embedding it in a hole drilled through the head of the bolt. Using a special strain gauge and adhesive, a simplified solution can be provided for measuring the axial load on bolt elements. 

Equipment Needed:

  • LB11 / TB21 - Cylindrical Strain Gauges: LB11 and TB21 are cylindrical strain gauges to measure strain, force and vibration in screws, bolts, or other construction elements.
  • EP70 Strain Gauge Adhesive: EP70 is an easy-to-handle, hot-curing epoxy resin adhesive especially suitable for applications with cylindrical strain gauges. 
  • Strain Gauge Cleaning Material: HBK provides different cleaning agents for cleaning measuring points and measurement objects prior to strain gauge installation.
  • Other equipment: Drilling machine, solder iron and equipment, bridge amplifier supporting strain gauge ¼ bridges

Step 1: Drilling a Hole in the Screw

1. In the first step, drill in the center of the screw with a diameter of 2 mm and an appropriate depth. 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.

The following sketch shows how this shall look like:

Bonding of bolt strain gauges step 1-sketch of the drilling

 

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.

Step 2: Preparation of the Strain Gauge

1. In the next step, take out one of the strain gauges from its package.

 Package of LB11 strain gauges from HBM

 

2. Cut an approximately 20mm long piece of teflon cord from the supplied roll with scissors. Grasp the strain gauge at the enameled copper wires with tweezers and insert the teflon cord into the cylindric strain gauge. Place the prepared strain gauge on a clean surface.

Step 3: Bonding of the Strain Gauge

1. A bench vise is helpful to fix the screw during the bonding process of the gauge.

Bolt on strain gauge step 5-Bench wise with fixed screw

 

2. Fill a syringe with EP70 (Order number: 1-EP70). EP70 is an easy-to-handle, hot-curing epoxy resin adhesive especially suitable for applications with cylindrical strain gauges. During all installation steps, the occurrence of air bubbles must be avoided.

 

3. 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.

Fill predrilled bolt with adhesive

 

4. Clean the surface on the bolt with cleaning pads from residual adhesive.

Clean the surface of the bolt

 

5. Insert the previously the prepared strain gauge in the hole of the screw. Take care not to compress the strain gauge during insertion. The cylindric strain gauge and teflon cord should rest at the bottom of the hole. Loosen air bubbles by slightly moving the strain gauge up and down in the adhesive.

Insert the strain gauge in the bolt

Step 4: Curing of the Adhesive

1. Cure the adhesive for the specified time in an oven. The curing times of the adhesive are specified in the instruction manual.

Temperature [°C/°F]Curing Time
60/1403 hours
Optinal post-curing at 100/2124 hours

 

2. After the adhesive has cured, the teflon cord protruding from the hole can be cut off with scissors. The remaining teflon cord stays in the installation. The bolt strain gauge is now ready for further installation!

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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