STRAIN GAGE INSTALLATION

1. PREPARE SHAFT SURFACE:

2. MARK SHAFT SURFACE FOR STRAIN GAGE ORIENTATION AND ALIGNMENT.

• The strain gage should be installed flat and square to the axis of the shaft.  For a straight reference line square to the axis, wrap a piece of paper around the shaft, align the edges and then mark or etch the straight reference line onto the shaft.

• Part # CEA-06-250US-350 is a common torque-pattern strain gage.
• Remove strain gage from package.
• Apply cellophane tape to top side of gage.  Use the tape as a “handle.”

• Tape gage in place on the shaft using reference marks on shaft and gage for alignment.
• Lift one end of tape and “hinge” back to expose bottom of gage.
• Apply an even, thin layer of catalyst to bottom side of gage and let dry.

• With the gage/tape still hinged back, apply bead of M-Bond 200 Adhesive where the tape meets the shaft surface.  (M-Bond 200 is a special Cyanoacrylate which has been pretested and certified for use in bonding strain gages – manufactured by Vishay Measurements Group.)
• Using Teflon film or equivalent as a non-stick barrier between your thumb and the adhesive, press the gage/tape to shaft surface with single rolling/wiping motion of the thumb.
• Apply thumb pressure for one minute, followed by a minimum two-minute wait before removing the cellophane tape. Bond strength increases rapidly during first five minutes.

• Discard Teflon film and carefully peel back cellophane tape from the strain gage.
• Tin with solder the solder pads of the strain gage plus the lead wires.  Attach lead wires to solder pads.
• Clean solder joints with rosin solvent.
• For waterproofing – Apply polyurethane coating to strain gage and solder joints.
• For mechanical protection – Cover the gage with butyl rubber or equivalent.
• For electrical shielding – Cover the strain gage and lead wires with aluminum tape or equivalent.
• M-Coat J from Vishay Measurements Group is a two-part polysulfide liquid polymer compound for environmental protection of strain gage installations.  When fully cured, it forms a hard rubber-like covering that provides an effective barrier against water and many other fluids. The tough coating also protects installations from mechanical damage.

If installing the strain gage on a material other than steel, then the surface preparation requirements will be different from the steps listed above. See Vishay’s Strain Gage Surface Preparation Guide for detailed instructions for different materials.

• Installing a strain gage on carbon fiber for torque measurement is not recommended. Installing the strain gage on carbon fiber shaft material presents a difficult challenge – calibration.  Calibrating the torque/strain measurement is not possible using a mathematical equation because carbon fiber is not “homogeneous and isotropic”, as are steel, aluminum and common metals.  The carbon fibers are oriented in the shaft, so torque/strain sensitivity at 0° does not necessarily equal sensitivity at 90°, for example. Instead, a metal portion of the shaft (such as a coupling) should be used.

• Welded Seams may induce some non-linearity. If it is an extruded section then the output should be closer to linear. Placing the gages on the faces that do not have the seam would be best.
• For couplings, a chevron strain gage that cancels bending is recommended with the following considerations (See Figure 1).
• For tapered shafts, use the same gage as would be used on a normal shafts. For calculations, use the average diameter at the gage location.
• A typical keyway (i.e. one that is not unusually deep or wide) has a negligible effect (less than a few percent) on the accuracy of the torque measurements presuming the gage is mounted 180° from the key slot