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Force-Measuring Clamps

Clamping forces can be measured easily and quickly.

Force-measuring clamps have been invented to facilitate and simplify the task of measuring the forces or pressures applied to clamped parts. There is a critical need to measure clamping forces or pressures in some applications — for example, while bonding sensors to substrates or while clamping any sensitive or delicate parts. Many manufacturers of adhesives and sensors recommend clamping at specific pressures while bonding sensors or during adhesive bonding between parts in general.

In the absence of a force-measuring clamp, measurement of clamping force can be cumbersome at best because of the need for additional load sensors and load-indicating equipment. One prior method of measuring clamping force involved the use of load washers or miniature load cells in combination with external power sources and load-indicating equipment. Calibrated spring clamps have also been used. Load washers and miniature load cells constitute additional clamped parts in load paths and can add to the destabilizing effects of loading mechanisms. Spring clamps can lose calibration quickly through weakening of the springs and are limited to the maximum forces that the springs can apply.

The basic principle of a force-measuring clamp can be implemented on a clamp of almost any size and can enable measurement of a force of almost any magnitude. No external equipment is needed because the component(s) for transducing the clamping force and the circuitry for supplying power, conditioning the output of the transducers, and displaying the measurement value are all housed on the clamp. In other words, a force-measuring clamp is a complete force-application and force-measurement system all in one package. The advantage of unitary packaging of such a system is that it becomes possible to apply the desired clamping force or pressure with precision and ease.

Like many other load-measuring devices, a force-measuring clamp contains strain gauges and exploits the well-known proportionality between strain and applied force or pressure. More specifically, a force-measuring clamp contains four strain gauges electrically connected in a Wheatstone bridge. The bridge output is fed to zero and span circuitry, the output of which is digitized and displayed. The span and zero circuitry make it possible to calibrate the bridge output to indicate force or pressure in any suitable unit of measure.

The strain gauges can be installed by use of Measurements Group M-Bond 610 (or equivalent) epoxy-phenolic adhesive or Measurements Group AE 10 (or equivalent) epoxy adhesive. The strain gauges are connected to a terminal strip for incorporation into the bridge by wires of 34 American Wire Gauge [≈6.3 mils (≈0.16 mm) in diameter]. Wires of the same size are used for connections between the terminal and a printed-circuit board. The printed-circuit board contains a voltage-regulation circuit, the span and zero circuits, two watch batteries, and a power switch. The final-stage output of the printed circuit is fed to a digital-display device that is plugged into the printed-circuit board, and is controlled by the zero and span circuits. Operation of the force-measuring clamp is easy: One simply slides the power switch to "on," adjusts the display to zero if necessary, applies a clamping force, and reads the display.

The functionality of a "breadboard" prototype force-measuring clamp was tested in a laboratory by use of the combination of certified weights, a load washer, a strain indicator, and a voltmeter. Some alternate or future embodiments of force-measuring clamps may include smaller batteries and/or smaller digital displays for the sake of compactness, more options, and better packaging of all components. Force-measuring clamps and/or similar devices could also be incorporated into other mechanisms.

This work was done by Mark Nunnelee of Dryden Flight Research Center.

This invention has been patented by NASA (patent pending). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to Yvonne Kellogg, Technology Commercialization Specialist, Dryden Flight Research Center, (661) 276-3720. Refer to DRC-99-37.