Spanners are general used hand tools. To have higher precision in driving an object, strain gauges and displays are added to the spanner so as to show the driving forces of the spanners.
Generally, the strain gauge is installed at a weakest area of the spanner so that the bridge circuit will lose of balance and thus the strain gauge can be measured precisely. When the applied twisting force has over a predetermined value set in the strain gauge, alarm voices will emit so as to inform the user that the operation has achieved to a desired one.
However from the experience, it is known when the object to be driven has achieved to a desired value and then the spanner is released from the object, then the object will rotate back a little so that in fact, the object is not at a desired driving angle. That is, the object is not at a tighten position which is predetermined set. However this will induce some danger in many mechanic devices, such as airplane, boats, etc.
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