US Patent 7448284 Spanner with strain alarm function

A spanner with a strain alarm function comprises a spanner body having at least one driving head at one end thereof; a strain gauge installed at the spanner body; a display installed in the spanner body electrically connected to the strain gauge; an alarm unit installed in the display; when a spanner body drives an object to be tightened and a twisting force applied to the object has achieved to a predetermined value, the alarm will emit alert signal to users so as to inform the user that the object to be tightened has achieved to a predetermined position. The alarm unit is one of a light emitting body and a buzzer. The predetermined value is a summation of a desired value for tightening the object and an added value under the consideration of a retuning level of the object as the spanner is released from the object.