Pivotal structure for driving head of wrench

A wrench comprising:

• a handle and a working end which is formed to one end of the handle and has top and bottom surfaces configured as flat cutting surfaces, two arms extending from the working end and a room formed between the two arms, each arm having a locking hole defined therethrough;
• a driving head having two recesses defined in an outside thereof, the two recesses located corresponding to the two locking holes of the two arms;
• an aperture is defined in an inner end of the room and located between the two arms, a positioning unit is located in the aperture and includes a spring and a bead, wherein the bead is semi-spherical, the spring is biased between an inner end of the aperture and the spring, the bead partially protrudes beyond the aperture, the driving head includes multiple positioning holes defined in the outside thereof and each configured as a concave arc-shaped groove, the positioning holes are located in a pivotal direction of the driving head relative to the working end, the bead is engaged with one of the positioning holes to secure a pivotal position of the driving head relative to the working end, and
• two bolts respectively and threadedly connected to the locking holes in the two arms and inserted into the two recesses of the driving head to pivotably position the driving head between the two arms, a wave washer mounted to each of the two bolts and being sandwiched between the driving head and the arm, each of the wave washers including multiple convex portions and multiple concaved portions formed to each of two sides thereof, the multiple convex portions and the multiple concaved portions located alternatively to each other, the convex portions of one of two sides of each of the wave washers partially contacting the outside of the driving head, the convex portions of another one of the two sides of each of the wave washers partially contacting an inside of the arm corresponding thereto so that a friction between the driving head and the arms is reduced when the driving head is pivoted relative to the arms.