Pruning shears according to the starting point for the pruning shears of the present invention and thus the closest related art background are disclosed in U.S. Pat. Nos. 5,950,315 and 6,748,663. Since so many structural features of the pruning shears according to the present invention correspond to the pruning shears mentioned in the above-mentioned patents, said patent is incorporated in this application by reference.
In pruning shears known from said patents, a spring-loaded pivoted blade, which forms a cutting jaw with a counter blade having a fixed position, is driven by a drive mechanism, which transmits power to the moving cutting blade by means of a chain. In the drive mechanism the chain is fastened to a chain wheel associated with a drive wheel coupled to a drive line. Some of the drive line is wound on the drive wheel. The chain wheel and the drive wheel are arranged to rotate together about a common axis, and when the drive line is drawn, it makes both the drive wheel and the chain wheel connected thereto rotate about their common axis. The chain thus applies the drawing to the spring-loaded blade. When the drawing is stopped, the blade spring restores the drive line to its original position. Such a drive mechanism is simple and reliable.
There is, however, a problem related to the drive mechanism described above. In the pruning shears described in said U.S. Pat. No. 5,950,315, the cutting head is pivoted with respect to the operating handle of the pruning shears in such a manner that the angular position of the cutting head may be adjusted relative to the handle. As a result of this adjustment, the drive line is either wound on or off the drive wheel. If the drive line is further wound on the drive wheel, it also goes around the chain wheel, which, for its part, applies the drawing along the chain to the moving blade by changing the width of the cutting jaw. To compensate for or cancel out this change, the fastening between one end of the drive line and the handle of the pruning shears must be released and more of the drive line is freed thereby to restore the maximum size of the cutting jaw. Even though the length adjustment of the drive line were simple with respect to the knot by which the drive line is fastened to the handle, the length adjustment of the drive line always causes some extra work.
U.S. Pat. No. 6,748,663 solves this disadvantage by a complex drive mechanism and a drive line arrangement enabling the adjustment of the angular position of the cutting head relative to the elongated handle without changing the width of the cutting jaw during the adjustment. Such a solution is, however, complex, expensive to manufacture, and the use is unreliable due to its complexity. It is also very difficult for a user to re-assemble the drive mechanism of the patent, if it must be opened and dismantled because of maintenance or other repair work.