Rotary lawn mowers which have a mulching function have been available in recent decades, but have met with limited commercial success. Mulching devices cut the clippings in sufficiently small size that they will drop down through a stand of grass rather than collect on the top surface of a lawn as do normal grass cuttings. The finely ground cuttings form a valuable mulch which not only diminishes evaporation but also functions as a fertilizer or compost thereby requiring reduced amounts of fertilizer. Dropping the clippings back on the lawn also eliminates the disposal problem as well as the time spent unloading grass catchers and disposing of the clippings.
One of the more common types of mulching systems utilized on a rotary mower comprises a stationary grid having uniform size openings therein positioned directly above the cutting blade which prevents the cuttings from passing therethrough until they are sufficiently recut that the fine cuttings will pass through the openings in the grid. Screens or grid type mulchers are very susceptible to plugging in conditions of high water content which quickly blocks all flow through the mower.
The idea of separate mulcher blades positioned co-axially over the cutting blade is illustrated in the patent to Perry (U.S. Pat. No. 4,196,568); Lalonde (U.S. Pat. No. 4,292,791) and Iaboni (U.S. Pat. No. 4,263,771). In the Perry patent, the mulching blade is located in the same chamber and turns in the opposite direction from the cutting blade like the action of a scissors, while in Lalonde both blades turn in the same direction with the cutting blades pumping the air and clippings upward while the elevated mulching blade and its respective trailing edge tip pumps the air downward back into the cutting blade. In Iabonia, the cutting and mulching blades have separate compartments one on top of the other to perform their functions. The general idea of increasing the speed of the downstream cutting blades in multiple blade mowers is taught in Brudnak (U.S. Pat. No. 3,916,606). In the last mentioned patent, the increased speed of the downstream cutting blade improves the flow of grass cuttings through the mower by increased velocity, however, there is no attempt to shred the clippings. Both patents to Jetzinger (U.S. Pat. No. 4,711,074) and Russell (U.S. Pat. No. 3,974,629) both illustrate different types of hammermill type shredders wherein the mulching blades beat the grass or crop against a stationary cutter which are somewhat similar to the above-mentioned screen type mulchers and have the same inherent disadvantages.
The broad concept of offsetting the mulcher blades axis of rotation from that of the cutting blade and directing the flow of grass clippings from the cutting blade into the mulching blade is generally taught in the patent to Niemann (U.S. Pat. No. 2,956,386). Niemann also teaches utilizing a plurality of mulching blades on a single spindle. Turning through different cutting planes.