In the first decade Donaldson Company made two important moves, it acquired the Wilcox-Bennet air filter license and introduced a new kind of filter the Simplex, used to prevent engine damage. Deere & Company was the company's most important client during the first couple decades. The early 1930s brought additional prosperity when William Lowther joined the company and designed the NS Filter, a tractor performance enhancing filter invented by Frank Donaldson. The 1930s were a transitionary period, at the beginning of the decade it was forced to contract in size due to tough economic conditions in the United States causing business from farmers to fall however, the sale of patents to influential investors provided the funds and market exposure it needed to survive and grow (part of the agreement was that Donaldson Company produce the filters). World War II provided opportunities for the company to expand into aerospace and military hydraulic equipment and device filters.