Basic earthmoving machine concepts were developed before 1950. In the early 1950's, three advancements initiated major refinements of construction machines: High-strength steel, nylon cord for tires, and high-speed lightweight high-output diesel engines. This expansion of technology led to larger capacity machines in the 1950's, expanded product lines in the 1960's, and machines with more productivity in the 1970's. Machine evolvement, a day-to-day year-to-year process, ends when man has no further need to move the earth he lives on. The end is not in sight, for there is only the future.
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Edwin H. Klump (1975) studied this question.