When construction equipment mechanization took over from horse power and manual labor, steam power reigned supreme. The famous steam shovel led the way, but all kinds of other equipment used steam as its motive power to increase productivity and lower costs. One of the pioneers of steam-powered concrete mixing and placing equipment was the Koehring Machine Co. of Milwaukee, Wisc., founded in 1907. Although Koehring later found fame and fortune with its well-known line of cranes and excavators introduced in 1921, the concrete mixer was actually Koehring's first product.
Key to Koehring's success with its early concrete mixers was the design and placement of the baffle blades and discharge chute within the drum. In the Koehring mixer, several distinct mixing actions occurred within the drum because it was fitted with two sets of baffle blades. In addition, the discharge chute — reaching well inside the drum — could move 90 degrees on a horizontal axis, so in one position it discharged the drum contents or, in the opposite position, aided the mixing process by directing the rotating material to the opposite side of the drum. This chute position, combined with the two sets of baffle blades, caused the material to move continually from front to back of the drum, as well as being rotated and showered to the bottom of the drum at each rotation.
According to Koehring's advertising, this double baffle design avoided separation of aggregates into its different sizes, which would occur in mixing systems that depended on a single series of baffle blades.
In the 1910s, Koehring marketed a line of six models of portable concrete mixers with capacities from 10 to 30 cubic feet based on dry batched material. Steam engines furnished on these models ranged from 5 to 12 horsepower and operating weights from 7,300 to 15,000 pounds. Before the end of the decade, gasoline and electric power was offered, and requests for steam machines began to decline.
A popular steam-driven mixer was the Koehring No.15 with 15 cubic feet dry batch capacity. According to specifications, it was equipped with a 30-inch-diameter vertical boiler 6 feet 6 inches high, and a vertical steam engine with 5½-inch bore and 7-inch stroke, developing 7 horsepower at 253 rpm. The side loader bucket could be raised in 8 seconds and the mixer emptied of "sloppy" concrete in 18 seconds. Koehring claimed the No. 15 could produce 180 cubic yards of concrete in a 10-hour day.
You can read more about the evolution of construction equipment in Keith Haddock's latest book release, a soft-bound version with updates of his fully illustrated Earthmover Encyclopedia available in bookstores now. Also, consider a membership in the Historical Construction Equipment Association, www.hcea.net .
