In the late 1950s and early 1960s, cable-shovel manufacturers began to feel threatened by developments in competitive types of equipment. The trend to hydraulic excavators had begun, and improved wheel loaders were claiming higher productivity with lower initial capital cost. Increasing sales of hydraulic excavators and wheel loaders were cutting into traditional cable-excavator markets.
Many shovel manufacturers developed their own hydraulic excavators, some with greater success than others. Koehring designed attachments that promised greater productivity from the traditional cable excavator.
Before developing its successful line of hydraulic excavators in the 1960s, Koehring launched the Skooper attachment for its ½-yard model 205 cable excavator in 1957. This hybrid machine was part cable and part hydraulic and carried a loader bucket that quadrupled the capacity of the standard shovel to two cubic yards.
With the Skooper attachment, the Koehring 205 retained its draw works (cable drums) as it could be readily converted to a standard crane, dragline, hoe or shovel. When operating as a Skooper, the hoisting action was provided by the cable hoist drum while the crowd motion was provided by a pair of double-acting hydraulic cylinders. Another hydraulic cylinder tilted or dumped the bucket.
The Skooper's increased capacity was derived from a light front end with no heavy boom to swing, and the high breakout force of the hydraulic cylinders. The machine boasted a 7-foot ground level crowding action and a choice of rock, general purpose and rehandling buckets. The 15-ton machine was powered by a 64-hp diesel engine.
Koehring designed the Skooper to compete with wheel loaders at the quarry face or in stockpile work. It was advertised as "a stand-still loader resulting in economy of motion. With ability to revolve 360 degrees, it digs, swings and loads from a stand-still position without the usual drive-in, back-out inefficient motion of front-end loaders."
In 1963, Koehring applied the Skooper principle to its first full-hydraulic excavator, the 4-yard 505. Instead of hoist cables, this 40-ton machine utilized an additional pair of single-acting hydraulic cylinders to raise the entire front-end assembly through a pantograph-styled linkage. Koehring offered Skoopers in its excavator line for well over a decade until improved front shovel geometry and more sophisticated hydraulic systems rendered them obsolete.
You can read more about the evolution of construction equipment in Keith Haddock's illustrated book "The Earthmover Encyclopedia" available in most bookstores. Also, consider a membership in the Historical Construction Equipment Association, www.hcea.net.