Road grader attachments to adapt relatively short wheel-base four wheel tractors for smoothing and grading road beds and the like are known in the art. Each such attachment attempts to, as best as possible, achieve the results of a dedicated road grader where a relatively long spaced wheel base has a movable positional scraper blade mounted intermediate the front and the rear wheels.
Since the purpose of a grader is to smooth an uneven ground surface, the geometry of any practical grader must address the problem of grading an average slope based on the position of the vehicle itself as it traverses the ungraded or the partially graded terrain. It has long been known that a long wheel base is advantageous in damping out small undulations in the ground while still preserving a track for the blade roughly approximate to the average terrain and slope of the ground. The blade in this case acts as a cutting device cutting an average slope of smoother characteristics in the preceding ground.
It is equally well known that a long wheel base severely inhibits the mobility and maneuverability of a rough terrain machine, and thus a relatively short wheel base of a tractor is considered advantageous for providing easy maneuverability of the tractor and its attachments over rough ground. Thus, in tractors, a short wheel base is advantageous, providing high maneuverability.
Therefore, it is advantageous that a grader attachment for a tractor, which has a relatively short wheel base, should somehow increase the smoothing effect over that which will be achieved by merely putting a scraper blade on the front of a tractor in the manner of a common bulldozer.
Thus, U.S. Pat. No. 4,236,587 discloses a towed groundwork apparatus and hitch in which a scraper blade is mounted through an upper and a lower link so that the tool or scraper blade moves vertically independently of the rear wheels of the vehicle. An extended arm connects the tool to a trailing rear wheel to provide a leveling frame for positioning the tool on the already smoothed ground.
U.S. Pat. No. 3,881,563 discloses a grader attachment for tractors having an elongate frame ending in a single front wheel and axle structure, which is steerable. A scraper blade attachment structure positions the scraper blade between the front wheels of the apparatus and the rear wheels of the tractor. A mechanism raises the front wheel of the tractor off the ground so that the apparatus may be steered through the attachment's front wheel. The overall geometry of the invention emulates, as closely as possible, the geometry of a standard long frame road grader.
U.S. Pat. No. 3,716,105 discloses an attachment structure having an elongated extension for attaching on the front of a tractor. This patent discloses the use of a dual wheel front wheel pivoted about an axis midway between the front and the rear wheels for damping minor undulations which otherwise would effect the position of the front wheels. The twin wheel structure tends to assume a position intermediate local high and low spots; the front axle, which positions the front of the grader device, is always located intermediate the local higher and lower positions.
Thus with the exception of U.S. Pat. No. 3,881,563 nothing in the prior art maintained a fixed long wheel base on a rigid frame for the grader or scraper mechanism. Rather, these particular structures flex with the attendant lack of smoothness that could be obtained by a long wheel base grader.
Each such device shows a structure in which the mount of the grader blade is that encountered upon a standard road grader. The grader is provided with a circular or closed framework, rigidly mounted for vertical positioning on the frame, which in turn rotates the blade. The blade is tilted for grading by tilting the rotational assembly, usually by providing that the entire blade rotational assembly is mounted to the main frame of the unit through a plurality of powered actuators which are individually extended or retracted to achieve a tilt. This produces a structure in which blade movement and the reacting forces upon the blade are tightly coupled into the -support structure which positions the blades, and can produce a chattering or washboard effect on the graded surface.