A conventional motor grader comprises a blade adapted to be moved and held in a vertically selected position of operation by at least one cylinder. The cylinder is integrated into a fluid circuit, including a main control valve which directs fluid under pressure to the rod or head ends of the cylinder to raise or lower the blade to a desired position, and a lockout valve which is adapted to trap fluid in the rod and head ends of the cylinder so that the blade is held in a semi-rigid selected position on the motor grader during a finish grading operation, for example.
It is oftentimes desirable to employ the blade or other auxiliary work tool mounted on the motor grader for an additional operation, such as snow-removal. In such case, it is then desirable to "float" the blade by providing means in the fluid circuit whereby a degree of reciprocal movement of the blade is allowed by the cylinder.
For example, U.S. Application Ser. No. 560,058, filed on Mar. 20, 1975 by Joseph E. Dezelan for "Blade Lift Float Circuit for Motor Graders" discloses a fluid circuit as above described, for permitting a work tool, such as the blade employed on a motor grader, to be either held in a selected work position or to be placed in a "float" condition of operation by at least one double-acting cylinder. Such application is assigned to the assignee of this application.
In more particular, such application discloses a work tool, at least one double-acting cylinder operatively connected to the work tool for selectively moving the same, a pressurized fluid source, a first control valve connected between the cylinder and the fluid source, the first control valve having a manually operable handle with head end, rod end and neutral positions for communicating fluid from the fluid source through the first control valve to the head or rod ends of the cylinder when the handle is in head end or rod end positions, respectively, and for preventing fluid flow through the first control valve when the handle is in neutral position, a normally closed second control valve connected between the cylinder and the pressure source for exhausting fluid from both the head and rod ends of the cylinder when the second control valve is open, and means including a solenoid for opening the second control valve when the solenoid is energized.
In the operation of a system as disclosed in the above-mentioned U.S. application Ser. No. 560,058, the operator closes a "float" switch to energize the solenoid and move the second control valve to open position so that the work tool will function in "float" mode, as, for example, in a snow-removal operation. If the operator sees an object buried in the road or protruding up in the road which he does not want to hit with the blade he must first open the float switch to take the operation out of the float mode and must then actuate the main control valve to raise the work tool so that it will miss the object. These operations take time and require skill to carry out in the proper sequence, and thus the object is not always missed as intended.