1. Field of the Invention
The present invention relates to variable displacement over-center hydraulic motors (VDOCM's) and, more particularly, to the use of two or more VDOCM's in conjunction with a torque-summing gearbox so as to overcome swashplate stick-slip zone problems experienced by single VDOCM's operating under light load or no load conditions.
2. Description of the Prior Art
The use of VDOCM's in closed loop position, speed, or hydraulic power control systems is well known in the art. For example, U.S. Pat. No. 4,534,271 discloses a means for combining plural VDOCM devices in an aggregate, U.S Pat. No. 4,426,911 discloses an actuator device using dual VDOCM devices coupled together by drive gears, and U.S. Pat. No. 3,241,319 discloses the use of dual VDOCM devices in a hydraulic apparatus. Furthermore, the use of one or more VDOCM's in hydrostatic transmissions is common, as illustrated in U.S. Pat. Nos. 4,739,618, 3,643,433, and 3,620,130.
Although all of the above-mentioned prior art patents disclose certain applications where it is beneficial to use one or more VDOCM devices, none address the situation where a single VDOCM device experiences swashplate stick-slip problems. Such a situation occurs when the resident swashplate is positioned at a small angle (which results in a small displacement), which results in the VDOCM having a frictional torque loss that exceeds its torque output capability. Thus, in this situation it is extremely difficult for the VDOCM device to maintain adequate position and speed controlled responses under light load or no load conditions.
It is therefore desirable to overcome the shortcomings of the above-mentioned prior art patents in this area, while still incorporating the positive benefits associated with VDOCM device use.