Off-road vehicles are commonly equipped with endless tracks rather than tires. Tracks have been found to be more useful than tires in rough or marshy terrain, in that they are less prone to stress-induced failures and to becoming bogged down. Tracks are desirable because they spread their load (several thousand pounds for large agricultural power drives) over a larger surface area than do tires for similar loads, so that the tracked vehicles do not sink into the ground. For this reason, tracked power drive vehicles have become widespread in off-road applications such as construction, the military, recreational vehicles, snow grooming, and some tracked vehicles have been used in agricultural harvesting. See, for example, U.S. Pat. No. 5,176,573 issued to Paul Dow on Jan. 5, 1993.
Tracked vehicles commonly employ endless linked steel belts as tracks. These steel tracks are usually cleated on the outside to improve traction. Steering is commonly accomplished by immobilizing one of the tracks and driving the other, causing the vehicle to pivot on its stationary track. This maneuver requires high horsepower and also does significant damage to the surface beneath the vehicle. Therefore, steel-tracked vehicles are generally prohibited on paved roadways and must be transported over the road on flatbed trucks.
Agricultural harvesters are used in plowed fields and even during wet weather; then, tracks are superior to tires for traction. An added advantage of tracks is that they do less damage to the soil by way of compaction and rutting. Recently, tracks made of elastomeric materials such as rubber or plastic have become available as alternatives to steel tracks. In agricultural harvesting, for example, rubber tracked vehicles can be driven in the fields without significant damage to the soil and then be driven over paved roads from field to field. It is a feature of the invention to provide improved tracked vehicles which can travel over the road, and at reasonable speeds.
Tracked vehicles are known to be powered hydraulically, as disclosed in, for example, U.S. Pat. No. 3,447,619. A common drive utilizes a low-speed high-torque (LSHT) hydraulic motor. Such drives are powerful but are incapable of road speeds above about 15 miles per hour, whereas many portions of the harvesting industry depend upon a road speed of 20 miles per hour or higher to meet the demands of the harvesting schedule. Other drives use high-speed hydraulic motors with gear reduction, but these are bulky, costly, and subject to high maintenance.
A common means of driving a rubber track is by forming a drive wheel within the power unit as essentially a sprocket and engaging the teeth of the sprocket with holes formed in the track. This design has the drawback that the sprocket teeth protrude through the rubber track and can dig up the field and, more seriously, do damage to paved roadway.
Off-road tracked vehicles must handle rough, rocky terrain without conventional ride-softening suspension elements such as springs and shock-absorbers, which would interfere with precise location of the picking and threshing mechanisms with respect to the crop and with respect to each other. Track designs which have heretofore been proposed include designs which ride very roughly over objects in their path. The track assembly must ride up on the object, reach its balance point, and then fall forward, with no means within the action of the track to absorb the shock of encountering the obstacle.
A drawback of conventional two-track vehicles resides in the long length of track required to support the vehicle. This exacerbates the aforementioned steering difficulty and surface damage. While four-track vehicles having a two-part articulated chassis have been proposed, for example, in U.S. Pat. Nos. 3,435,908; 3,741,331; 3,789,942; 3,937,289; and 4,072,203, they have not been suitable for heavy loads as required for agricultural harvesting applications. It is a feature of the invention to provide improved power drives which enable four-track vehicles to haul heavy loads over rough terrain and with positional stability needed to locate harvesting heads and threshing components with respect to the ground and to each other.
An additional drawback of conventional four-track vehicles is the need to limit the freedom of articulation of the joint between front and rear elements to left and right horizontally. This limits the ability of the vehicle to adapt to irregular terrain. This is because vertical rigidity must be maintained between the two elements, particularly in designs in which independent vertical oscillation of each of the four power units is permitted. Without vertical rigidity, the articulating joint is vertically unstable in such conventional two-track vehicles. For this reason, a simple ball-joint connection cannot be used. It is a feature of the invention to provide a joint for an articulated tracked vehicle which permits both horizontal (left and right) and rotational (about a longitudinal axis through the rear element) relative motion of the front and rear elements, while maintaining vertical rigidity between the elements.
Track drive units commonly are designed with rugged drive elements such as sprockets and chains which are exposed for easy repair and maintenance, which has the drawback of making these elements vulnerable to damage. It is a feature of this invention to provide a power unit for a tracked vehicle wherein the drive elements are encased and located, within the unit for maximum protection of the elements, but in a way which does not interfere with operation and which still permits ready access for repairs and maintenance.