FIG. 1 is a schematic representation of a track system. An endless track 10, which may be a chain or a rubber belt, passes over a drive wheel or sprocket 12 and is guided over a track roller assembly 14 that can pivot about an axis 16. The assembly 14 carries rollers 18 which guide the track 10 and maintain it in contact with the ground.
In such a track system, it is important to maintain the correct tension in the track. If the track is allowed to become too slack, it can come away from its drive wheel 12 and guide rollers 18. On the other hand, if excessive tension is allowed to develop in the track, then damage can be caused to the track system.
The track therefore requires a tension management system which must be capable of allowing for the various events encountered during vehicle operation that affect the tension in the track. In particular,
Vibrations caused by uneven rotation of the drive wheel or uneven resistance in the guide rollers;
Variation in the length of the track due to wear and strain elongation;
Variation in the path followed by the track resulting from ingested debris becoming trapped between the track and its drive wheel or guide rollers;
Changes in the geometry of the track as it goes round, for example if the drive or guide elements are polygonal instead of being circular or when the track is a chain having long rigid links;
Changes in the attitude of the track roller assembly if its pivotal axis does not coincide with the axis of the drive wheel; and
Changes in ground inclination especially when the vehicle is driven over the brow of a hill.
It will be noted that some of these changes, notably vibrations in the drive train, are of relatively low amplitude and high frequency. On the other hand, when the vehicle is driven over the brow of a hill, the transition from driving uphill to engine braking downhill causes a high amplitude step change in the track tension.
The prior art solutions to this problem include tensioning systems that use a screw to adjust the position of a track tensioning idler roller as well as more complex mechanical arrangements that allow recoil but these have not proved satisfactory.
U.S. Pat. No. 6,106,082, which represents the closest known prior art to the present invention, describes a tension management system for controlling the tension of an endless track by regulating the hydraulic pressure in a hydraulic jack 20 acting on a idler 22 serving to tension the track. The tension management system comprises two accumulators of different capacity that are connected to the jack. An accumulator normally consists of a tank of which the interior is divided by a diaphragm into two separate chambers, one filled with hydraulic fluid and the other with gas. The gas filled chamber is preloaded to a given pressure and acts as a spring. The operation of an accumulator is analogous to a hydraulic cylinder in which movement of the piston is opposed by a mechanical spring, the stiffness of the spring being determined by the pressure preloading of the gas filled chamber.
In U.S. Pat. No. 6,106,082, the smaller accumulator permanently communicates with the hydraulic jack and acts as an air spring to absorb low amplitude vibrations and shocks. The larger accumulator is connected to the jack through a pressure relief valve and a non-return valve arranged in parallel with one another. The second accumulator only comes into play when the volume of the hydraulic fluid displaced from the jack exceeds the capacity of the first accumulator, as may occur in the example given above of the vehicle being drive over the brow of a hill. When the capacity of the first accumulator is exceeded, the pressure in the jack will continue to rise until the pressure of the relief valve is reached. At this point, the relief valve opens to allow hydraulic fluid to flow to the second accumulator in order to limit the pressure rise in the jack and hence the tension in the track. When the pressure in the jack later drops, the hydraulic fluid stored in the second accumulator is returned to the jack and the first accumulator through the non-return valve.