Tension Device for Drive Belt

1. Field of the Invention

The invention concerns a tension device comprising a tension roller which is supported and free to rotate on a supportive member, a pivot bearing outside an axis of rotation of the tension roller and capable of being locked in position with respect to an engine block or the like, a tension spring pressing the tension roller, for adjustment, against the drive belt or the like, and of an additional spring pressing against the drive belt.

2. Description of the Prior Art

A tension device of the aforementioned type has already been disclosed in DE-OS No. 2,617,368. In this design a swiveling lever is pivotably supported on a supporting plate. A tension roller is arranged outside a pivot bearing and is tensioned against a drive belt by a working spring acting on the other end of the swinging arm. This working spring is constantly active during operation. To permit an optimal basic adjustment, the supporting plate is for its part connected with the engine block via another pivot bearing and may be fixed in the proper place by a fastening screw. An additional tension spring arranged between the engine block and the supporting plate permits the basic adjustment to be made with constant tension against the drive belt when the tension roller is arrested in its swivel motion with respect to the supporting plate. After release of the arrest, operating shocks and swings of the drive belt are absorbed and equalized by the working spring. Assembly of the tension device in the manner described is relatively costly and, due especially to the arrest required, complicated. It requires a complicated procedure. In addition, the known tension device has a great range of adjustment of the swiveling lever, which may result in damage having serious consequences. Strong pulse shocks in the operating behavior of the engine cause the drive belt to stretch severely in the region of the tension device. This leads to deflection of the tension roller in the direction away from the drive belt. Since the known tension device exhibits no damping of any kind in this direction of motion, the drive belt may be slack for at least a short time. In this phase the likelihood of slippage between the drive wheels and the drive belt is very great. In this case, especially in toothed belts, a failure to maintain a correct angle of rotation between, for example, crankshaft and cam shaft occurs due to skipping of one or more teeth, which may lead to shutdown of the engine, and at the least to defective operation.

The object of the invention is to provide a tension device of the type mentioned at the beginning which consists of few individual parts, one which may be simply assembled and adjusted and by which slippage between the drive belt and the drive wheels, which may result in skipping of drive belt teeth during operation, is reliably prevented.

This object is accomplished in that the tension of the working spring operative against the drive belt exceeds that of the tension spring and the adjusting motion of the tension roller is in operation limited to one region, limitation of the adjusting motion being provided in the direction toward the working spring pressure, which limitation results from the position of the tension roller at maximum engine temperature, and limitation of the adjusting motion in the direction toward the drive belt is given by the maximum increase in the section of the drive belt arranged in the region of the tension device, under the most unfavorable operating conditions.

Owing to the working spring having a force in the direction of the tension roller, other means for arresting the tension roller within its range of adjustment effective in operation may advantageously be omitted. In addition, this range of adjustment is limited to a small extent. The optimal adjustment results from considering various operating values or parameters, such as required tension, greatest corrected variation of drive-belt length as a function of engine temperature, elongation of drive belt due to aging, running in, etc., greatest elongation in the loaded side of the drive belt as a result of pulse shocks, instantaneous loads, etc., and elongation of the drive belt required to obtain skipping of a tooth due to slippage, to mention only a few influences. These values are different for virtually each application and, as a basis for computation, in cases of doubt supported by experiments, result in the required degree of to the invention.

A limitation of adjusting motion before a tooth is skipped under the most unfavorable operating conditions is sought in every case. The limit in the other direction results from the maximum required readjustment of the tension roller owing to elongation of the drive-belt section in the unloaded half. In an example adapted to practice, the limited range of adjustment of the tension roller amounted to about 2 mm.

The dissimilar spring forces of the tension spring and the working spring are selected, as characterized by additional features of the invention, so that the working spring, at about half its deflection within the adjusting motion, produces the required tension for the drive belt and this spring force matches that of the tension spring. Under these conditions, upon assembly care must only be taken to see the supporting member, with the tensioned tension spring, is locked in position by the fastening screw, to be sure that the working spring, on the one hand, is tensioned and the range of adjustment of the tension roller is optimally arranged for later operation. This may be achieved most simply by providing for the tension spring a constant spring force over the required range. This can be done by, for example, a spiral or helical spring torsionally loaded. The working spring, on the other hand, should have a progressive tension characteristic, wherein at a working point within the range of adjustment of the tension roller in operation equilibrium of forces prevails between the two springs. Starting from this, the spring force of the working spring rises increasingly in the direction of motion away from the drive belt and decreases in the direction of motion toward the drive belt. In this way, correct adjustment and location of the working point and/or range of adjustment is effected automatically in assembly. The location of the working point within the range of adjustment may of course be varied as desired by selection of the spring forces.

The tension device pursuant to the invention is distinguished by few individual parts and particularly simple handling in assembly. In addition, advantageously constant results are obtained in series assembly.

The drawing discloses a sectional elevation view of the tension device of the subject invention by the drawing.

The single figure shows a tension device for a drive belt with a tension spring 1 and a working spring 2. A tension roller 4 is supported and free to rotate on a supporting member 3. The supporting member 3 has available a pivot bearing 6, arranged eccentrically to the axis of rotation 5 of the tension roller 4, which bearing is formed by a threaded pin 7 and a corresponding bore in the supporting member 3. The threaded pin 7, in a mounting plate 8, is fixed to an engine block or the like. On the other hand, the supporting member 3 may be fastened securely against twisting by a nut 9. Between the supporting member 3 and the mounting plate 8 is arranged, as tension spring 1, a spiral spring torsionally loaded, whose free ends 10 are bent at an angle and engage in recesses 11 of the supporting member 3 and/or the mounting plate 8. The tension spring 1 has a virtually constant tension over the entire range of travel. The ball bearing 12, provided for the rotary motion of the tension roller 4, is radially movable on the supporting member 3 via the working spring 2 which in this embodiment is a corrugated washer spring embedded in a groove 13. The radial range of adjustment results from the radial dimension of an annular slot 14 and is limited by the surface 15 of the supporting member 3 and the surface 16 of the bore of the inner ring 17. The radial spring characteristic of the working spring 2 is progressive, i.e., the spring force increases with increasing spring deflection. Axially, the inner ring 17 is clamped, by means of a disk spring 20 and a retaining ring 21, between annular disks 19 resting on both sides on its faces 18. In the radial spring deflection of the inner ring 17, its faces 18 slide on the annular disks 19, whereby a frictional damping is produced.

In assembly, the entire tension device is fastened by, for example, screws to the engine block via the mounting plate 8. There the drive belt, not illustrated, already rests on the tension roller 4. The nut 9 is loosened, so that the supporting member 3 can adjust to the course of the drive belt by swinging about the threaded pin 7 against the force of the tension spring 1. In this position the nut 9 is tightened. The tension roller, via the inner ring 17 and the working spring 2, is then fitted in radially by about half the annular slot 14 shown, so that in operation one half of the annular slot 14 remains for deflection when the drive belt is compressed and the other half for deflection out when the drive belt is elongated.

To replace the drive belt, the nut 9 only needs to be loosened. With an auxiliary tool, not represented, engaging in adjusting bores 22, the tension roller 4 can be swung, against the force of the tension spring 1, out of the range of travel of the drive belt.

The embodiment illustrated is only one example. Numerous variations, wherein a limitation of the range of adjustment and two springs coordinated with one another are provided, are of course possible within the idea of the invention and within the scope of the appended claims.