Endless belt power transmission systems or components – Friction drive belt – Including plural interconnected members each having a drive...
Reexamination Certificate
2002-07-01
2004-11-23
Charles, Marcus (Department: 3682)
Endless belt power transmission systems or components
Friction drive belt
Including plural interconnected members each having a drive...
C474S201000
Reexamination Certificate
active
06821224
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a drive belt. The invention further relates to a method for producing a continuous band of such drive belt and to a continuously variable transmission wherein such drive belt is utilised.
DESCRIPTION OF THE RELATED ART
Drive belts of the present type are generally known through their application in continuously variable transmissions intended for the transmission of mechanical power at continuously variable speed and torque ratios between an engine and a load in particular for automotive purposes. Such drive belts are also known from the patent specification EP-0.279.645 B1. The known drive belt generally comprises one or two endless rings and an array of plate-like transverse elements oriented mutually parallel transverse to a longitudinal direction of the drive belt, whereby the endless ring Is provided in a slot of the elements such that the elements may freely slide along the ring in the longitudinal direction thereof. The endless ring typically is laminated comprising a number of concentrically stacked continuous bands. Through this measure, the ring may have a considerable tensile strength, whereas it is still relatively easily bendable in its longitudinal direction.
Because of the nature of use in continuously variable transmissions, where it rotationally connects two pulleys each having two pulley discs that define a V-groove of variable width, the known drive belt is subjected to tensioning, bending and stretching during operation, resulting in high internal stress levels that vary in dependence on the rotational speed of the pulleys and the torque applied to the transmission. The trajectory of the belt thereby includes two longitudinally straight parts where it crosses over from one pulley to the other and two longitudinally bent parts where it runs between the discs of a pulley at a respective radius of curvature for each of the two pulleys, which radii define the transmission ratio of the transmission. As a result of the tensioning, bending and stretching, a tensile stress in the continuous band near its radially inwardly oriented surface and a tensile stress near its radially outwardly oriented surface varies cyclically between a maximum stress level and a minimum stress level during operation of the drive belt in the transmission. Such cyclical variations render the drive belt prone to fatigue cracking, which may ultimately cause the drive belt to break apart and fail. To minimise the risk of belt failure due to fatigue cracking, or put alternatively to extend functional belt life as much as possible by improving its resistance against fatiguing, the known continuous bands are pre-bent, i.e. they are provided with an internal residual stress distribution during manufacturing. According to the known art the internal residual stress distribution is provided such that during operation of the drive belt the maximum tensile stress near the radially inwardly oriented surface and the maximum tensile stress near the radially outwardly oriented surface of the continuous bands are equal and, consequently, that the overall maximum tensile stress is at a minimum. The above-mentioned situation occurs when the internal residual stress distribution of a continuous band corresponds to a stress distribution under the influence of which the continuous band would be longitudinally bent at a radius of curvature that is twice a minimum radius of curvature at which it may be bent during operation. The radius of curvature at which a continuous band would be curved under the influence of the internal residual stress distribution, e.g. when cut, is hereby denoted as a pre-bending radius. This relation between pre-bending radius and minimum radius of curvature accurately holds, particularly when a thickness of the continuous band as seen in the radial direction of the curvature is relatively small compared to the minimum radius of curvature, which is normally the case for the drive belt. It is remarked that it is known from EP0.283.303 B1 to determine such internal residual stress distribution of a continuous band by transversely cutting the continuous band and by measuring the radius of the curvature in the longitudinal direction of the posture assumed by the cut continuous band.
Thus according to the known art the desired pre-bending radius is defined as twice the minimum radius of curvature at which the endless ring is bent in its longitudinal direction during normal operation of the transmission in which the drive belt is applied. It is noted that generally speaking and at least for drive belts to be applied in passenger car transmissions, such minimum radius of curvature occurring during operation corresponds fairly accurately to a minimum physical radius of curvature of the drive belt that is determined by the transverse elements having a taper defining a maximum amount of mutual rotation of adjacent and mutually contacting elements about an axial of the drive belt in combination with a dimension of the elements in the longitudinal direction of the drive belt, alternatively denoted element thickness. Of course, the latter minimum radius is somewhat, though usually only slightly, smaller than the minimum radius of curvature actually occurring during operation to allow the full range of transmission ratios of the transmission to be realised. In practice, the optimum pre-bending radius of the continuous bands may be accurately approximated by increasing the minimum physical radius of curvature of the drive belt by about 10%, at least for typical automotive application of the drive belt such as in passenger cars.
Although pre-bent at such pre-bending radius the continuous bands should provide the drive belt with a longest possible functional life, it surprisingly appeared in practice that the drive belt is still prone to early failure with respect to what was to be expected theoretically. Accordingly, currently applied drive belts are over dimensioned with respect to their nominal torque transmission capacity, which means that they are provided with an endless ring or rings that has or have a larger longitudinally facing cross sectional surface area than that what would theoretically required according to the known art. Such increased cross sectional surface area favourably decreases the maximum stress level in the continuous bands, which may for instance be realised by increasing the number of continuous bands applied in a ring or by increasing the transverse width thereof. These measures, however, adversely affect the drive belt cost price and size and, therefore, are principally undesirable.
SUMMARY OF THE INVENTION
It is an object of the invention to improve the functional life of the known drive belt without increasing its cost price, or, alternatively, lowering the cost price of the drive belt for a given nominal torque transmission capacity. According to the invention this objects is surprisingly realised with the drive belt according to the below.
Extensive—fatigue—testing, both with assembled drive belts and with separate continuous bands running around two cylindrical rollers, and analysis of the results thereof surprisingly appeared to reveal that, at least in those cases where drive belt failure could be indisputably attributed to fatiguing of a continuous band, the drive belt functional life could be improved by adopting a pre-bending radius that is considerably larger than twice the minimum radius of curvature that occurs during operation, which was previously considered the optimal value. According to the invention, this phenomenon may be accounted for by the observation that in the known drive belts fractures appear to initiate more often near the radially inwardly oriented surface of the radially innermost continuous band of the ring than near the radially outwardly oriented surface. From these observations, it is hypothesised that as a result of he interaction between the transverse elements in the curved trajectory part of the drive belt and the radially inwardly oriented surface of the radially innermost c
Brandsma Arjen
Pennings Bert
Van Der Kamp Johannes Haaije
Van Der Meer Cornelis Johannes Maria
Van Dijnsen Gerardus Petrus Maria
Charles Marcus
Van Doorne's Transmissie B.V.
Young & Thompson
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