Transmission system comprising an elastic driving belt having residual elastic elongations when stretched between two pulleys

Abstract: An elastic driving belt (1) stretched between two pulleys (7,8) of fixed distance between centers and having a predetermined starting tensioning, undergoes a permanent elongation such that the residual elastic elongation has a percent value included between the detectable percent contraction values along the inner edge of the belt itself when the latter is respectively wound according to two predetermined bending lines. The residual elastic elongation is detected by measuring the residual tensioning value on the belt when the permanent elongation has occurred, and reading the elongation value corresponding to the residual tensioning value on a load-elongation graph relating to the belt.

Description: The present invention relates to a transmission system comprising an elastic driving belt having residual elastic elongations when stretched between two pulleys. Said elastic belt being of the type comprising: an annular body made of elastomeric material; a plurality of elastic filiform elements extending lengthwise within the annular body and disposed in mutual side-by-side relation according to a line substantially defining the neutral deflection axis of the belt in a free state. It is known that driving belts in general are essentially comprised of an annular body made of elastomeric material within which a plurality of filiform elements disposed in mutual side-by-side relation extend, said elements substantially performing the function of resisting the tensile stresses imposed to the belt in use. In very recent embodiments, the filiform elements are practically inextensible. This feature has been found to give rise to big problems when the belt is mounted to pulleys of very reduced diameter. This is due to the fact that the deflection to which the annular body of the belt it subjected when the belt is passed over the respective pulleys causes a contraction of the elastomeric material at the inner edge of the belt, which contractions are increasingly greater as the bending undergone by the belt becomes more marked. Therefore, when the pulleys have a very reduced diameter, the contractions can reach such an extent that important localized deformations may be produced in the whole section of the elastomeric material, the attachment areas to the filiform elements inclusive. Under these use conditions, the belt is always submitted to an early wear due to the fact that the elastomeric material is liable to become unglued from the filiform elements and to get cut against the same. Therefore, as a rule the belts provided with inextensible filiform elements are not used on pulleys having diameters smaller than 40 millimeters. In order to obviate the above restrictions, the same applicant has accomplished driving belts in which the filiform elements are elastically extensible so that a predetermined elastic elongation of the belt takes place when the belt is stretched, in use, on the respective pulleys. The elastic tensioning thus produced in the whole section of the elastomeric material serves to greatly reduce the contractions undergone by said material when the belts are passed over the pulleys. A driving belt of the above type has been the object of EP-A-0 339 249, to which one may refer for further information. In this patent application it is also pointed out that when the filiform elements are submitted to a tension equal to one tenth of their ultimate tensile stress, they must have a percent elongation neutralizing a contraction value of the innermost surface of the belt included between two limit-values corresponding to the belt curving according to two predetermined bendings. Said bendings are defined by respective circumferences the width of which corresponds to 60% and 35% respectively of the width of the circumference according to which the filiform elements in the belt extend. The belts having the features described in the above patent application enable an important reduction in the pulley size and the diameter of said pulleys can reach values on the order of 15 or 16 mm, without early wear problems occurring. However it has been found that, following the trend of reducing the pulley diameters as far as possible, belts adapted for use on pulleys having still smaller diameters are required. For meeting these requirements, the availability of very elastic belts should be necessary, capable of undergoing an important elastic elongation at the mounting so as to resist the strong contractions that the elastomeric material will tend to undergo on passing over the pulleys. However although this solution does not seem complicated at first sight, there are technical problems due to the fact that if the belt elasticity overcomes given parameters, the belt itself for several reasons is subjected to undergo important elongations which become permanent in time and will bring to a gradual decrease of the belt pre-tensioning. Thus the adhesion of the belt to the pulleys would be impaired, as well as, which is still more important, the belt capability of resisting the contractions of the elastomeric material when the belt is passed over the pulleys. FR-A-2 610 056 discloses an endless power transmission belt comprising a matrix (21) made of elastomeric material and a longitudinal reinforcement comprising twisted cords (20) embedded in said matrix, which have an overall extension modulus sufficiently low to allow mounting over pulleys of a transmission, while it mantains a minimum tension suitable to allow operating said trasmission. FR-A-2 610 056 does not teach or suggest a transmission system comprising at least a pulley having a diameter less than a predermined value and an endless belt having at a preset value of its ultimate tensile stress a residual elongation value that balances a contraction value of a radially innermost surface ranging between predetermined contraction values, wherein one of said contraction value corresponds to a bending line of said belt having a minimum preset percent value of the circumferential development of said belt. The main object of the present invention is substantially to solve the above technical problems. The foregoing and further objects which will become more apparent in the course of the following description, are substantially attained by a transmission system according to the characterizing portion of the claim 1. Further features and advantages will best be understood from the detailed description of a preferred embodiment of the present invention, given hereinafter by way of non limiting example with reference to the accompanying drawings, in which: Fig. 1 is a frangmentary perspective view of an elastic belt for a transmission system in accordance with the present invention; Fig. 2 is a load-elongation diagram referring to two different elastic belts; Fig. 3 shows an elastic belt stretched between two pulleys and showing permanent elongations and residual elastic elongations; Fig. 4 is a diagram representing the variation in time of the elastic reactions of the belt disposed in the conditions shown in fig. 3, as a result of the gradual increasing of the permanent elongations. Referring particularly to Fig. 1, an elastic belt for a transmission system in accordance with the present invention, has been generally identified by reference numeral 1. The elastic belt 1 conventionally comprises an annular body 2 made of elastomeric material and extending according to an endless line of predetermined length. In the example shown, the annular body 2 has a section of the so-called "poly-V" type, that is provided with several longitudinal grooves on its base surface. Actually, it exhibits a rectilinear outer side 3, two opposed lateral sides 4 defining the belt 1 width as a whole, and a lower side 5 having a plurality of substantially V-shaped notches defining respective grooves on the longitudinal extension of the belt. A number of filiform elements 6 is incorporated in the annular body 2 made of elastomeric material and they extend lengthwise in the body itself being disposed in side-by-side relation according to a substantially cylindrical surface at a predetermined distance "h" from the inner side 5. The line according to which the filiform elements 6 are distributed conventionally defines the neutral deflection axis of the belt 1 in a free state. In other words, when the belt is bent without any pre-tensioning being produced thereon, the elastomeric material disposed under the filiform elements 6 with reference to Fig. 1, undergoes an elastic contraction, whereas the elastomeric material disposed over the filiform elements is elastically stretched. In accordance with the present invention, the filiform elements 6 appear elastically extensible to such an extent that the belt 1, stretched between two elements having a fixed distance between centers with a starting tension equal to one tenth of its ultimate tensile stress, undergoes a permanent elongation in time at the end of which it will exhibit a residual elastic elongation of a predetermined value. In greater detail the residual elastic elongation is provided to have a percent value included between the percent contraction values detectable at the inner edge of the belt defined by the extension of the base side 5 thereof, when the belt is respectively bent according to two predetermined bending lines. One bending line can be defined by a circumference equal to 60% of the circumferential extension of the belt at the filiform elements 6, whereas the second bending line is defined by a circumference equal to 20% of said extension of the belt. If the above described features are respected, the belt 1 will have the possibility of being mounted to pulleys of very reduced diameter, without running the risk that the permanent elongations undergone by the belt at the mounting may give rise to an excessive reduction in the elastic tensioning of the same, bringing it to such values that it cannot conveniently resist the elastic contractions tending to be set up on the inner edge of the belt due to the belt deflection about the pulleys. The identification of the permanent elongations and the detection of the residual elastic deformations in the belts are carried out by a process in which it is provided that a load-elongation diagram be first drawn in connection with the belt under examination. Shown in Fig. 2 is a diagram representing two load-elongation curves marked "a" and "b" respectively, relating to two belts having different elastic features. The ordinates "y" in the graph of Fig. 2 define the loads relating to curves "a" and "b" whereas the abscissas "x" define the corresponding elongations. In addition, also represented on the abscissas "x" of said graph are the percent values "ε1" and "ε2" (with inverted signs) of the elastic contractions that can be detected on the inner edge of the belts under examination wound in a free state according to said first and second bending lines. Said contraction values can be calculated on the basis of the description made in the above mentioned patent. Also provided in the process is a step in which the belt 1 under examination is engaged between two pulleys 7, 8 having a diameter ranging between 10 and 50 mm. Pulleys 7 and 8 are pushed apart from each other in order to submit the belt 1, on each of its branches 1a, 1b, to a tensioning equal to one tenth of its ultimate tensile stress (see Fig. 3). As can be seen from Fig. 2, at one tenth of the tensile stresses "1/10 Fa" and "1/10 Fb", a starting elastic elongation "εa" and "εb" is identified on each curve "a" and "b". The distance between centers of pulleys 7 and 8 is kept unchanged while the pulleys are set in rotation and the tensioning of the belt 1 is constantly checked in time. Pulleys are driven in rotation at speeds substantially of the same range of magnitude as the maximum speeds of use, for example between 500 and 1000 rpm, as in the case of belts used for washing machines. Under this situation, it can be seen that the belt 1 is subjected to undergo permanent elongations causing a reduction in its starting elastic tensioning. Said tensioning therefore will be gradually reduced as far as it is stabilized to an almost constant value. The graph drawn in Fig. 4 shows, by two curves "Ka" and "Kb", the tensioning decreasing "F" in time "t" exhibited by the belts to which the curves "a" and "b" of the load-elongation diagram of Fig. 2 correspond. Both curves "Ka" and "Kb" prove that the tensioning produced at the beginning on the belt is subjected to decrease rather suddenly, afterwards reaching the above specified stable constant value. The stable constant values of the residual tensioning relating to curves "Ka" and "Kb" are respectively denoted by "Fca" and "Fcb" on the ordinates "F" in the paragraph of Fig. 4. The values "Fca", "Fcb" of the obtained residual tensioning is then transferred to the load-elongation graph corresponding to said tensioning value. The elongation, denoted by "εfa", "εfb", represents the residual elastic elongation exhibited by the belt once the constant value "Fca", "Fcb" of the tensioning between pulleys 7, 8 has been reached. The values "εfa", "εfb" can be immediately compared with values "ε1" and "ε2" to make possible to ascertain whether they are within the range defined by "ε1" and "ε2" themselves. With reference to figure 2, it is noted that the value "εfa" is inside to the range ε1, ε2 and the corresponding driving belt will be therefore suitable to be used for the above specified purposes. On the contrary the residual elongation "εfb" is outside of range defined by values "ε1" and "ε2", which fact means that the corresponding driving belt is not suitable to be on pulleys of very small diameter, unless elastic elongatin values higher than those closely necessary to resist the elastic contractions on the inner edge of the belt are accepted. The outstanding features of a particular elastic belt made in accordance with the invention are hereinafter described, by way of example only. longitudinal grooves, 1200j6: Primitive extension: 1178 mm. Radius "R" corresponding to the primitive extension: R = 1178/2 π = 187,5 mm. Number of V-shaped grooves: 6. Groove outline: "j" as per specifications. Groove height: 2 mm. Distance h: 3 mm. Minimum percent contraction value: "ε1": Maximum percent contraction value: "ε2": Filiform inserted elements: 17 coils of cord disposed in side by side relation according to a cylindrical surface spaced apart from the belt edge by a distance "h". Cord: made up of two strands each having 940 monofilaments (940 x 2), twists per cm of the 48z type for each strand and twists per cm of the 48s type on the two strands, one strand with the other. Cord diameter = 0.55 mm. Ultimate tensile stress of the cord: 145 N (Newton). Percent elongation of the cord at one tenth of the tensile stress: 3.5%. Tensile stress of the belt: 4,000 N (2000 N per branch) Elastic percent elongation of the belt at the beginning at one tenth of the tensile stress: 2.27%. Constant tensioning value of the belt obtained after the previously described dynamic test starting from a load of 400 N: 233 N. Value of the residual elastic percent elongation at a load of 233 N (Newton) included between 1.08% and 6.5%, measured with a value of: 1.35%. The present invention attains the intended purposes. The belts manufactured using the innovative measures suggested by the invention are in fact capable of working with great reliability also when combined with pulleys of very reduced diameter, in the range of 8 - 10 mm.

Claims: A transmission system comprising an elastic belt (1) comprising in turn an annular body (2) made of elastomeric material and a number of elastic filiform elements (6) resistant to tensile stress, embedded in said annular body (2) along a surface substantially corresponding to the position of a neutral axis of said annular body (2), said belt (1) being bent along a first bending line defined by a circumference having a radius equal to 60% of a radius (R) of a circumferential development of said annular body (2) corresponding to the position of said elastic filiform elements (6) undergoing bending to which derive a first conctraction value (E1) of a radially innermost surface (5) of said annular body (2) and being bent along a second bending line defined by a circumference having a radius equal to a preset percent value of said radius (R) of said circumferential development undergoing bending to which derives a second contraction value (E2) of said radially innermost surface (5) of said annular body (2), said elastic belt (1) at 1/10 of its ultimate tensile stress having an elongation value that balances a contraction value ranging between said first and said second contraction value, and at least two pulleys (7, 8), said belt (1) being mounted between said pulleys (7, 8) in a state of elastic pretension caracterized in that i) said preset percent value is equal to 20%, ii) said elongation is the value of a residual elastic elongation and iii) the diameter of the at least one of said pulleys (7, 8) is less than 15 mm.

Inventors: ROBECCHI EDOARDO, ROBECCHI, EDOARDO
Applicants: DAYCO PTI SPA, DAYCO PTI S.P.A.
Application year: 1990
Main classification: F16G1
IPC codes: F16H7, F16G1