TOOTHED PULLEY AND MEANS OF TRANSMISSION RELATED THERETO

Abstract: Toothed pulley (1) comprising a plurality of grooves (2) alternating with a plurality of teeth (3, 4). The flank (6, 7) of each groove is joined to the head of the adjacent tooth (3, 4) by means of a connection (16, 17) formed of at least two convex curvilinear segments (18, 19) and a recess (20). The curvilinear segments are tangent to the end of the flank and to the terminal point of the tooth head delimitated by the outer circumference (12) of the pulley respectively. The pulley engages with a toothed belt comprising in a preferred embodiment a connection between the tooth flank and the adjacent groove, for example in the form of a parabolic arc.

Description: The present invention relates to a toothed pulley and the means of transmission related thereto and more particularly to a toothed pulley with an improved connection between the teeth and the grooves adjacent to the teeth. It is known that a toothed pulley comprises a plurality of grooves alternating with a plurality of teeth and that each groove is joined to the teeth by a connection in the form of a circular arc. The toothed belt required to mesh with the pulleys in a synchronous transmission system comprises an elastomeric annular body delimitated by two faces, from at least one of which projects a plurality of teeth alternating with a plurality of grooves. Each tooth is also joined to the adjacent grooves by means of a connection in the form of a circular arc. It is usual in the transmissions referred to for the connections of the pulley and the belt engaging with the pulleys to have circular arcs whose radii are substantially identical or at all events of comparable values (see e.g. GB-A-2 159 602 which discloses the features of the preamble of claim 1). During the transmission of motion, a state of stress is generated by the relative mechanical thrusts between the belt teeth, which are made of elastically deformable material, and the pulley teeth, which are made of rigid material. This stress is distributed over the body of the belt tooth. Practically speaking, the belt tooth, which is anchored along its base to inextensible longitudinal strands, behaves in the same way as a beam restrained at one end and subjected to a vertical load at the opposite end. In the proximity of the base of the tooth, therefore, the elastomeric material receives the thrust transmitted by the pulley tooth and transfers it to the longitudinal strands. Unfortunately, the state of stress at the base of the belt tooth is unevenly distributed and there is an accentuated localisation of the forces acting on the rubber in the area of connection between the flank of the loaded belt tooth and the adjacent groove. The said localisation of forces, and particularly their intensity may over the course of time result in irreparable impairment or the correct operation of the transmission and the working life of the belt itself. Attempts have been made to overcome the drawback mentioned by creating circular connections arcs at the base of the belt and on the head of the pulleys with different radii, giving the radius of the pulley head connection a greater value than that of the belt connection with a view to preventing the pulley head connection from compressing the base connection of the belt tooth in a confined area. This solution, however, may prove insufficient, since when very heavy loads are transmitted by the flank of the pulley tooth to the flank of the belt tooth there is a tendency for the connection zone in elastomeric material at the base of the belt to divaricate. In particular, even if the preceding solution is adopted, there is still a risk of extremely localised pressures on the base of the belt tooth and hence an irregular distribution of forces along the base of the belt. It must also be added that resort to the preceding solution inevitably results in the transfer of loads at a higher point on the belt tooth. Unfortunately, this circumstance has an adverse effect, since it can readily be understood that the belt tooth is increasingly subject to deformation in flexion the further the load is applied away from its base. Attention must equally be drawn to the fact that circular arc connections between grooves and teeth in a pulley could be unsuitable for employment with toothed belts such as those disclosed in EP-A-0 368 099 also filed by the present Applicant. Briefly, it may be stated that the belts described therein comprise a base connection formed of a curvilinear segment whose length is greater than that corresponding to a circular arc. In essence, according to the solution referred to, the curvilinear connection segments at the base of the tooth could, for example, be formed with a parabolic arc so as to permit the build-up of a greater quantity of elastomeric material compared with a circular arc without altering the dimensions of the tooth. Even though adoption of the toothed belt referred to in a transmission system composed of pulleys with a conventional profile might continue to be satisfactory, account must be taken of the fact that under particular operating conditions the belt tooth could rise up from its correct position of engagement with the pulley. Since the toothed belt connection, in the form of a parabolic arc, for example, would in fact be closer to the ciruclar arc connection of the pulley tooth, there would be a consequent reduction of the free space in the proximity of the opposed connections when the belt tooth is inside the pulley groove. Subsequently owing to load variations occurring during the transmission of motion, the flat head of the pulley tooth could shift with respect to the positions where it is centred with the central axis of the groove between the belt teeth with the result that the circular arc of the pulley tooth connection by striking the parabolic connection of the belt tooth would exert a mechanical thrust, possibily followed by shifting of the tooth from its correct meshing position. Furthermore and in general terms, it has been observed that in known pulleys there may be interference between the summit of the belt tooth and that of the pulley tooth in the transient access phase under a particularly heavy load. This interference is generated by elongation of the pitch between the belt teeth resulting in unwanted approximation of the pulley tooth and the belt tooth that has to mesh. The purpose of this invention is thus to provide a toothed pulley and the transmission related thereto capable of preventing the concentration of forces in the connection area at the base of the belt tooth with which the pulley meshes and any unwanted interference during engagement under heavy loads, and also of permitting their dependable and correct employment with toothed belts whose connection at the base of the tooth involves the provision of a greater quantity of elastomeric material compared with a circular arc connection for a belt tooth of the same dimensions. The object of the invention is a toothed pulley of predetermined external circumference, the said toothed pulley comprising a plurality of grooves alternating with a plurality of teeth, each groove being symmetrical with respect to a first central radial line and formed of a base and two extended flanks starting from the base and continuing as far as two ends located on a second predetermined circumferential line with a smaller radius than that the outer circumference, each tooth being symmetrical with respect to a second central radial line and formed of a head delimitated by the outer circumference over a segment contained within two terminal points, each flank having one end connected by means of a connection to the terminal point of the head of the adjacent tooth, the said pulley being characterised in that the said connection comprises two convex curvilinear segments and a recess between them, the said curvilinear segments, the first and the second respectively, being tangent to the flank at the final end thereof and to the head at the terminal point thereof respectively. Another object of the invention is a means for the transmission of motion between a toothed belt made of elastomeric or similar material and at least two pulleys such means being characterised in that it comprises at least one pulley of the type hereinbefore described. The present invention will be more readily understood from the following detailed description of some of its specific embodiments given by way of non-limiting examples with reference to the accompayining drawings, in which: figure 1 is a longitudinal section of a toothed pulley showing a groove and the teeth adjacent to the same; figure 2 is an enlarged view of the head connection of the pulley shown in figure 1; figure 3, 4 and 5 show some variants of the connection illustrated in figure 2; figure 6 shows the pulley illustrated in figure 1 and a toothed belt in partial longitudinal section; figure 7a and 7b are further variants of the head connection of a toothed pulley according to the invention; figure 8 shows a transmission between a toothed belt and a pulley with a head connection as illustrated in figure 7. As can be seen in figure 1, the pulley comprises a groove (2) between adjacent teeth (3 and 4). The groove (2) is symmetrical with respect to the central and radial axis L-L and is formed of a base (5) of length D and of two curvilinear flanks (6 and 7) each of which extends from initial ends (8 and 9) located on the base of the groove to final ends (10 and 11). The curvilinear profile of the flank at the final ends (10, 11) is defined by the tangent t forming the pressure angle c with the radial lines (ℓ) passing through the initial ends (8, 9) on the base. The groove is also defined by a height H measured along axis L-L between the groove base (5) and the outer circumferential (12) of predetermined radius R. The profile delimitating the flanks is tangent to the flank at initial ends (8, 9) and is for example a profile in the form of a parabolic arc drawn with respect to axes x and y passing through one of the ends at the base of the groove. Ends (10, 11) of the flanks are located on a second circumferential line with a smaller radius than the outer one. In general terms, if account is taken of the difference between the outer and the inner radius, it is found that the ratio between this difference and the height H lies between 0.08 and 0.43. Each of the teeth (3, 4) adjacent to the groove (2) is symmetrical with respect to a second central radial line L′-L′ and is formed of a head (13) delimitated by the outer circumference (12) over a segment contained within two terminal points (14, 15). The fundamental features of the pulley are the connections (16, 17) between the final ends (10, 11) of the flanks and the terminal points (14, 15) of the heads (13) of the adjacent teeth (3 and 4). In its more general form, each connection is constituted of two curvilinear convex segments (18, 19) and recess (20) between the said segments. In addition, the two convex curvilinear segments are tangent to the flank at the final end thereof and to the head of the adjacent tooth at the terminal point thereof respectively. The two curvilinear segments may have a variety of circular or substantially circular profiles, for example parabolic, elliptical or similar. Figure 1 illustrates one of the possible embodiments of connections (16 and 17) based on two circular curvilinear segments that intersect at a point to form the recess (20). As can be seen in the enlarged view in figure 2, connection (16) comprises a first circular arc (18) and a second circular arc (19). The first arc is defined by a curvature centre (21) located on the perpendicular N to tangent t at the lower end point of the flank and the second arc (19) by a curvature centre (22) located on the diametral line F passing through the terminal point (14) of the head of tooth (3). In a preferred embodiment corresponding to the drawing in figure 2, the first and second circular arcs have the same curvature radius. Furthermore, the first and second circular arcs also have particular radius values in relation to the connection of known type, which is defined, as can be clearly seen in figure 2, by a single circular arc (23) having its curvature centre (24) located on the intersection of straight lines N and F. In more precise terms, the radius of circular arcs (18, 19) is less than the radius of the single circular arc and the ratio between one of the radii of the two circular arcs (18 or 19) and the radius of the single circular arc (23) lies between 0.3 and 0.8, for example such ratio is 0.5. Figure 3 illustrates a further embodiment of connection (16) characterised in that the two circular arcs (18, 19) are tangent to each other at the point (25) of maximum depth of recess (20). The connection illustrated in figure 4 differs from those already described by virtue of the fact that it comprises two circular arcs (18, 19) that differ from each other, circular arc (18) having the greater radius. The connection illustrated in figure 5 is even more different from those already described insofar as it comprises two circular arcs (18, 19) that are no longer intersecting nor tangent to each other, but united by a third circular arc (26) whose curvature is opposite to that of the lateral arcs so as to form a recess (20). The pulley in figure 1 with the head connections hereinafter described can be employed with constant pitch toothed belts, for example with belts forming parts of the transmissions described in U.S. patent 4,696,665 granted to the present Applicant, the text of which is incorporated herein to provide the dimensional data for the belts and pulleys open for use in combination with the characteristics of the invention. A possible embodiment of a transmission with a toothed pulley according to the invention is illustrated by way of example in figure 6. In this schematic representation, the pulley 1 with a substantially step-like connection (16) meshes with a toothed belt (27) comprising a tooth (28) with parabolic flanks (29, 30) joined to the adjacent grooves by means of circular arc connections (32 and 33). With reference to figures 1 and 2 and solely by way of example, the values for a pulley according to the invention are set out below: - Pitch p = 9,525 - Number of teeth = 20 - Radius of outer circumference R = 29.57 mm - Radius of inner circumference passing through final ends (10,11) of the flanks = 0.97 R - Height H of the groove = 3.45 mm - Distance D between initial ends (8, 9) of the groove base = 2.26 mm - Distance T between tangents t = 5.8 mm - Pressure angle c = 16 degrees - Curve of parabolic arc flanks (8, 9) y = 1.1 x² - Curvature centre C of the single circular arc (23) located on a circumference whose radius relative to that of the outer circumference (12) is 0.34 R - Distance of curvature centre (24) from axis L′-L′ = 1.05 mm - Ratio between radius of circular arc (18) and radius of arc (23) = 0.5 Figure 7 provides an enlarged view of a further embodiment of connections (16, 17) illustrated in figure 1. As can be clearly seen, in this solution the curvature radius of the first and that of the second circular arc are still located on straight lines N and F respectively. However, the radius of the first circular arc (18) is appreciably smaller than the radius of the second circular arc (19). In particular, the ratio between the radius of arc (18) and that of arc (19) is less than 0.1 for example it is equal to 0.08. The pulley illustrated in figure 7 is preferably employed with toothed belts such as those disclosed in EP-A-0 368 099 also filed by the present Applicant. Briefly, the said belt comprises teeth alternating with grooves and each tooth comprises two flanks and two roots connecting the flanks to the adjacent grooves. Each root is formed of a curvilinear segments whose ends, first and second, are located on the end of the flank and on the base line delimitating the grooves respectively. The tooth is defined by a tangent on the flank at the point of intersection between root and flank forming an angle of predetermined value with the groove base line. The fundamental feature of such belt lies in the fact that the first end of the curvilinear root segment coincides with the first terminal point of a circular arc tangent to the flank and to the base line with a curvature centre on the bisectrix of the said angle of predetermined value. The second end of the curvilinear segment is located between the second terminal point of the circular arc referred to and the point of intersection between the central line of the groove and the base of the groove. Figure 7b illustrates a further embodiment comprising a connection (16) for example, for pulleys with a 14 mm pitch, once again formed of a first curvilinear segment (18) tangent to the flank at the terminal point and a second curvilinear segment (19) tangent to the head of the tooth at the terminal point. In this arrangement, however, there is also a third convex curvilinear segment (19′) between the first and the second curvilinear segments, and the recess (20) is formed by the intersection between the first and the third curvilinear segment (18 and 19′). Preferably, the first and the third segment (18 and 19′) are circular arcs, whereas the second segment (19) is a parabolic arc. In more specific terms, the first circular arc has its curvature centre (21) on the perpendicular N to the tangent t at the lower end point of the flank, the second curvilinear segment (19) is a part (14′ 14) of a single parabolic arc (23′) that originates at the lower end point of the flank, where it is tangent, and terminates tangent to the terminal point of the tooth head, and the third circular segment (19′) has its curvature centre on a perpendicular F′ to the tangent at a point on the single curve (23′). In particular, when the curvature centre of the third circular arc segment (19′) is on the perpendicular at the end of the single parabolic curve (23′) coincident with the terminal point of the tooth, the solution in figure 7b becomes the same as that in figure 7a. The curve of the single parabolic arc (23′) in figure 7b is none other than the form of the connection of the pulley head (7) described in the EP-A-0 368 099 filed by the present Applicant already mentioned. The pulley in figure 7b, therefore, thanks to the presence of the recess (20), is suitable for engagement with a toothed belt having a connection at the tooth base like that described in the preceding patent application with the advantages and overcoming of the drawbacks associated with the prior art as explained in what follows. When the tooth of the belt with the parobolic base connection is inside the pulley groove, as shown in figure 7b, the final end of the flank of the groove relatively to the final end of the belt tooth is preferably at a greater distance with respect to the plane of the longitudinal strands of the belt. Figure 8 next shows a pulley according to the invention with a toothed belt comprising a curvilinear connection as described in the previously referred to application filed by the present Applicant, that is to say a curvilinear connection extending over a greater distance than a connection in the form of the arc of a circle. In greater detail, the toothed belt (35) in figure 8 comprises teeth (36) with curvilinear flanks (37, 38) joined to the adjacent grooves (39 and 40) by means of connections (41, 42). The said connections (41, 42) can be more particularly constituted of parabolic arc profiles tangent to both the tooth flank and the base line delimitating the grooves between the belt teeth. Alternatively, the said connections (41, 42) may be arcs of an ellipse, a hyperbola or other similar curves. Moreover, the teeth of the belt can be coated as usual with one or more layers of fabric, for example in nylon. The pulley (1) in figure 8 comprises a groove (43) with adjacent teeth (44 and 45) and connections (16 and 17) between the flanks and the teeth. Dashed lines are also included in figure 8 to indicate connections with a conventional circular arc profile both at the base of the belt tooth and on the head of the pulley tooth. The invention attains all the aims proposed. To provide an explanation of the results achieved, consideration will now be given to figure 6, in which the belt tooth (28) is shown in the position corresponding to the absence of a load to be transferred between belt and pulley. In the loading condition, flank (30) of belt tooth (28) shifts from the condition illustrated in the figure and brings itself into contact with the flank of the pulley groove. In this condition, the ends of the circular connection (33) of the belt find themselves in contact with the ends of connection (16) between groove flank and pulley tooth. None the less, the corner areas of the two connections are free from contacts thanks to the presence of the recess (20). The elastomeric part around the corner area of the connection (33) (marked with a dashed line in the figure and indicated with the letter Z) is free from the localised stresses observed in known toothed transmissions. Consequently, the present invention permits the attainment of substantially uniform tangential stress along the base of the belt tooth. In particular, the solution offered by the invention makes it unnecessary to resort to a circular pulley tooth connection with a radius greater than that of the circular connection at the base of the belt tooth. As the result, a belt tooth meshing with the present pulley is subjected to less deformation in flexion compared with the solution referred to from the prior art. Next, examination of the transmission illustrated in figure 8 permits substantial repetition of the remarks concerning the absence of stresses localised at the base of the belt tooth along the corner connection area made in connection with figure 6. To provide a better explanation of the result obtained, consideration will first be directed to a transmission comprising a belt (35) with a base connection (41) identified by an arc of greater extent than a circular arc, namely an arc constructed in such a way as to greater build-up of elastomeric material compared with a circular arc, as is moreover evidenced by the free space between the profile (41) and the underlying pecked profile in the form of a circular arc. It is again assumed that belt (35) with connection (41) is meshed with a pulley groove in which the head connection is formed by the single circular arc drawn in figure 8, in other words a connection known in state of-the-art pulleys. During the change from the condition shown in figure 8 to loading conditions, there is an approximation of the profiles of the belt flank and the pulley flank. In the course of the said relative movement, the circular arc profile of the belt connection, on finding a curvilinear profile at a greater inclination with respect to profile of a circular arc, would tend to transmit upward-directed mechanical thrusts onto connection (41) with the risk of tending to shift the belt out of its correct attitude for proper engagement. Connection (16) according to the present invention overcomes the drawback thus referred to. In effect, the formation of a connection (16) consisting of two circular arcs instead of a single circular arc, or alternatively as illustrated in figure 7b, and in particular the presence of the recess (20), removes material with respect to the conventional circular connection, and thus does away with the presence of unwanted mechanical thrusts tending to shift connection (41) and with it belt tooth (35) from the correct meshing position. Furthermore, in the final position, connections (41 and 16) only come into contact in the proximity of their respective ends, thus leaving their respective corner areas free from unwanted localised stresses. Mention must also be made of the fact that the present solution overcomes the drawback of unwanted mechanical interference between the summit of the belt tooth and that of the pulley tooth during meshing under a heavy load. On the assumption that the pitch is lengthened, in fact, even the presence of a relative shift between the belt tooth and the pulley tooth, it turns out that the particular solution offered by the invention, thanks to the recess (20) facilitates meshing, and hence reduces belt tooth wear and deformation.

Claims: Toothed pulley (1) of predetermined external circumference, said pulley comprising a plurality of grooves (2) alternating with a plurality of teeth (3, 4), each groove being symmetrical with respect to a first central radial line (L-L) and formed of a base (5) and two extended flanks (6, 7) starting from the base (8, 9) and continuing as far as two final ends (10, 11) located on a second predetermined circumferential line with a smaller radius than that (R) of the external circumference, each tooth being symmetrical with respect to a second central radial line (L′-L′) and formed of a head (12) delimitated by the outer circumference (12) over a segment contained between two terminal points (14, 15), each flank having a final end connected by means of a connection (16, 17) to the terminal point of the head of the adjacent tooth, said pulley being characterised in that said connection comprises at least two convex curvilinear segments (18, 19) and a recess (20) between them, said curvilinear segments, first and second respectively, being tangent to the flank at the final end thereof and to the head at the terminal point thereof respectively. Pulley as in claim 1 characterised in that said two convex curvilinear segments meet at a point (20) and form said recess. Pulley as in claim 1 or 2 characterised in that said two curvilinear segments have the same profile. Pulley as in claim 1 or 2 or 3 characterised in that said two curvilinear segments are of equal length. Pulley as in claim 1 characterised in that said curvilinear segments are constituted of substantially circular arcs (18, 19). Pulley as in claim 1 characterised in that said two curvilinear segments and said recess are originated by two circular arcs, first and second respectively, said first circular arc being tangent to the flank and having its curvature centre (21) located on the perpendicular (N) to the tangent (t) at the end point (10) and said second circular arc being tangent to the head at the terminal point (14) and having its curvature centre (22) on the diameter (F) of the outer circumference of the pulley passing through the terminal point (14) of the head of the tooth (3). Pulley as in claim 6 characterised in that the radius of said circular arcs, first and second, is smaller than the radius of a single circular arc (23) having its centre (24) at the point of intersection between said perpendicular (N) to the tangent to the flank and said pulley diameter (F) passing through the terminal point (14) of the head, said single circular arc being drawn between the flank and the head of the tooth. Pulley as in claim 7 characterised in that the ratio between the radii of the two circular arcs, first and second, and the radius of the single circular arc lies between 0.3 and 0.8. Pulley as in claim 7 characterised in that said circular arcs intersect at a point and form said recess (20). Pulley as in claim 7 characterised in that said circular arcs are tangent to each other at a point (25) and form said recess. Pulley as in claim 1 characterised in that said two convex curvilinear segments are two circular arcs devoid of reciprocal points of contact and that said recess is formed by a third concave curvilinear profile (26) tangent to said lateral curvilinear segments. Pulley as in claim 6 characterised in that the ratio between the radius of the first circular arc and the radius of the second circular arc is less than 0.1. Pulley as in claim 6 characterised in that the ratio between the radius of the first and second circular arc is 0.08. Pulley as in claim 1 characterised in that there is comprised between said two curvilinear segments (18, 19) tangent to the flank at the final end thereof and to the head at the terminal point thereof respectively a further convex curvilinear segment (19′), said first (18) and said further (19′) segments being successive circular arcs and said second segment (19) being a parabolic arc and constituing the final part of a single parabolic curve (23′) having its vertex at the final end of the flank, said first circular arc (18) having a radius of curvature (21) on the perpendicular to the tangent to the flank at its final end point, said further circular arc segment (19′) having its curvature centre on a line perpendicular to the tangent at a point of said single parabolic curve. Transmission of motion between a toothed belt (27) made of elastomeric or similar material and at least two pulleys characterised in that it comprises at least one pulley as claimed in any of the preceding claims. Transmission of motion between a toothed belt and at least two pulleys,said toothed belt comprising a plurality of teeth (28) and a plurality of grooves alternating with said teeth, each tooth being formed of two roots (32, 33), two extended flanks (29, 30), each root comprising a curvilinear segment (33) whose ends, first and second, are located on the end of the flank and on the base line of the groove respectively, each tooth having a tangent at the point of intersection between root and flank forming an angle of pretermined value with the groove base line, said transmission being characterised in that it includes at least one pulley as claimed in claim 1 engaging with a toothed belt in which the first end of the curvilinear root segment coincides with the first terminal point of a circular arc tangent to the flank (29, 30) and to the base line with a curvature centre on the bisectrix of said predetermined angle and said second end of the curvilinear root segment is located between the second terminal point of said circular arc and the point of intersection between the central line of the groove and the base of the groove and at most is located on said point of intersection on the base of the groove. Transmission of motion between a toothed belt (35) and at least two pulleys, said toothed belt comprising a plurality of teeth (36) alternating with a plurality of grooves (39, 40) delimitated by a base line when the belt is stretched, each tooth being joined to the adjacent grooves by a curvilinear connection (40, 41), said transmission being characterised in that it comprises a belt in which said curvilinear connection segment between the tooth and the adjacent groove is a parabolic arc tangent to the flank and the base line, and at least two pulleys as claimed in claim 12. Transmission of motion between a toothed belt and at least two pulleys characterised in that said belt comprises teeth that alternate with grooves and are joined to each other by a parabolic arc segment and said pulleys are as claimed in claim 14.

Inventors: DI GIACOMO, TOMMASO, MACCHIARULO, VINCENZO
Applicants: PIRELLI TRASMISSIONI INDUSTRIALI S.P.A.
Application year: 1989
Main classification: F16H55
IPC codes: B65G15, F16H7, F16G1, F16H55