GB2232222A

GB2232222A - Pulley for a toothed belt power transmission system

Info

Publication number: GB2232222A
Application number: GB8912184A
Authority: GB
Inventors: Francis Albert Shaw Innes
Original assignee: Gates Rubber Co
Current assignee: Gates Rubber Co
Priority date: 1989-05-26
Filing date: 1989-05-26
Publication date: 1990-12-05
Anticipated expiration: 2009-05-26
Also published as: GB8912184D0; GB2232222B

Abstract

A toothed pulley 1 for a synchronous power transmission system; the pulley being provided with teeth 2 and grooves 3 around its circumference in which the radius of the envelope of the pulley teeth is greater at a first axial face 11 of the pulley than at a second axial face 12 of the pulley. The cross-section in any radial plane through the tip of any pulley tooth may be a continuous curve from the first to the second axial face. Alternatively the cross-section may include a straight line followed by a continuous curve (as shown). <IMAGE>

Description

POWER TRANSHISSION SYSTEM AND PULLEY THEREFOR This invention relates to a synchronous power transmission system and to a toothed pulley for use in such system. Synchronous drive systems utilising toothed pulleys and toothed belts are now used extensively in industry, and particularly in the automotive industry.

One major problem that is experienced with such systems is the generation of noise. This arises from the regular engagement and disengagement of the belt teeth with the pulley grooves, which causes vibration of the belt. In any drive system it may well be the case that a driving, driven or idler shaft carrying a toothed pulley has a natural frequency which is the same as, or a multiple of, the frequency of the belt vibration. Under these circumstances, resonance occurs and can create high and quite disturbing noise levels. When synchronous belts are employed on drive from an automotive engine it will generally be found that resonance occurs in some portion of the speed range of the engine.

There have been many proposals in the past to achieve noise reduction. The principal work done has been directed to the shape of the individual belt teeth and pulley grooves, and simply as examples of these there can be mentioned US patent no. 4037485, UK patent no. 2084688 and US patent no. 4371363. However, all these known belts vibrate when employed in synchronous power transmission systems, and no successful means for solving the vibration problem has yet been found.

The present invention thus seeks to provide a synchronous power transmission system, and a pulley for use in that system, which will mitigate the generation of unpleasant noise.

According to the present invention a toothed pulley for a synchronous power transmission system has teeth and grooves around its circumference, and the radius of the envelope of the pulley teeth is greater at a first axial face of the pulley than at a second axial face of the pulley. The invention also extends to a synchronous power transmission system comprising a pulley as aforesaid together with an endless flexible belt provided with teeth and grooves along its length, the belt teeth being engageable with the pulley grooves.

Proper application of this principle to any pulley and groove configuration may lead to a desired noise reduction at certain transmission speeds. Without wishing to be limited to any theory of operation, the applicants at present consider that there are three factors which appear to contribute to noise reduction. Towards the first axial face of the pulley it will generally be the case that the belt is supported by engagement of the belt grooves with the tips of the pulley teeth. Towards the second axial face of the pulley, however, where the pulley diameter has been relieved, it may be the case that belt support is by engagement of the tips of the belt teeth with the bottom of the pulley groove, and this contact serves to damp the vibration generated towards the first axial face of the pulley.Secondly, contact between the flank of each belt tooth and each pulley groove is spread over a short time interval, rather than occurring substantially instantaneously over the whole width of the belt as is the case with the conventional constant radius pulleys. Flank impact, and thus noise generation, may thereby be reduced.

Thirdly, it will be appreciated that the belt tension towards the second axial face of the pulley is somewhat less than the belt tension towards the first axial face of the pulley. This tension change causes the resonant frequency to differ across the belt width, and can again lead to noise reduction.

The power transmission capacity of a synchronous power transmission system according to the invention is somewhat reduced in comparison to a similar system using constant diameter pulleys. Nevertheless, there are many applications in which this loss will be acceptable, particularly if accompanied by noise reduction.

The invention may be applicable to pulleys having any of the tooth and groove configurations found in the art of synchronous power transmission systems. These generally range from simple trapezoidal teeth and grooves to more complex configurations for tooth tips, tooth flanks and groove bases.

In one embodiment of the invention the cross-section in any radial plane through the tip of any pulley tooth is a continuous curve from the first to the second axial face of the pulley. In other embodiments the cross-section is a discontinuous curve, although it is preferred to avoid configurations where the diameter changes in step fashion at any part of the tooth.

In one preferred embodiment, the cross-section in any radial plane through the tip of any pulley tooth includes a straight line parallel to the pulley axis and extending from the first axial face of the pulley to an intermediate axial plane of the pulley, and a continuous curve extending from the intermediate axial plane towards the second axial face of the pulley.

The distance between the first axial face and the intermediate plane may be from 0.1 to 0.4 the axial width of the pulley, more preferably from 0.2 to 0.25 the axial width of the pulley.

The continuous curve may be an arc of a circle having a centre lying in the intermediate axial plane and a radius equal to at least 1.5 times the diameter of the envelope of the pulley teeth at the first axial face of the pulley. The curve may, of course, have a configuration other than that of the arc of a circle, for example a part-elliptical configuration.

The continuous curve may extend to the second axial face of the pulley, or it may extend to a second intermediate axial plane of the pulley, with the tooth cross-section from the second intermediate axial plane to the second axial face of the pulley being a second straight line parallel to the pulley axis. Alternatively, the crosssection from the second intermediate axial plane to a third intermediate axial plane may be a second continuous curve, with the cross-section from the third intermediate axial plane to the second axial face being a second straight line parallel to the pulley axis.

In any of these configurations the difference in radius of the envelope of the pulley teeth at the two axial faces is preferably substantially equal to the pulley tooth height. Effectively this means that the teeth have "disappeared" at the second axial face of the pulley, the section of the pulley adjacent to that face being substantially smooth surfaced.

In order that the invention may be better understood, examples of specific embodiments of pulleys and transmission systems in accordance therewith will now be described in more detail, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a cross-section in a radial plane through a first embodiment of pulley according to the invention; Figure 2 is a section through one pulley groove on the line II-II of figure 1; Figures 3 and 4 illustrate belt engagement with the pulley adjacent to opposite axial sides of the pulley; Figure 5 is a part section in a radial plane through a second embodiment of pulley; Figure 6 is a cross-section on the line VI-VI of figure 5, showing one pulley groove; and Figure 7 shows two graphs illustrating test results.

Referring now to figures 1 and 2 these show a pulley 1 designed to be mounted on a shaft (not shown) for use in a synchronous power transmission system. The pulley has a plurality of teeth 2 and grooves 3 provided at a predetermined pitch around its circumference. Each pulley groove is of a curvilinear section as shown in figure 2, each flank 4a, 4b of the groove merging into an adjacent tooth tip by a circular arcuate section 5a, 5b.

Each pulley tooth tip has a substantially identical cross-section in any radial plane taken through the pulley tooth, that cross-section being shown in figure 1. As will be apparent from that figure the cross-section from a first axial face 11 of the pulley to an intermediate axial plane A-A is a straight line parallel to the axis of the pulley.

From the plane A-A to the second axial face 12 of the pulley the cross-section is a continuous curve in the form of an arc of a circle having radius R, drawn from a centre 13 lying in the plane A-A, and intersecting the second axial face 12. The radius R is chosen so that the tooth height reduces from its maximum value h at the first axial face 11 to zero at the second axial face 12.

For a pulley of radius r and of axial width w it is preferred that the distance from the first axial face 11 to the plane A-A is from 0.2 to 0.25 W, and that the radius R is at least 3r.

The effect of using with the pulley of figures 1 and 2 with a toothed belt having teeth and grooves provided at the same pitch as the pulley teeth and grooves, the belt teeth being of a shape designed to cooperate effectively with the pulley grooves, is shown in figures 3 and 4. Figure 3 shows the relationship adjacent to the first axial face of a pulley driven by the belt in the direction of the arrow, with the belt 20 being supported by virtue of engagement between the tips 21 of the pulley teeth and the bases of the grooves 22 between adjacent belt teeth. It will be noted that the tips of the belt teeth do not engage the bases of the pulley grooves, although there is of course flank to flank contact between the belt teeth and pulley grooves in order to effect transmission to the pulley.

However, as one moves closer to the second axial face of the belt, as shown in figure 4, it will be seen that the tips 23 of the belt teeth do come into engagement with the bases 24 of the pulley grooves, so that the belt gains its support from this region. A clearance therefore appears between the bases of the belt pulley grooves and the particular section of the pulley tooth tips. The effective diameter of the belt around the pulley thus reduces towards the second axial face, with corresponding reduction in belt tension towards that axial face.

As already explained, it is thought that this tension reduction across the belt, together with the different belt/pulley contact regions across the pulley width contribute to noise reduction. It is also thought that noise reduction may occur due to the fact that the belt teeth move into engagement with the pulley flank over a short time interval, rather than having immediate impact across the whole width of the pulley flank.

Figures 5 and 6 illustrate a particular pulley design that has been tested on the water pump drive of a Rover engine. Reduction in pulley radius from the rear face 30 of the pulley to the front face 31 thereof is indicated on the figure. It will be noted that the profile so generated is a straight line section 32 from face 30 to a first intermediate plane A-A, a second continuous curve section 33 from plane A-A to plane B-B, which is, or approximates to, a circular arc having a radius equal to at least three times the pulley radius at the rear face 30, and a further section 34 between plane B-B and a further intermediate plane C-C which is another continuous curve, again drawn to a circular arc. From plane C-C to the front face the cross-section is a straight line 35 parallel to the pulley axis.The distance between the straight line sections at the two faces of the pulley is equal to the pulley groove height h, so that at the front face of the pulley the surface thereof is effectively free of teeth. The radius of the pulley tooth tips 5a, 5b is maintained across the full width of the pulley. despite the changing pulley tooth height.

Figure 7 shows the results of tests using the pulley of figure 6 in comparison with a standard pulley having teeth and grooves of the same pitch, but the teeth being of a constant tooth height h across the full width of the pulley. The graph on the left shows the two systems operated at low belt tension, i.e. 200 Newtons. It illustrates a significantly reduced noise level utilising the modified pulley, over a speed range from 700 to 1100 rpm. The graph on the right shows a similar comparison with the two systems operating at high belt tension, i.e. 300 Newtons, and illustrates a significant noise reduction between 800 and 1200 rpm.

It will be understood that the optimum curve for reduction of the tooth diameter may vary according to the tooth and groove shape of the pulley, to the pulley diameter and to the environment in which the pulley is used. The optimum configuration may thus be determined empirically for any given situation. It will further be understood that in any given drive transmission system only one pulley may need to be a modified pulley in accordance with the invention, or more than one pulley may need to be so modified. If more than one pulley is modified then the tooth cross-sections of those pulleys may be the same or different.

Claims

1. A toothed pulley for a synchronous power transmission system; the pulley being provided with teeth and grooves around its circumference in which the radius of the envelope of the pulley teeth is greater at a first axial face of the pulley than at a second axial face of the pulley.

2. A toothed pulley according to claim 1 in which the cross-section in any radial plane through the tip of any pulley tooth is a continuous curve from the first to the second axial face of the pulley.

3. A toothed pulley according to claim 1 in which the cross-section in any radial plane through the tip of any pulley tooth includes a straight line parallel to the pulley axis and extending from the first axial face of the pulley to an intermediate axial plane of the pulley, and a continuous curve extending from the intermediate axial plane towards the second axial face of the pulley.

4. A toothed pulley according to claim 3 in which the distance between the first axial face and the intermediate axial plane is from 0.2 to 0.25 the axial width of the pulley.

5. A toothed pulley according to claim 3 or claim 4 in which the continuous curve is an arc of a circle having a centre lying in the intermediate axial plane and a radius equal to at least 1.5 times the diameter of the envelope of the pulley teeth at the first axial face of the pulley.

6. A toothed pulley according to any one of claims 3 to 5 in which the continuous curve extends to the second axial face of the pulley.

7. A toothed pulley according to any one of claims 3 to 5 in which the continuous curve extends to a second intermediate axial plane of the pulley, and the crosssection from the second intermediate axial plane to the second axial face of the pulley is a second straight line parallel to the pulley axis.

8. A toothed pulley according to any one of claims 3 to 5 in which the continuous curve extends to a second intermediate axial plane of the pulley, the cross-section from the second intermediate axial plane to a third intermediate axial plane is a second continuous curve, and the cross-section from the third intermediate axial plane to the second axial face is a second straight line parallel to the pulley axis.

9. A toothed pulley according to any one of the preceding claims in which the difference in radius of the envelope of the pulley teeth at the two axial faces is substantially equal to the pulley tooth height.

10. A synchronous power transmission system comprising an endless flexible belt provided with teeth and grooves along its length, and a toothed pulley provided with teeth and grooves around its circumference, the belt teeth being engageable with the pulley grooves, in which the pulley is as claimed in any one of the preceding claims.

11. . A synchronous power transmission system according to claim 10 in which the belt tooth height is less than the pulley tooth height, and the difference in radius of the envelope of the pulley teeth at the two axial faces is greater than the difference the pulley tooth height and the belt tooth height.