Synchronous belt and pulley drive

Abstract

A synchronous belt and pulley drive in which the drive between spaced pulleys is primarily by frictional contact of a belt on the pulley peripheries; synchronization is insured by providing spaced teeth on the belt which teeth are accommodated by tooth gaps in the periphery of the pulleys. The drive is further characterized by matching the pitch of the driver pulley with the belt pitch under a first tension and matching the pitch of the driveN pulley with the belt pitch under a second tension, wherein the first tension is different and usually greater than the second tension.

Description

BACKGROUND OF THE INVENTION

Current design practice for designing synchronous belt drives follows the same general principles used for designing inverted tooth chain drives wherein the chain (or belt, as the case may be) teeth carry the load imposed on the drive. However, a belt differs from a chain in at least two important respects, i.e., belts, because of their construction of elastomeric material, usually with a reinforcing cord and/or cloth covering, elongate much more than chains under load; and the resilient belt teeth deflect much more than the relatively rigid teeth of a chain. In a belt drive, friction between the belt and the pulley peripheries can be utilized to carry a major portion of the load.

The Invention

This invention relates to synchronous belt drives as, for example, those in automotive timing arrangements. Other uses will be apparent to those skilled in the art.

One of the primary purposes of this invention is, in a drive using a toothed belt and toothed pulleys, to take advantage of belt-pulley friction as the primary load-carrying means. The principal function of the teeth is to eliminate excessive slip and insure and maintain synchronization between the pulleys. By doing so, belt tooth deflection and wear are minimized. This, in effect, transforms the toothed pulleys into pulleys having a "variable" pitch. The teeth on the "tight" strand of belt (from the driveN to the driveR pulley) cannot interfere with the pulley teeth because the pulley tooth gaps are larger in depth and length than the teeth of the belt, thus minimizing belt tooth deformation and wear. Therefore, because of the relatively small loads on the belt teeth, they can be spaced further apart, (the belt pitch extended) than in the normal, prior art toothed belt-sprocket drive. For example, a belt according to this invention may have approximately one-third (or less) the number of teeth than a conventional toothed belt, both of which are usable for the same purpose. Because of the fewer teeth on the belts, fewer teeth or tooth gaps are necessary on the pulleys. The fewer gaps generally mean lower manufacturing costs. By reducing the number of tooth gaps in the pulleys without increasing the size of the gaps, the area of contact between the outer periphery of the pulleys and the belt is increased, as compared to prior art drives. Thus, the friction effect between the belt and the pulleys is enhanced.

One of the important aspects of the invention is that the pitch of the driveR pulley substantially matches the belt pitch under a first tension and the pitch of the driveN pulley substantially matches the belt pitch under a second tension. The second tension is less than the first tension and equals the "slack" side tension of the belt while the first tension equals the tight or "taut" side tension of the belt. This relationship will be more fully discussed in the detailed description of the invention.

THE DRAWINGS

FIG. 1 of the drawings illustrates a side elevational view of a drive constructed to this invention;

FIG. 2 is a graph in which elastic elongation is plotted against belt load;

FIG. 3 illustrates schematically a drive according to this invention in which a belt tensioner is used; and

FIG. 4 is a schematic illustration of a typical drive according to this invention and marked for reference to the calculations in the specification.

DETAILED DESCRIPTION

Looking at the drawings and especially FIG. 1, there is shown a belt drive comprising a driveR pulley 10 and a driveN pulley 12, each rotatable about its center and in the direction indicated by the arrows. The pulleys 10 and 12 are connected by a flexible belt 14 having a plurality of spaced teeth 16. The pulleys 10 and 12 are each constructed with tooth gaps 18 and 20, respectively, each of which will accommodate a belt tooth at the proper time. Each tooth gap 18 and 20 has a depth and length greater than the depth and length of a belt tooth 16.

When the pulleys are rotating, the belt is subjected to a first tention (T₁) at the tight or taut side, i.e., from the driveN pulley 12 to the driveR pulley 10, and to a tension (T₂) at the slack side, i.e., from the driveR pulley 10 to the driveN pulley 12. The pitch of the driveR pulley 10 approximately matches the pitch of the belt under tension T₁, while the pitch of the driveN pulley 12 approximately matches the pitch of the belt under tension T₂, which means that the pitch of the driveR pulley 10 differs from the pitch of the driveN pulley 12.

To design a belt drive based on the principles of this invention, the pitches of the pulleys are determined by the pitch of the belt strand entering the pulley. It is therefore necessary to know the elastic elongation--load characteristics of the belt. A reasonable size for the pitch diameter of the smallest pulley is selected, and the number of tooth gaps is chosen such that only two or three are provided to accommodate the belt teeth; the lengths of the tooth gaps being one half to one quarter of the spacing therebetween.

The total number of equally spaced tooth gaps (pitches) around the periphery of the smallest pulley is (illustrated for example, in FIG. 1 as the driveR pulley) such that only two or three pulley tooth gaps accommodate the teeth in the portion of the belt wrapping the pulley. Pulleys having five to eight tooth gaps are feasible, depending on the wrap. The number of pitches (tooth gaps) in the other pulleys is governed by the speed ratios required.

Having selected a reasonable pitch diameter (d) for the smallest pulley and the number of tooth gaps thereon (n), an approximate pitch (p) for the drive (pulleys and belt) can now be determined by dividing the chosen number of tooth gaps into the selected pulley and pitch circumference, i.e.

p=πd/n.

When only two pulleys (besides an idler if necessary or desirable) are involved, the number of pitches in the belt can be determined by reference to Center Distance Factor Tables, e.g. Catalog 189 (1969) published by Uniroyal, Inc. In as much as such tables list center distances in terms of pitches, it is necessary to convert the given center distance (in inches) into pitches by dividing it by the approximate pitch (p). The number of pitches in the belt is chosen to make the required center distance (in pitches, determined above) match as closely as possible a listed center distance value in the tables. Dividing the latter value (in pitches) into the given center distance (in inches) obtains a more exact value for belt pitch, hereinafter referred to as "assumed" belt pitch.

Since the pitch of the pulleys should match the pitch of the belt strands entering them, the pulley pitches are equal to the belt pitch plus the elastic pitch elongation of the entering belt strand due to the load on it. The pulley pitches will therefore differ from each other and from the "assumed" belt pitch. The linear length of belt required can be figured and compared with the actual belt length based on the number of pitches and the assumed belt pitch. If the actual belt length exceeds the calculated wrapped belt length, the calculations is repeated using a smaller assumed belt pitch (and vice-versa). Usually, not more than three trial calculations are necessary to obtain a correct solution.

FIG. 1 illustrates the relationship of the belt teeth and pulley teeth. For example, all the tooth gaps 18 or 20 have the same length; the length of the belt teeth 16 are approximately three-fourths of the length of the tooth gaps. The designations A to H represent the entry wall of the tooth gaps 20, from belt entry to belt exit, on the driveN pulley 12, and A' to H' represent the exit wall of the tooth gaps 20 from the belt entry to belt exit also on the driveN pulley 12.

There is no clearance at A and H'; if the belt tooth length is approximately three-fourths of the tooth gaps length, then A'=H=approx. 1/4 tooth gaps length; and A<B<C<D<E<F<G<H=1/2 tooth gap length, and

A'>B'>C'>D'>E'>F'>G'>H'=0

It is apparent that a similar relationship exists with respect to the tooth gaps 18 of the driveR pulley 10 and the length of belt teeth 16.

FIG. 1 of the drawing illustrates a belt-pulley drive constructed according to this invention wherein the pulley pitches correspond with the pitch of the belt strand entering it. The belt is wrapped around the pulleys; the tooth gaps in the pulleys are larger than the belt teeth, so that the belt can be mounted on the pulleys without interference. If an idler is used to apply an installed tension (as in FIG. 3), as is generally the case when the drive is used for automotive applications, the installed tension will move the belt slightly in a clockwise direction (as viewed in the drawing) on the driveR pulley, and contra-clockwise (as and an "extended pitch" belt, wherein the belt tooth size would be that of a standard smaller pitch belt, but the spacing between the teeth extended to something considerably larger, and not an integral number of the pitch defining the belt tooth size as is illustrated in FIG. 1. The procedure to follow is (a) assign an approximate diameter for the smallest pulley and arbitrarily fix the number of teeth in each pulley to meet the specified speed ratio; (b) calculate the approximate belt pitch required to meet the center distance specification; (c) reduce these approximations to precise dimensions by trial and error. The elastic elongation- load characteristics of the belt must be known, and the minimum installed tension selected such that the ratio of belt tensions, T₁ /T₂, at maximum drive load equals or exceeds e^.mu.θ where e is the base for natural logarithms, 2.718, μ is the coefficient of friction between the belt and pulley face, and θ is the angle of wrap on the pulley. The choice of this initial tension will not permit the belt to slip, so that friction alone should be able to carry the load.

Claims

1. A synchronous belt and pulley drive comprising:

a driver pulley;

a driveN pulley;

said pulleys being spaced from each other and each having a plurality of spaced tooth-gaps;

a belt engaging the pulleys and having spaced teeth which are accommodated by the tooth gaps of said pulleys;

the drive between said driver pulley and said driveN pulley being substantially by friction of the belt thereon, the relationship of belt teeth and the pulley tooth gaps insuring synchronization of the drive between the pulleys and substantially eliminating slippage of the belt on the pulleys;

the pitch of the driver pulley substantially matching the pitch of the belt under a first tension; and

the pitch of the driveN pulley substantially matching the pitch of the belt under a second and different tension.

2. A drive as in claim 1 wherein said first tension is greater than second tension.

3. A drive as in claim 1 wherein said first tension is that of said belt from the driven pulley to the driver pulley and said second tension is that of said belt from the driver pulley to the driveN pulley.

4. A drive as in claim 1 wherein the spacing of the teeth on said belt is substantially extended to provide extensive frictional contact of the belt with the outer peripheries of said pulleys.

5. A synchronous belt and pulley drive comprising:

a driver pulley and a driveN pulley which are spaced from each other;

spaced tooth gaps located around the periphery of each pulley; the spacing of which is different on one pulley than the other pulley;

a toothed belt wrapped between the pulleys, the teeth of which are accommodated within the pulley tooth gaps;

said belt having a taut strand and a slack strand when said drive is transmitting power from the driver pulley to the driveN pulley; and

the spacing of the tooth gaps of said driver pulley being substantially equal to the spacing of the belt teeth on the taut strand thereof and the spacing of the tooth gaps of said driveN pulley being substantially equal to the spacing of the belt teeth on the slack strand thereof.

6. A drive as in claim 5 wherein the spacing of the tooth gaps on the driveN pulley is less than the spacing of the tooth gaps on the driver pulley.

7. A synchronous belt and pulley drive comprising:

at least a driver pulley and a driveN pulley which are spaced from each other,

each pulley having equally spaced tooth gaps located in the periphery thereof, the spacing of which are different on each pulley;

a toothed belt wrapped between the pulleys, the teeth of which are accommodated within said pulley tooth gaps;

said belt having a taut strand and a slack strand when transmitting power from the driver pulley to a driveN pulley;

said tooth gaps being of larger dimension than said teeth to permit the belt to wrap said pulleys without substantial tooth gap-belt teeth interference under belt tension;

the spacing of the tooth gaps of said driver pulley being substantially equal to the pitch of the belt teeth on the taut strand thereof and the spacing of the tooth gaps of a driveN pulley being substantially equal to the pitch of the belt teeth on a slack side thereof.

8. A drive as in claim 7 wherein the spacing of the tooth gaps on a driveN pulley is less than the spacing of the tooth gaps on the driver pulley.

9. A synchronous belt and pulley drive comprising:

a plurality of spaced pulleys each having a plurality of spaced tooth gaps in the periphery thereof;

a drive belt engaging the peripheries of the pulley and having spaced teeth which are accommodated by the tooth gaps of the pulleys, said belt having taut portions and slack portions when providing the drive for said pulleys such that the pitch of the belt in the taut and slack portions is different;

the pitch of said pulley tooth gaps being equal to the pitch of the belt at the entry of said belt to the pulley.

10. A synchronous belt and pulley drive as recited in claim 9 in which the pitch of the tooth gaps of one pulley is different from that of the next succeeding pulley.

11. A synchronous belt and pulley drive as in claim 9 in which there are two pulleys.

12. A synchronous belt and pulley drive comprising:

a driver toothed pulley and a driven toothed pulley, said pulleys being spaced from each other, and having spaced tooth gaps therein;

a toothed belt running over and engaging said pulleys and being constructed of elastomeric material at least on a surface thereof, said belt having a taut strand and a slack strand with its teeth equally spaced with the spacing defined by the tension applied thereto, the spacing of said teeth being greater than the width of said teeth to provide extensive frictional contact with the outer peripheries of said pulleys;

the spacing of the tooth gaps of said driver pulley being equal to the spacing of the belt teeth corresponding to the design tension in the taut strand of said belt and the spacing of the tooth gaps of said driven pulley being equal to the spacing of the belt teeth corresponding to the lesser tension in the slack strand of said belt;

said pulley tooth gaps being of larger size than said belt teeth in order to permit the belt to wrap said pulleys without interference on the bottom

and sides of the tooth gaps.

13. A synchronous belt and pulley drive comprising:

a driver pulley and a driven pulley;

said pulleys being spaced from one another and each having bearing surfaces separated circumferentially by a plurality of uniformly spaced recesses for accommodating belt teeth, an endless belt engaging the pulleys and having on its inner surface a plurality of spaced teeth, the spacing between adjacent teeth being uniform in the circumferential direction when the belt is free of tension,

the drive between said respective pulleys being primarily by friction between the belt and said bearing surfaces, the relationship of belt teeth and pulley recesses insuring synchronization of the drive,

said pulley recesses being circumferentially wider and radially deeper than said belt teeth,

the circumferential extent of the bearing surfaces being substantially greater than the circumferential width of the recesses,

the circumferential extent of the bearing surfaces on the driver pulley being matched to the spacing between adjacent belt teeth when the belt is stretched under a first tension, and

the circumferential extent of the bearing surfaces on the driven pulley being matched to the spacing between adjacent belt teeth when the belt is under a second and different tension.