CA1115090A

CA1115090A - Pulley for positive drive systems

Info

Publication number: CA1115090A
Application number: CA307,520A
Authority: CA
Inventors: Frank L. Wujick
Original assignee: Uniroyal Inc
Current assignee: Uniroyal Inc
Priority date: 1977-12-23
Filing date: 1978-07-17
Publication date: 1981-12-29
Also published as: BR7808421A; DE2855748C2; FI783911A7; PL128922B1; GB2011578B; IT7869946A0; GB2011578A; NL7812282A; GB2092704B; JPS5491655A; SE442139B; SE7813224L; ATA902578A; YU303178A; IT1108296B; PL212040A1; GB2092704A; MX148323A; HK50684A; NO784175L

Abstract

ABSTRACT
A pulley configuration which increases the contact area between the pulley tooth tips and the land area of the belt teeth. The in-creased contact area reduces land area wear between belt teeth. The pulley tooth tip has a longitudinal cross-sectional contour partially composed of two substantially circular arcs connected by a line seg-ment which is substantially straight or slightly curved. The length of the line segment is from 5 percent to 100 percent of the width of the belt tooth on which the pulley is used, and preferably between 6 percent and 33 percent of the width of the belt tooth. The pulley in combination with a toothed belt forms a positive power transmission system.

Description

BACK~ROUND OF ~HE INYENTION

This Inventlon rol~t~s to ~ tooth~d pow~r tran~mlsslon b~le and pulley drTve.
U.S. Patent No. 2,507,~52, to R. Y. Case, describes a power transmission belt comprising an inextensible tenslle ~mber haviilg teeth bonded to one side and a protective Jack~t fabric covering the teeth. Ths t~eth are preferably made of an elastomeric material, such as rubber, and the belt may also include a backing layer of iden~lsal or similar material to that which the teeth are construct~d.
lQ Many different elastomeric materials have been utilized for the constructîon of belts made in accordance with tho Case patent, some of the more common materials being neoprene and polyur~thane.
These belts are designed to mesh with toothed pulleys that are con- -structed of a material havlng a higher Young s modulus than the elastomeric material used for the construction of the belt. The conventional toothed belt, as described in tho Case patent, util;zes a ~ooth cross-sectional configuratTon that is essentially trapezoidal and which is very si~Tlar to a conven~ional rack tooth. Many ~ -attempts have be~n made to alter the belt and pulley teeth configura- ~ `
2Q tions to relieve the problem of belt failure. In such trapezoidal tooth belts, the common failure is that of tooth shear due to stress concentration. In seeking to reduce tooth shear, UOS. Patent No.
3,756,091 to H. Miller, discloses belt teeth having a cross-sectional `~configura~ion which approxima~es the contour of the one-half order ;~
isochromatic fringe in a bele tooth under a defined rated load. The pulley grooves in accordan e with the Mill~r patent are tn matlng engagement with and ar~ s~bstantially conJugate to the belt teeth.
Th0 belt having substantlally curvilinear teeth in accordanco with the Miller patent reduced bclt tooth shear and ~ncreased horsepoweF
capacity. One mode of failure tn the belt configuration accord;ng to Mi~ler may occur becaus~ of land wear In the belt, especially with smal I diamet~r pulleys. Land we~r between th~ belt teeth is due to abrasion of the protective layer and exposure of the tensile member by action of the pulley tooth against the belt. this land wear leads to premature failure due to a detachment of the teeth from the tensile member and/or a break in the tensile member. :
U.S. Patent ~o. 4,037,485, to Hobackg propos~s a solution ~:
to the land wear problem. As disclosed ln the Hoback pat~nt, the dimensio~al relatlonship of tho belt teeth and grooves and the pulley taeth and grooves is such that in th~ longitudinal extent of the be)t between the pulleys, the height of the belt teeth is ~ :
greater than the ~epth Gf the pulley grooves while as the belt travels around the pulleys, the extreme outwardly facing portions of the belt teeth wh kh con~ront the pulleys comes into contact with the portions of the toothed pulley which define the bottom of the pulley grooves. At the sam~ tims, Hoback discloses that the belt teeth are compressed to reduce thoir height so that the extreme radially outwardly facing portlons of the ~ulley teeth co~e into contact with the portions of the belt disposed between the belt teeth which define the bottoms of th~ belt grooves.
Relatively early in ths operat;ng life of th~ toothe~ power transmission bele and pulley as disclosed in Hoback, the compressive engage~ent of the elasto~eric teeth against the bottom of the pulley groove results in a substantially perm2nent deformation of the belt tooth. This d~fofmation is acceleraeed at elevated operating ~em-peratures such as occur in automotive applications. The deformation results in a significant permanent decrease in the tooth beight.
The deformed teeth no longer support the tensil~ member as desired by Hoback. Subsec;uently the same wear pattern develops ;n the toothed belt and pulley drive according to Hoback as in the too~hed power ' transmission configuration according eo the Hiller patent.

5~
It has been found in accordance with the present inv~ntion that the land wear problem of the Miller and Hoback patents can be sub-stantially reduced. The reduction results in an increase in the operating life of the toothed belt. Furthermore, in accordance ~lith the present invention the toothed power transmission drive has improved belt 1ife, even under severe operating conditions such as elevated temperature and torsiona) vibration environments. The improve~ent is most beneficial with small pulleys.
The present invention is a pulley for use in conjunction with a flexible drive belt, such as disclosed in Miller.
As used herein, the terms describing the features of the present invention are defined in the patent to Miller.
The present inventlon reduces the problem of land area wear between belt teeth by changing the pulley configuratio~ to increase the contact area between the pulley tooth tips and the land area of the --belt teeth. It further relieves the pressure in the belt tooth lænd area by providing support ~or the belt tooth in the pulley groove.
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The pulley of the present invention is a non-conjugate form of the b~lt. Each of the pulley grooves is form~d by a substan~ially cir-cular arc ~onnecting the tips of adjacent pulley teeth. The belt teeth, as the belt travels around the pulley, is Tn compressiveengagement with the pulley groove. Each of the pulley tooth tips has a longitu-dinal cross-sectional contour generally as disclosed tn Miller and is partially composed of two substantially circular arcs having centers of curvature displaced from each other. The centers of curvature ar~
each 10cated on the same side of ~he tooth tip as its respective arc.
The tooth tip arcs ~re connected by a line segment which is substantially straight or sllghtly curved. The line seqment defines the pulley tooth tlp wldth. In accord~nco wi~h th~ pro~ont Inv~nttQn, tho s~rf3ce contact r~tlo of tho ~olt tooth wldth to th~ lln~ ~ogm~ne Is b~twcon 20:1 ~nd 1:1. Tho l~ngeh of thls lln~ s~gm~nt 15 from 5~ to 100% of the width of the belt tooth on the belt wlth which the pulley Is to be used, and preferably botween 6% and 33% oF the width of the belt tooth.
Both the cQmpresslve engage~cnt of th~ belt tooth with the pulley groove and the surface con~act ratio as definecl, d~ act ~ndivi- ~:
dually to reduce land wear ;n the b~lt. However, in combination, as ~oro fully descr;bed herein, they reduce l~nd wear In the be!t to an extent greater than the s~m of their Individual effects.

In accordance with its broadest aspect, the present invention relates to a power transmission system con-sisting of at least one small diameter toothed pulley in combination with a flexible toothed belt having belt teeth of cross-sectional contour substantially composed of two su~stantially circular arcs, the pulley teeth being com-posed of tip portions connected by cavity portions, each tip portion having a longitudinal cross-sectional contour partially composed of two substantial circular arcs, said arcs having centers of curvature displaced from each other, the center of curvature of each arc being located on the same side of the center line of said tip portion as its respective arc, the outermost portion of each toot,h tip joining the two arcs forming a line - . :
segment, the length of the line segment being from 6~ `~
to 33~ of the width of the belt teeth, and the distance between the outermost portion of the pulley teeth cavitles and the innermost portion of the pulley teeth cavities being less than the depth of the belt teeth which are not in contact with the pulley.
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BRIEF n~s~cR~ T~oh~ ~ Y~
__ The invention wlll be more clearly understoot from the following description read together with the drawlng in wh;ch:
Fig. 1. is a long;tudinal cross-sectional view 700king trans-versely of the preferred embodlment of the cooperatlng pulleys in accordance wlth the present tnvention Tn ~ngagement with a belt to form a positive drive system;
Fig. 2. ts an enlarged fragmen~ary longitudlnal cross~seGtional vtew of the pulley in accordance wlth the preferred embodtmKnt;
Fig. 3. 1~ an enlarged fragmentary longitudinal cross-secttonal view of the pulley of Fig. 2 superimposed on a view of a corresponding conjugate pulley accordlng to the Miller patent; and Ftg~ 4. is an enlarged fragmentary longttudTnal cross-sectional vtew of the pulley of Ftg. 2 tn m3tTng engagement with a belt constructed in accordance with the Miller patent under a no~load condition.
D~ETAILED DESCRIPTION OF THE l~iVENT!ON
As seen in Fig. 1., the eldless belt 10 engages the driving and driven pu11eys 11 and 12. ~eit 10 Is provided with a tensile me~ber 13 ccmpr;sing a plurality of turns of a contlnuous stra~d of ftlamentary _ 4 (a) ~ :`
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.~ matcri 1. The tcnsTle member 1~ car.ies substantially ~he cntirc workin9 load ;mposed on the belt 10, and up to the max;mum load for whtch the belt is designed, the sensile member 13 is substantially Inextensible. The above ci~ed Case and Miller paten~s may be con~
sulted for a mor~ detailed description of the princ~ples of this ~ -general class of ~o~thed belt and pulley system. The entire contents of the Case and Miller patents are each incorporated herein by reference. The belt further includes a backing layer 14 and a protective jacke~ (not shown) extending over the entire ~oothed sur-face of the belt. A thin 1ay~r of elastomeric or other material (not shown) between the jacket and the t~nsile member 13 may be added to improve adhesion Tn the land areas of the beltD The belt may be : :
made in any one of a number of ways, but i~ is pre~erable to use the ~ethod described in Canadi~n Patent ~o. 637,926 to W. S~m ra An alternative method for m~nufacture is described in the above cited Case patent.

The pulleys 11 and 12 of Fig. I ac seen in Fig. 2 eaoh compr7se a hody portion 20 having ourvilinear teeth 21 separated by curvilinear cavlties 22. The tooth tip 23 viewed in 10ngitudinal cross-section has an outer configuration which is composed of two circular ares 24 ant 2~ having displaced centers of curvature 26 and 27 displaced from a tooth tip center line 28. Arcs 24 and ~5 have two equal radii 29 and 30 with centers of curvature 26 and 27 displaced equal amounts on opposite stde of the center line 28 of the tooth 21 but on the same stde a5 their cnrresponding arcs. Both centers of curvature are within the pulley tooth. The non-intersecting arcs 24 and 25 are connected by a line 5e~ment 31. The arc 24 extends from potnt A to polnt B. The arc 25 extends from point C to point D. The centers 26 and 27 lie at equal distances fram a tangent to the outside d;ameter :,. .
.~ . ~ 5 ... ..

of the pulley at the c~nt~r ltn~ 28 o~ the tooth, llo at ~q~al dls-eances to opposlt~ sld~s of th~ e~nter llna 28 and lle at oqual radial dista~ces from the center of the pulley. The line segment 31 is part of an arc of a circle having a radius drawn from a center Iying on the extension of the center line 28. The center of curvature of line 31 may lie on center line 28, i.e. within the body of the pulley. Alternatively the center of curvature of line segment 31 may lie on a~ extensTon of center line 28 which is outside ~he body of the pulley. The line segment 31 is shown as being a straight line, .
but n~y be concave or convex wlth respect to the body of the pulley.
Fig. 3 showc an enlarged fragmentary longitudinal cross-section vlew of the pulley of Flg. 2 superTmposed on a view of a corresponding conj~gate pulley according to the Miller pa~ent. ThTs figure emphasizes the difference in the longitudinal cross-sectional profiles of the respective pulley grooves and teeth. FTg. 3 shows the cavity 22 of this invention which is formed by the arc of radius 32 at a center of curvature 33. The arcs 24 and 25 which partially form the pulley tooth tTp of this 7nvention ~re generated by respective radii 29 and 30 havTng center~ of curvature at 26 and 27. The line segment formTng the outermo~t ~ortion of the tooth tip and connectTng the arcs ~4 ~nd 25, accordTng to the ~nvention, has a length equal to the distance between points ~ and C. The arc forming the cavity 22 may intersect with ~he arc~ 24 and 25 of this invantton at points A and D.
in contrast~ the conjugate pulley according to the Miller patent, ha~ a cavity 41 formed by arc having a radius 40 and a center of curvature 39. The arcs 46 and 47 which partia~ly form the ~ulley tooth tip according to the HTller p3tent are generated by respective radii 44 and 45 having centers of curvature at 42 and 43. The line segment forming th~ outermost portion of the tooth tip and connecting the arcs 46 and L7 has a length e~ual to the distance between poin~s .
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a~

E and F. Th~ arc forming the cavlty 41 ;ntersects the arcs 46 and 47 at points G and H~
The length of the line segm~nt BC of the present Tnventlon is greater than the length of the line segment EF of the pulley tooth ~ip according to the ~iller patent. The centers of curvature 26 and 27 of the present 5nvention are displaced on opposite sides of the can~er line 28 a distanca greater than the displacement of the centers of curvaturè 42 and 43 of the pulley tooth tip according eo the Mi11er patent. The cross-hatch~d area 48 represents the addi-tlonal material present in the pulley groove and ~ooth tlp profile according to this Tnventlon as compared to the same profile according to the Miller patent.
Fig. 4. is an enlarged fragmentary longitudinal cross-sectional view of the pulley o~ Fig. ~ in ~ating engagement wlth a belt con-s~ructed In accordance wlth the Miller patent under a no-load con-di~ion. in longltudinal cross-sec~lon each tooth 49 of the belt 50 is composed of two circular arcs 51 and 52 of equal radius 53 and 54 Ineersecting at a polnt 55 on the center line 34. The centers of curvature 56 and 57 from which the arcs 51 and 52 are drawn are iocated on a ITne 58 as shown in the Miller p~tent. The extent of th~ arcs 51 and 52 ts such that they extend to the line 58. The centers of curvature 56 and 57 for the arcs 51 and 52 are displaced on opposite sides of the center line 34 from their corresponding arcs by an a~ount that Ts generally equal to or less than 10% of the radlus of curvature of the aros 51 and 52u The depth of the groove between adJacent pul1ey teeth in the pulley of the inventlon is less than the depth of the groove in the conjugate pulley and is thus less than the depth of a t~oth of the belt.
This Ts shown clearl~ In Fig. 4t where the tlp of the belt tooth Is shown as overlapping the base of the p~iley groove. Obviously this will not happen in practice and the belt tooth wlll be distorted due to contact with the pulley groove. It is preferred that the :.
pulley groove depth be between 1% and 15% less than the belt too~h depth.
In designing a drive according to the invention, Tn addition to the criteria already de~cribed, it is desirable that the surface contact ratio of belt tooth width (th~ width represented by the length of broken line 59 between the points J and K in Fig. 4) to the length of the line segment 31 (the wtdth represented by the length of the llne BC in F~g. 2.~ be between 20:1 and 1;1 and desirably between 15:1 and 3~
The cross-sectional confTguration of cavity ~2 has a radius 32 much larger than radil ~9 and 30 and has a eenter 33 wh7~h is outside the pulley body and is on a center ltne 34. The centers of radii ~;
26, 27 and 33 are located on the same or slightly spaced clrcles 35 and 36 which are concentric with and within a circle connecting the outermost line segments 31 of the tooth tTps. The circles 35 and 36 are spaced radially inwardly from this circle a distance equal to or less than 30 per cent of the total tooth depth. The total tooth depth . ~ ~ :
is the radial distance between the intersectlon of line segment 31 and center line 28,~;.e. a po;nt on the addendum 37 and a circle con-necting the innermost points of cavtty 22, The Tnnermost point of :
cavity 22 is at the intersection of the arc generated by the radius 32 and the center Itne 34, i.e, a point on the dedendum 38.
The cavity 22 Ts formed by an arc of a circle having a radlus 32

2~ drawn from a center 33 on center line 34. The arc for~ing cavity 22 may Intersect the arc 24 on one side of the overall pulley groove profile and intersect arc 25 on the other side of the profile. The profile cf the pulley groove ~rom the intersec~ton of line segment 31 ~
and eenter line 28 So a corresp~nding intersection of the next adJacent . .

3 tooth Is repeated around th~ circumference of the pulley go deflne .
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the other teeeh and grooves4 The ~otal depth of tho pullsy gr~ove 13 ~qu~l to th~ dlstanc~
measured along center line 34 between the Tntersections of llne 34 and the addendum 37 and the dedendum 38. As shown in ~ig. 2., the to~al depth of th~ pulley groove Ts tho sum of the length of the radius 32 and the distance d. Preferably the total depth of the pulley groove is not more than IS% less than the depth ~ ~he belt tooth which en~ages wTth the pulley groove. Consequently ~he belt tooth may be in compress7ve engagement with the cav;ty 22. Alter-natively the dimensional relationship between the p~lley yroove depeh and the depth of the bèlt tooth may be as describad Tn the patent to Mlller.
In designing the pulley groovo and tooth form, It is desTrable that the arc formTng ths cavity 22 smoothiy intersect with the arcs 24 and 25. However, ia order to provide a smooth transition from the arc forming the cavity 22 to the arcs 24 or 25 forming a portion of the p~lley tooth tip, it may be desirable to provlde a connecting line. This connecting line may be sTmilar to lino segment~31, EXAMPLE
~ In order to compere the performance of power transmisslon systems having pulleys with the novel dlmansional relattonship of this tnventton wtth those having convsntional dlmenslonal relationships, the following procedure was followed. Several positive or synchronous drlve belt samples were manufactured by conventlonal methods ustng conventional matertals well known in tho art. All the belts were formed of a neoprens rubber compositton having a nylon fabrtc facing on the belt teeth, and tnc~uding a tensile member of fiber glass cords disposed substaneially on ~he dedendum line of the belt teeth.
The belt samples after manufacture were dynamically tested on toothed 3 pulleys of appropriate dimsnsion and con~Tguratton as will be here;nafter described.
Three power transmission syst~ms werè tested. These combina-tions are identified as A, B and C: -Combination A comprises belts and pulleys manufactured strictly in accordance wtth the Miller patent. These belts and pulle~s are sold commercially by UNIR~YAL, Inc. as its 8 mm, pitch Powergrip HT~
Belt and Pulley system, The elghteen belt samples produced had the fo11Owing dimensTons as measured in the longitudinal extent of the belt between the pulleys:
a pltch between belt teeth of 0.315 inches; a belt tooth width of 0.260 inches; a distance between the belt teeth of 0.055 inches; and a belt tooth depth of 0.135 inches. The belts were 15 mm. wide and 936 ~m. in pitch length.
The pulleys had a tooth depth of 0.142 tnches and were substan-. .
tially conJugate to the belt teeth~ As taught in the Miller patent, the tooth tips had a longitudinal cross-sectional contour composed of two circular arcs conneceed by a line segment. The line segment -length was O.all inches. In ~he resul~ing power transmission system the line segment is 4.2% of the width of the be1t too~h.
Combination ~ comprises a poh~r transmissTon system exactly as found in Comb;nation A except that the belt tooth depth had been increased by 0,012 inches (8.9%) to 0.147 inches, This corresponds to the teachings in U,S, Patent No. 4,037~485 to Hoback. Seven belt samples were made. ~ ~ -Combina~ion C comprTses a power trans~ission system in accordance wTth the present invention. The belt has the same dimens;ons as the belt in Combination Ao The pulley tooth had a depth of 0.130 inches;
the pulley cavity had a radius of curvature of 0~106 inches. Th~
line segment length was G.020 inches.
Table 1 is a comparison of the ~y dim~nsions of Ccmbinations A, B and C and the resultTng compression of the belt teeth.
TABLE I Comblnations A B C
eelt Tooth Width .Z60 .260 .260 ~elt Tooth Depth .135 D147 .13 Pulley Tooth Depth .142 .142 ~130 Line Segment Length .011 .Oi 1 .020 Line Segment Length 4.2% 4.2~o 7.7Yo (as percentage of belt tooth width) Percent Belt Tooth Compression 0% 3.4% 3.7%
(Belt Tooth Depth-Pulle~ Tooth Dep~th) Pulley Tooth Depth Each belt sa~plo was mounted on a dead w~ight~flex tester havlng ., a two pulley arrangement consisting of a drlver and a driven pulley each having an outslda diameter of 2.160 7nches. The drtver pulley as operated at 350 rpm wTth a f~rce between pulley shaft centers of 250 Ibs. The test was conducted at ambient te~peraturc. The belts , were tested ts failure unless o.therwlse tndicat~d. The laps~d tTme .
to ehe nearest hour of each group o~ belt sa~ples to failure ;s shown in Ta~le~TI.
TABLE I I
Llfe on Test, Hours . ~ .
_A B _ _ __~
83.459.0 298.~ (test suspended) :
46.9117.2 690.7 65.599.1 18706 75.7120.8 23304 Average 65.1 99.0 34501 ' The test was repeated 7n an environmental chamber held at 190F~
Th~ results are shawn in Table 111 TABLE ~
Life on Test" H_urs A a c _ 93.0 118.2 lg~.2 ~7.~ 98.5 105.7 82.~ 117.5 1~9.3 ::
.4 _ ~LZ! ~
, Average9i.6 111.4 162.0 The results in Tables 11 and 111 clearly show that the pulley~
belt Comb1nation C having the pulley to belt relationship of this ~;
invention provides a significantly improved belt life over Comblna-tions A and B. This 1mprovement was shcwn on the tests made at ;~
amblent temperature and on the tests ma~de on accelerated tests at 190 deg. f.
The only signlficant varlable in ~he comparative teats was the relatlonship o~ the pulley dimensTons to the belt dimens10ns. Comr :
btnatlon B had belt teeth~that were greater In depth than th~ depth of the corresponding pulley grooves~ Combin~tion C also had belt teeth .
that were grsater In~depth than the d~pth of the correspondlng pulley grooves. However ~the line segment length.of the Combinatlon C
pulleys was substantially greater than that of the line segment lengths .
of the Combinatlon A or the Combinatlon B pulleys. The pulley of the prlor arc Comblnatlons A and B had tnadequate line segment lengths causing the belt to be:subjec~ed to excess stress on the tensile .
members. This excess stress caused abraston of the belt:betw~#n the belt teeth and the failure resulted prim3rlly from excessive wear tn th7s area.
in the pulleys o~ Combination S ehe llne segment length was of , , ':
, ~

:

su~ficient l~ngth to distrtbute the load over a larger area and thus reduce the stress in the araa betwoen the belt teeth. Consequently, wear in the area of the balt between the teeth was reduced, and ~he belt life was signlficantly increased at both ambient and e)evated tempe rat ures .
The pulley described herein may be uied in combination with toothed belts other than that dtsclosed In th~ pa~ents to Case~
M~11er and Hoback. Tho longitudln21 cross^seceTonal con~our of the Jndlvidual belt teeth may be trapezoldal~ curviiinear or wedge-shaped or a comblnation or a modlflcation of these contours9 The belt may also have ~eeth on both sldes of ehe tens~le member.
While there has been descrlbed what 5s at present considered to be preferred embodimants of thls Tnvention, it will be obvious to thoseskllled in the art that various changes and modiftcations may be ~ade thereln w7thout departing fro~ the spirlt of the inventlon, and it lsr therefore, ai~od to cover all such changes and mociificatTons as fall withln the scop~ and spirtt of this Invention.

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Claims

What is Claimed is:

1. A power transmission system consisting of at least one small diameter toothed pulley in combination with a flexible toothed belt having belt teeth of cross-sectional contour substantially composed of two substantially circular arcs, the pulley teeth being composed of tip portions con-nected by cavity portions, each tip portion having a longi-tudinal cross-sectional contour partially composed of two substantially circular arcs, said arcs having centers of curvature displaced from each other, the center of curvature of each arc being located on the same side of the center line of said tip portion as its respective arc, the outer-most portion of each tooth tip joining the two arcs forming a line segment, the length of the line segment being from 6%
to 33% of the width of the belt teeth, and the distance between the outermost portion of the pulley teeth cavities and the innermost portion of the pulley teeth cavities being less than the depth of the belt teeth which are not in contact with the pulley.

2. The power transmission system of claim 1, wherein said pulley teeth line segments are circular arcs.

3. The power transmission system of claim 1, wherein said pulley teeth arcs are tangent to their respec-tive line segments at their points of intersection

4. The power transmission system of claim 1, wherein the pulley teeth line segments are straight lines.

5. The power transmission system of claim 1, wherein the longitudinal cross-sectional contours of the pulley teeth cavities are composed of substantially circular arcs.

6. The power transmission system of claim 1 wherein the distance between the outermost portion of the pulley toothtip and the innermost portion of the pulley tooth cavity is not more than fifteen percent less than the depth of a tooth of said belt which is not in engagement with the pulley.

7. The power transmission system of claim 1, wherein the belt teeth in contact with the pulley are in compressive engagement with some of the pulley teeth cavi-ties.

8. The power transmission system of claim 7, wherein the pulley teeth cavities are non-conjugate with the belt teeth.

9. The power transmission system of claim 7, wherein the pulley teeth line segments are circular arcs.

10. The power transmission system of claim 7, wherein said arcs partially forming the contours of the pulley teeth are tangent to the line segments at their points of intersection.

11. The power transmission system of claim 7, wherein the pulley teeth line segments are straight lines.

12. The power transmission system of claim 7, wherein the longitudinal cross-sectional contours of the pulley teeth cavities are composed of substantially circular arcs.