CA1182663A - Torsionally elastic power transmitting device and drive - Google Patents

Abstract

Abstract A shock-absorbing torsionally elastic single or multiple V-belt or V-ribbed belt power transmission friction drive assembly is described having:
a plurality of sheaves, at least one of which is shock-absorbing and includes;
a rotatable hub having at least two lugs protruding radially therefrom; a rotatable rim with at least two radially inwardly extending ears adapted to matingly engage the lugs of the hub, and having a peripheral surface configured to engage the belt, resilient cushions interposed between the lugs and ears, comprised of high hysteresis elastomeric material; and an endless power trans-mission belt trained about the sheaves in driving relation in serpentine configuration. This is used to absorb torsional shocks and minimize heat build-up.

Description

Ihis is a divisional application of application Serial No. 300,820 filed on June 29, 1981.
This invention relates to rotary driven members and more particularly to torsionally elastic power trallsmission assemblies capable of absorbing or isolating torsional shocks and vibrations in a power drive train.
Power transmitting devices are known which are capable of dampen:irlg or isolating torsional shock loading and millimiæi.llg noise and vibration by the use of resilient cushioning means. Rubber cushions, for instance, are adapted to yieldingly transmit rotary motion between mating lugs of an integral hub and rim assembly. Typical known applications include cushioned sprockets for use with roller chain or synchronous belting (timing belts), direct gear drives, and torque transmission between shafts (flexible couplings), for instance.
Various industrial applications are contemplated including those set forth in Koppers Company "Holset Resilient Couplings" catalog, March, 1973. Additional relevant prior art includes, for instance, Croset United States Patent No.

2,873,590, and Kerestury United States Patent No. 3,314,512.
Prior rubber cushions used in torsionally elastic couplings were effective in smoothing out vibrations and modulating torque peaks for the primary drive of motorcycles using synchronous drive belts. However, it was found that when subjected to abusive driving techniques, such as "speed shifts"
where gear shifts are made without lctting off on the throttle, the driven sprocket experienced very high torque peaks. During the speed shift the tor-sionally elastic driver sprocket assembly would wind up (along the "soft"
portion of the torque deflection curve) allowing the driven sprocket to slow down. Subsequently when the flattened or "stiff" portion of the torque deflec-tion curve was reached, as the cushions filled their associated cavities, a large torque was suddenly applied, causing a very high peak load on the drive due to inertial efEects. In some cases the belt Eailed by bo-th tooth shear or breaking of the tensile reinforcement.
The problems associated with such abusive conditions are believed to be overcome by provision of an elastomeric cushion spring havi~g desirable spring ra-te and damping properties allowing much higher torques to be trans-mitted while still operating on the initial sloped ("soft") portion of the torque deflection cu-ve, and simultaneously accommodatillg relatively large angular deflections for the drive.
In another vehicular application there is a trend toward extensive use o:E dynamically unbalanced four and six cylinder diesel and other engines exhibiting severe speed excursions a-t low rpm, especially for automobiles and trucks which are particularly vibration prone. Accessory drives for these engines transmit power from the crankshaft sheave to various driven sheaves usually linked with a number of separate V-belts or V-ribbed belts, or in the case of the so-called serpentine drive with a single V-ribbed belt, all working on a friction drive principle. These rough running engines, particularly the four and six cylinder diesels, have such high rpm excursions at idle speeds ~below about 1000 rpm) that V-ribbed belts and other friction drive belts under-go tremendous slippage and elastomeric material shear relative to the tensile section, causing the belt and sheaves to heat up to such temperatures that in severe cases the belts have failéd after only a single hour of operation on the drive.
In this latter aspect it is an object of this invention to overcosne the slippage and heat build-up problems aforementioned associated with V-belt or V-ribbed or similar type friction drive. Examples of serpentine drives and other belt configurations useful in this aspect of the invention, include those disclosed in Fisher et al. United States Patent No. 3,951,006.

Briefly described~ in one aspect the invention relates to a shock--absorbing torsionally elastic power -transmitting device including a rotatable imput drive means; a rota-table output driven means; and resilient spring cushions linking the input drive means -to the outpu-t driven means in power transr~itting relation and dispaceable unde:r load, the cushions comprising bodies of elastomeric material loaded with modulus and hysteresis increasing fibrous reinforcement.
In another aspect, the i.nvention relates to a shock-absorblng tor-sionally elastic bel-t sprocket or sheave assembly including a rotatable hub having at least two lugs protruding radi~lly therefrom; a rotatable rim with at leas-t two radially inwardly extending ears adapted to matingly engage the lugs of the hub, and having a peripheral surface configured to engage an endless power transmission belt; and resilien-t cushions interposed between~the lugs and ears, comprising elastomeric material in which is embedded discrete reinforcement fibers functioning to substantially increase -the dynamic modulus and hysteresis of the cushions.
~ he invention provides a shock-absorbing torsionally elastic single or multiple V-belt or V-ribbed belt power transmission accessor-y drive assembly for use with an engine exhibiting substantial speed excursions at low rpm, comprising: a plurality of sheaves each having a peripheral surface provided with a-t least one V-shaped groove on its driving surface adapted to receive the belt in driving relation: at least one of said sheaves being a shock-absorbing crankshaft sheave assembly, adapted to be coupled to the crank-shaf-t of the engine, and including a rotatable hub having at least two lugs protruding therefrom, a ro-tatable rim having at least two radially inwardly extending ears adapted to indirectly matingly engage the lugs of the hub and.
having a peripheral surface formed with said a-t least one V-shaped groove, and B

resilient elas-tomeric cushions i.nterposed is cavities be-tween -the lugs and ears in driving relation; said crankshaf-t sheave being provided with a captive void vol.ume defined as an individual cavity volume less the volume occupied by the cushion contained in tha-t cavi.ty of such amount to allow, in operation, the hub and rim to undergo relative angular deflections of from about .1 to abou-t 8 degrees, and a single of multiple V-belt or V-ribbed belt trained about the sheaves in friction driving relation.

P~.~ - 3a -Other aspects will become apparent :Erom a reading of the description ancl c:laims.
The invention in its preferred embodiments will be morc particularly described by reference to the accompanying drawings, in which like parts are designated by like numerals in the vari,ous figures~ and in which:
Figure 1 is a partial cutaway view of a power transmissiorl drive using a synchronous belt;
Figure 2 is a sectional view of the leftmost sprocket of Pigure 1, taken along section 2-2 of Figure 3;
Flgure 3 is a partial sectional view of the sprocket taken along section 3-3 o-f Figure 2;
Figure 4 is an enlarged view of a sectioned portion of a rubber cushion as shown at 4-4 of Figure 3;
Figure 5 is a schematic view of a serpentine V-ribbed belt drive for an automobile using a torsionally elastic sheave;
Figure 6 is a partial sectional view along 6-6 of Figure 5; and Figure 7 is a graph of angular deflection versus torque for the sprocket shown in E~igures 1-3, comparing d:iE-Eerent cushion:ing mater:ials.
The clescription w:ill refer to a primary dr:ive sprocket :Eor a motor-cycle in Fi.gures 1-3, and a crankshaft sheave :Eor an automobile in F:igures 5 and 6. ~lowever the power transmission assembly and cushioning means may be employed in various applications wherever torsional flexibility a:nd elastic isolation between the hub and rim members is requ:ired i.n the transmiss:i.on o:E
rotary motion. For example, the devices are suitable :Eor use in such d:iverse applications as transmission drives :Eor business machines, trac-tive drives, air conditioner compressor drives, dlrect gear drives, chain dr:ives, va-rious belt drives, and i.n flexi.ble couplings.
Re-ferring first to Figures 1-3, a power transmission belt drive 10 for the prirnary drive of a motorcycle (linking engine output to transmission/clutch input) includes a shock-absorbing torsionally elastic drive sprocket 12 con-figured in accordance with the invention, a driven sprocket 14 which may be of conventional design, and a positive drive power transmission belt 16 trained about and linking sprockets 12 and 14 in synchronous drivi.ng relationship.
Alternatively, the shock-absorbing sprocket may be the driven sprocket rather tnan the driver sprocket, either in the primary or secondary drive (linking transmission output to rear wheel) of a motorcycle.
The endless power transmission belt 16 is preferably formed of a poly-meric body material 17 in which is embedded a tensile member 19. A plurality of teeth 18 are formed on the driving surface of the belt of a predetermined pitch to make mating engagement with corresponding teeth 2C of the shock-absorbing sprocket, and with the teeth of sprocket 14 (not shown).
The shock-absorbing sprocket assembl.y 12 is generally composed of a central hub 24 linput drive), outer rim 26 (output driven means), resilient spring cushioning means 28 and 30, flange bearings 31, 32 sandwiching the hub ~z~

~with bearing 31 being integral Wit]l hub 24), and a pair of rim surfaces 34, 36 integral with the rim and forming radial bearing surfaces, e.g., 33 (Figure 1), with each of the flange bearings 31, 32.
The hub 24 has at least two generally radially protruding lugs 38, 40 which serve to transmit torque to the rim. Depending upon the appl:ication arld size of the sprocket assembly, it may be preferable to use at least three such lugs, to prevent any thrust-indllced wobbles in the assembly. The major diameter of the hub as measured Erom tip-to-t:ip of lugs 38 and 40 in Figure 2, is somewhat less than the interl~al diameter oE the rim to allow clearance for rotative movement. A portion oE the internal bore of the hub is splined to Eorm a journaled fit with splines 44 Eormed on the motorcycle crankshaft 42. The crankshaft, which protrudes from housing 46, is threaded at its tip 48 to receive lock nut 50 which, together with lock plate 51 holds the sprocket in retained assembly.
The hub and its radially ex-tending lugs are mounted for limited rotational movement within the internal cavity of rim 26. In addition to carry-ing teeth 20 about its circumference, the rim also has a plurality of inwardly extending ears 52, 54 adapted to mate with the lugs 38, 40 of the hub, torque being transmitted from the hub member lugs to the ears of the toothed rim member through the resilient cushioning means 28 in the forward direction and reversing cushions 3û in the opposite direction. It is preferred that the cushion means be precompressed (interference fit) as shown, with its side sur-faces exerting a biasing force against the lugs and ears oE the assembly, the advantages including initial elimination of free play, and reduction of free play due to compression set after extended use.
The cushioning means is configured with respect to the rim and hub to define a captive void volume or cavity (under no load) to permit, in use, angular deElec-tio1l of the hub rolative to -the -rim. The void volume and angul-ar deflection may be tailored to the speciEic application. The cap-tive void volume shown in Figures 2 and 3 is determ:ined by -the total volurne occupied bythe resilient cushion 28 together with side clearances 56, 58 i.e., the bound volume between lug 40 of the hub and ear 52 o:E the rim, side bear:ing plates 31 and 32, and arcuate connecting portions 60 and 62 of the hub and rim, res-pectively.
The resilient cushioning means preEerably is conE:igured to have arcuate top and bottom su:rEaces 70, 72, d:ivergir1g side surEaces 74, 76 and substantially planar and preferably generally parallel front ancl rear faces 78,80.
In accordance with one aspect, the cushioned spring members 28, 30 are comprised o:E bodies of suitable elastomeric material loaded with modulus and hysteresis-increasing fibrous reinforcement. It is preEerred that the fibers are generally uniformly dispersed within the elastomeric matrix, as shown in Figure 4 which is a schematic drawing of an S.E.M. photomicrograph of a transverse section of the cushion, at 36X. The elastomeric matrix itselE
preferably has good compression set, high fatigue life, and resistance to any environmental materials, such as oil and grease in accordance with the appli-cation. For instance, nitrile rubber compounds have been found useful in the motorcycle sprocket application. Various natural and synthe-tic rubbers may be utilized and blends thereof, also with other compatible polymeric materials such as thermoplastic resins as well as some thermosets.
The reinforcing fibers must be compatible with the elastomeric matrix and for this purpose may be coated or treated to achieve adhesion with the base elastomer. Various organic and inorganic fibers are useful and the speci-fic choice will be again dictated by the particular application. In general, organic fibers made from polyester, nylon, rayon, cellulosics such as hard and soft woods, cotton linters, aramids, and the like are useful; typical inorganic fibers blendable with the elastomeric custlions i.nclude fiber glass, metallic fibers, carbon fibers and the like. The :fibers may typically have lengths aver-aging from about .4 to about 12.7 millimeters and more preferably from about 1 to about 6.4 millimeters, and aspect ra-tios pre:Eerably from about 30:1 to about 350:1 and more preferably from about 50:1 to about 200:1.
Although the loading levels of the fibers based on the flnish~d resilient cushions will vary in accordance with the application and modulus/
dampening characteristics required, in general it is preferred to employ :from about 3 to about 50, more preferably from 3 to about 30 and most preferably from about 4 to about 10 percent fibers by volume based on the overall cushion volume.
Although not narrowly critical, it has been found highly advantageous to orient the fi.bers 64 predominantly at right angles to the principal direction in which the cushions are placed under load. If directions of the principal strains are known, an efficient use of fibers is to orient them along these maximum and/or minimum principal stra.in axes. With respect to Figures 3 and 4, in use the cushions 28 and 30 are displaced both in a direction parallel to the axis of rotation of the assembly as well as circumferentially, so as to -tend to fill up the adjacent side cavities 56, 58 respectively and to scrub circumferent-ially along the inner surface 62, of the driven rim. As seen in Figure 4, the bulk 64 of the individual fibers are generally aligned in this axial direction, whereby under load the individual aligned fibers 64 in the cushions are placed in tension to thereby resist displacement of the cushions in the direction tending to fill the adjoining captive void cavities. This has the result of very substantially increasing the transverse modulus of the cushion in use.

In certain other applications and clepending upon -the shape oE the individual cushion, and the positioning of the adjacen-t cavities which will define the principal strain axes, a random orientation of the embecldecl fibers may be useful.
Although fiber loading is known to normally reduce fatigue resistance of an elastomeric part~ and produce a somewhat poorer compression set, these problems are ]argely overcome by des;gning the cushion W;til respect to the adjoining cavities so that minimum strain levels are imposed axially on the cushion. The higher modulus material thereby undergoes less strain and hence the fatigue resistance and compression set problems are substantially minimized.
In another aspect, it is preferred to utilize fibrous loading material which swells appreciably when contacted with enviro-nmental materials, such as oil and grease used in bearings of the hub and rim. In this manner the cushions swell appreciably and this swelling off-sets a portion or all of the compression set which is induced after extended flexing of the cushions under cyclic loading and unloading. As a working example, cellulosic fibers marketed under the trademark Santoweb K (Monsanto Company) formed of a hardwood cellulosic fiber of approximately .018 to .025 mm average fiber diameter, with a typical average aspect ratio of 55, was used. This fiber was treated for improved adhesion to nitrile and then uniformly dispersed at a level from about 4 to about 8 percent by volume in a banbury mixer with a typical nitrile (NBR) rubber compound addi-tionally comprising carbon black loading, oil extender, accelerators and vulcanizing agents. After thcrough dispersion the resilient blocks were molded so as to orient the fibers as shown in Figure ~. These blocks had a tensile strength at break of 1500 psi, elongation at break of 100 percent, a durometer of 88 Shore A, compression set of 32 percent after 22 hours at 212~F. (com-pressed to 25 percent of original height), a compression set of 57 percent after 9 _ 70 hours at 250~F., and an increase of 17 pe:rcent in volume after immerSiOJI
in railroad airbrake grease (A.A.R. SpeciEica-tion No. ~-914) for one week at 212F. Thus, significarlt swell took place which tends to offset compression set during use.
Al-though it is preferred to use cushions which are solid, in appro-priate applications per:Eorations or holes may be used to provide the advantages disclosed in -that application.
The torque/angular deElect;on curve ("D") Eor the resiLicn-t cushions of this aspect (Figures 1-4) compared with prior art cushions (Curves A and B) and an experimental cushion (curve C) without Eiber loading are shown in Figure 7. All curves were generated using the sprocket assembly of Figures 1-3 of the drawings. Curve A is the torque deflection relationship using a standard nitrile elastomeric stock with holes penetrating the cushions in the axial direction. Curve B utilizes the same material and construction as Curve A with the exception -that the blocks were solid (no holes). In each of Curves A and B
the breakover point or "knee" 66, 66' was less than about 50 lb-ft. torque.
Curve C on the other hand represents a special carboxylated nitrile stock of high modulus purposely compounded to attempt to achieve a much higher torque at the breakover point 71. Curve D, however, achieved a significantly higher breakover point 73 approximating 190 lb-ft. torque, providing a much greater ability to withstand torque transients in the drive train, with minimal backlash and simultaneously accommodating angular deflections of preferably greater than 8 degrees. The resilient cushions used with respect to Curve D employed the nitrile compound discussed in the above example loaded with 6% by volume Santoweb K. (Trademark).
The fiber-loaded cushion represented by Curve D above has success-fully solved the "speed shift" problem mentioned above, for motorcycle drives.

~ lthougllllot narrowly critica:L, -the slope of the torque/cleflection curve ~below -the "knee") is preEerab]y from about .02 to about .2, more prefer-ably from about .03 to about .09 for the function degrees/lb-Et. The high modulus cushion enables slopes within these desirable ranges to be obtained.
Simultaneously, the corresponding breakovcr point ("knee") is preferably at least about 100 lb-t. more preferably at least 150 lb-ft of torque.
In Figure 5, an alternative app:Lication is shown ;n which a shock-absorbing torsionally elastic belt sheave 12' is employecL in a serpentine accessory drive for a transverse mounted diesel automobile engi.ne. In th:is example, sheave 12' is coupled directly to the engine crankshaft. Shcave 12' is coupled in driving relationship to a backside water pump sheave 75 (to which an offtake belt, not shown, may couple a vacuum pump), air conditioner sheave 77, alternator shea-ve 79 and power steering sheave 81, a tension applying backside idler 83, all coupled by serpentine belt 85. In gelleral, at least three sheaves are li.nked by the belt in driving relation. As shown more clearly in Figure 6, belt 85 may be a typical reinforced endless power transmission belt of V-ribbed construction whose individual ribs 91 wedge into or make frictional contact with corresponding V-grooves 89 formed on the driving circumference of sheave 87.
The use of a torsionally elastic sheave 12' in this application constructed using cushions 28', 30' of high modulus (preferably fiber loaded) or unloaded cushions of relatively soft elastomeric material results in a number of advant-ages including reduction of engine rpm excursions and induced vibration or wobble, as well as damping torque peaks in the drive train. This has been found to translate into tremendously longer belt and drive life.
In an actual comparative test on a transverse mounted 90- V-6 diesel engine serpentine accessory drive of the type shown in Figure 5, three different crank sheaves 12' were employed. Change in rpm ~excursion) at a given rpm level approximating engine idle was measured as was sheave temperature (qualitatively).
The first crankshaEt sheave was the control, a standard sheave of the type shown partially in Figure 6~ without provision o-E ally elastomeric blocks or other torque compensation. The second and thircl crankshaft sheaves employed the com-pensator sheave 12' of the invention using, respectively, cushions 28', 30' loaded with from about 4 to about 8 volume percent Santoweb K. (-trademark) per the example mentioned previously herein, and unloaded relatively soft elasto-meric cushions 28', 30' made of 35 percent acrylonitrile ~BR reinEorced with carbon black and having a modulus of 600 psi at 100 percent elongation, and a tensile strength oE about 1800 psi at break (350 percent elongation). With the control, the excursion defined as the maximum change in rpm per engine cylinder from idle ~600 rpm) was about 13 rpm; with the compensator using high modulus fiber reinforced cushions the excursion was about 9 rpm; and for the compensator using relatively soft cushions the excursion was only about 2.5 rpm.
The control sheave (without torsional compensation) caused the belt to slip about G degrees and the sheave became so hot after a few minutes of engine opera-tion that it scorched the V-ribbed belt. The torsionally elastic sheave with the high modulus fiber-loaded cushion became hot but did not burn or scorch the belt. The third torsionally elastic sheave, with the softer cushions, heated only slightly whereby it was possible to hold onto the sheave (by hand) virtual-ly indefinitely. Of course, the lower the rpm excursionJ the lower the vibration level, belt slip and belt elastomeric material shear and the longer the expect-ed belt and drive train life. With low excursions it is also possible to reduce flywheel weight, etc.
In the drive embodiment of Figure 5 it is preferred to limit the cap-tive void volume between the cushions 28', 30' and adjacent sheave components (i.e., hub, rim, etc.) so as to achieve angular deflections from about .1 to about 8, more preferably from about 3 to about 7 degrees at 100 Ib-ft. torque.

Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A shock-absorbing torsionally elastic single or multiple V-belt or V-ribbed belt power transmission accessory drive assembly for use with an engine exhibiting substantial speed excursions at low rpm, comprising:
a plurality of sheaves each having a peripheral surface provided with at least one V-shaped groove on its driving surface adapted to receive the belt in driving relation: at least one of said sheaves being a shock-absorbing crankshaft sheave assembly, adapted to be coupled to the crankshaft of the engine, and including a rotatable hub having at least two lugs protruding therefrom, a rotatable rim having at least two radially inwardly extending ears adapted to indirectly matingly engage the lugs of the hub and having a peripheral surface formed with said at least one V-shaped groove, and resilient elastomeric cushions interposed in cavities between the lugs and ears in driving relation; said crankshaft sheave being provided with a captive void volume defined as an individual cavity volume less the volume occupied by the cushion contained in that cavity of such amount to allow, in operation, the hub and rim to undergo relative angular deflections of from about .1 to about 8 degrees; and a single or multiple V-belt or V-ribbed belt trained about the sheaves in friction driving relation.

2. The drive assembly of claim 1 in which the resilient elastomeric cushions are formed of high hysteresis elastomeric material.

3. The drive assembly of claim 1 which is rpm excursion reducing in rough running automotive or truck engines, has a serpentine belt, and comprises:
at least three sheaves having multiple V-groove surfaces, one of which is the engine crankshaft sheave which includes the at least two lugs protruding radially from the hub and the peripheral surface of the rim having a multiple V-grooved surface configured to engage the belt; and an endless V-ribbed belt trained in serpentine arrangement about the multiple V-grooved sheaves in friction driving relation.

4. The drive assembly of claim 3 in which the captive void volume is chosen to allow the hub and rim to undergo relative angular deflections of from about 3 to about 7 degrees.

5. The drive assembly of claim l; wherein the resilient elastomeric cushions are formed of high hysteresis elastomeric material.

6. A belt drive assembly according to claim 3 wherein the shock-absorbing sheave has a torque/deflection curve whose slope below the breakover point is from about .02 to about .2 degrees/lb-ft.

7. The belt drive assembly of claim 3 wherein the shock-absorbing sheave has a torque/deflection curve whose breakover point corresponds to greater than 100 lb-ft. torque.