Energy transfer device

Abstract: Clutch and brake components especially adapted for use in electromagnetic assemblies (50) include a first disc (56) or plate having a ground surface (58) comprising parallel surface features and a second disc or plate (64) having a turned surface (66) comprising circular or spiral features. Excellent static and dynamic frictional coefficients are achieved. Preferably, the ground (58) and turned (66) surfaces have roughness of between Ra 3 and 300. A wide variety of variously alloyed steels may be utilized to fabricate the clutch or brake components.

Description: This invention relates to frictional surfaces for selectively activated energy transfer devices, in particular for opposing brake and clutch components. There is continuing emphasis directed to improving power and torque transmission levels and performance through frictionally engaging clutch and brake components. This is especially true with regard to automotive related products where volume reduction is necessary due to reduced vehicle size and weight reduction is a constant though increasingly difficult goal. An area of brake and clutch technology receiving particular attention involves those frictionally coupled energy transmitting components associated with electromagnetic clutches. Such clutches are commonly used to couple and decouple air conditioning compressors from an associated drive member and portions of four-wheel drive schemes from one another. Such electromagnetic clutches are called upon to perform high torque transfer and cycle life but are faced with significant space and cost constraints. Patent art discloses significant activity in this field. For example, US-A-2,965,205 teaches a high torque capacity clutch or brake for automatic transmissions. The device includes a frusto-conical steel ring having a smooth surface which mates with a similarly configured aluminium ring which is first knurled and then coined such that the knurled surface conforms to the surface of the ring. US-A-3,750,788 teaches a clutch or brake configuration wherein a wear resistant material includes a contact face formed exclusively of adjacent isosceles triangular teeth in a circular pattern. The teeth deform the opposed friction lining material and create complementary triangular depressions in a circular pattern. In US-A-4,632,236, an electromagnetic clutch includes a frictional surface which is machined to a generally trapezoidal shape. The opposing frictional surface is polished and both surfaces are plated. US-A-4,727,974, discloses another electromagnetic clutch configuration in which one of the clutch surfaces includes a plurality of irregularly shaped projections and the other includes machined and plated concave channels and projections arranged in a spiral pattern. From a review of the foregoing patents and appreciation of the service life and torque requirements of brakes and clutches, it is apparent that improvement in such devices are both possible and desirable. Another prior art proposal is featured in US-A-4705973 which generally discloses a selectively activated energy transfer device comprising a first member disposed for rotation about an axis and having a first friction surface disposed perpendicularly to the axis, a second member having a second friction surface disposed in opposition to the first friction surface, and means for selectively translating one of the first and said second members and the respective friction surface into contact with the other of the members and its respective friction surface, one of the first and said second friction surfaces having generally circular surface irregularities. More particularly, the transfer device of the aforementioned US-A-4705973 has both of its friction surfaces formed with generally circular surface irregularities, its rotor and armature plate both containing spiral, concave grooves and projections. In accordance with the present invention as claimed, the aforesaid generally disclosed selectively activated energy transfer device is characterized in that the other of the first and second friction surfaces has linearly parallel surface irregularities. Preferably, the generally circular surface irregularities are produced by turning and the linearly parallel surface irregularities are produced by grinding. The turned and ground surface irregularities preferably have a roughness of between Ra 0.0762 and 7.62 micrometres (3 and 300 microinches). The second member may rotate about the rotational axis of the first member, in which case the transfer device functions as a clutch. Alternatively, the second member may be fixed, in which instance the transfer device functions as a brake. The brake or clutch components may be fabricated of a wide variety of variously alloyed steels. It is found that the combination of opposing friction surfaces, one with generally circular surface irregularities and the other with linearly parallel surface irregularities, exhibits excellent static and dynamic frictional coefficients, as will be discussed hereinafter. In order that the present invention may be well understood there will now be described an embodiment thereof, given by way of example, reference being made to the accompanying drawings, in which: Figure 1 is a full, sectional view of a portion of a four-wheel drive vehicle transfer case which incorporates the present invention ; Figure 2 is a plan view of the rotor face of an electromagnetic clutch assembly incorporating the present invention; Figure 3 is a plan view of an armature face of an electromagnetic clutch incorporating the present invention; Figure 4 is an enlarged view of the surface finish of the rotor face or coil housing illustrating the ground, i.e., straight line, finish pattern; Figure 5 is an enlarged view of the surface finish of an armature of an electromagnetic clutch according to the present invention having a turned circular or spiral, pattern; Figure 6 is a side, elevational view with portions broken away of a low velocity friction apparatus on which the data of Figures 7 through 10 were developed; Figure 7 is a graph showing the performance of a clutch having one ground and one turned surface according to the present invention achieved in a low velocity friction test apparatus of Figure 6; Figure 8 is a graph showing the performance of a clutch having two ground surfaces achieved in a low velocity friction test apparatus of Figure 6; Figure 9 is a graph showing the performance of a clutch having one turned and one tumbled surface achieved in a low velocity friction test apparatus of Figure 6; and Figure 10 is a graph showing the performance of a clutch having one ground and one turned surface according to the present invention achieved in a low velocity friction test apparatus of Figure 6. Referring first to Figure 1, an in-line vehicular clutch assembly incorporating the present invention is illustrated and generally designated by the reference numeral 10. The clutch assembly 10 illustrated is intended for use in the drive train of four-wheel drive vehicles to selectively provide power to the front wheels of said vehicle. The clutch assembly 10 includes a housing 12 rotatably supporting an input drive shaft 14 at one end and rotatably supporting a first output drive shaft 16 at its opposite end. The housing 12 includes a radially outwardly extending lobe 18 which rotatably supports a second output drive shaft 20. Disposed within the housing 12 is a planetary gear assembly 24. The planetary gear assembly 24 includes three sets of concentric and co-acting, relatively rotatable components. First of all, an internally toothed ring gear 26 is operably coupled through sets of interengaging splines 28 to the input drive shaft 14 and rotates therewith. Meshing with the ring gear 26 are pairs of pinion or planet gears 30A and 30B, one of each pair being illustrated in Figure 1. The pairs of planet gears 30A and 30B are rotatably disposed upon corresponding pairs of suitable stub shafts 32A and 32B which are in turn secured within a planet carrier 34. The planet gears 30A and 30B constitute two gear trains, the outer planet gear 30B of each train meshing with the ring gear 26 and the inner planet gear 30A of each gear train meshing with a sun gear 38 which is coupled to or integrally formed with a sleeve shaft 40. The sleeve shaft 40 is disposed concentrically about and rotates independently of the input drive shaft 14. At the opposite end of the sleeve shaft 40, a first gear or chain drive sprocket 42 is coupled thereto by interengaging splines 44. The first chain drive sprocket 42 receives and drives a chain 46 or other similar and suitable power transfer means which engages a second gear or chain drive sprocket 48 coupled by interengaging splines 49 to the second output drive shaft 20. As will be readily appreciated, rotary motion of the input drive shaft 14 effects rotary motion of the output drive shaft 16, the sleeve shaft 40, the planet carrier 34 and the output drive shaft 16 in predetermined ratios. Depending on the torque transfer characteristics desired, any one of the three gear sets of the planetary gear mechanism 24 may be coupled to the input shaft 14. Also disposed within the housing 12 is an electromagnetic clutch assembly 50. The electromagnetic clutch assembly 50 includes a stationary magnetic coil 52 mounted upon a formed mounting plate 54 which is received within and secured to the housing 12 by suitable means. Adjacent the coil 52 and concentrically disposed about the sleeve shaft 40 is a clutch rotor 56. Spaced from the rotor 56 and arranged in aligned opposition therewith is an armature 64. As illustrated in Figure 2, the rotor 56 includes a clutch or brake surface or facing 58 and defines a plurality of spaced apart, circumferentially extending slots 60. As illustrated in Figure 3, the armature 64 defines a clutch or brake surface or facing 66 which opposes and selectively contacts the surface 58 of the rotor 56 and further defines a plurality of spaced apart circumferentially extending slots 68. The single interrupted circle of the slots 68 is preferably disposed at a radial location midway between the two circles of slots 60 in the rotor 56 as illustrated in Figure 1. The surface 66 of the armature 64 defines a plurality of radially extending channels or grooves 70. Turning again to Figure 1, the armature 64 also includes an axially extending annulus 72 which is coupled to the planet carrier 34 by opposed interengaging sets of teeth 74A and 74B. A disc pack clutch assembly 78 operable disposed between the planet carrier 34 and the sleeve shaft 40 includes a first plurality of clutch plates 80 coupled by suitable splines to the planet carrier 34 and a second plurality of interleaved clutch plates 82 coupled to the sleeve shaft 40. A plurality of movable locking balls 84 are disposed between the rotor 56 and a connecting sleeve 86. The movable locking balls 84 are disposed in a like plurality of pairs of conically shaped recesses having ramps on the opposed faces of the rotor 56 and a connecting sleeve 86. Finally, the clutch assembly 50 includes a plurality of compression springs 88 operably disposed between the rotor 56 and a retaining ring 90 coupled to the sleeve shaft 40. The compression springs 88 bias the rotor 56 away from the armature 64. When the electromagnetic clutch assembly 50 is deenergized, no direct connection exists between the first and second output shafts 16 and 20. The torque delivered to each of the output shafts 16 and 20 is determined by the configuration and coupling of the planetary gear set 24. Application of current to the coil 52 of the electromagnetic clutch assembly 50 creates a magnetic flux which causes the armature 64 to translate toward the rotor 56 thereby initiating a frictional driving connection between the rotor 56 and the armature 64. Because the armature 64 is coupled to the first output drive shaft 42, it will rotate at the same speed. Likewise, because the rotor 56 is coupled to the second output shaft 20 through the drive chain 46, it will rotate at the same speed as the second output drive shaft 20. Therefore, if relative motion exists between the power output shafts 16 and 20, such motion will cause the balls 84 to roll up the conically shaped recesses and forcibly move the rotor 56 and the armature 64 to compress the friction plates 80 and 82 of the disc pack clutch assembly 78. When compressed, the plates 80 and 82 transmit energy between the output shafts 16 and 20, adding torque to the slower turning shaft and subtracting torque from the faster turning shaft. By modulating the current to the coil 52 of the electromagnetic clutch assembly 50, the relative rotation of the output shafts 16 and 20 may be controlled. The clutch assembly 10 just described and its operation are more fully described in co-owned US-A-4,718,303. Turning now to Figures 2 and 4, the rotor 56 and the greatly magnified surface 58 of the rotor 56 are shown, respectively. The rotor surface 58 is ground such that a plurality of linearly parallel grind marks, i.e., surface irregularities 96, are generally parallel and define generally parallel straight line ridges and valleys on the surface 58 of the rotor 56. Grinding is the preferable method of producing this finish of the surface 58 though other machining techniques which effect a pattern of substantially linearly parallel ridges and valleys may be utilized. The surface roughness may be in the range of from Ra 0.0762 to 7.62 micrometers (3 to 300 micro inches) and is preferably between Ra 1.524 and 5.08 micrometers (6 and 200 micro inches). Referring to Figures 3 and 5, the surface 66 of the armature 64 defines a similar surface finish. Here, however, the surface scratches or irregularities 98 are arranged in a circular or spiral pattern. Such surface finish is preferably achieved by turning though other machining techniques which produce a pattern of generally circular or spiral closely adjacent ridges and valleys are totally suitable. Again, the surface roughness may be in the range of Ra 0.0762 to 7.62 micrometers (3 to 300 micro inches), a preferable range being Ra 1.524 to 5.08 micrometers (6 to 200 micro inches). It should be clearly understood that the described and illustrated arrangement of surface treatments on specific components is exemplary and illustrative only. Either the rotor 56 or the armature 64 may include the linearly parallel (ground) surface 58 while the other may include the circular or spiral (turned) surface 66 or vice versa. In other words, the invention resides in the combination of the surface treatments and associated clutch componentry, not the specific arrangement of surface treatments on specific components. As those familiar with such apparatus will readily appreciate, the present invention may be incorporated into and utilized with a broad range of clutch as well as brake assemblies wherein it is desirable to achieve significant energy transmission and braking capability in relatively compact components. If the device is to be utilized as a brake, one of the two rotatable members, either the rotor 56 or the armature 64, is reconfigured as a stationary component such that when the electromagnetic clutch assembly 50 is activated, the frictional surfaces 58 and 66 engage and inhibit rotation of the other component. Several examples of the performance of clutch and brake components incorporating this treatment on the clutch or faces are set forth below. As mentioned previously it should be noted that the precise mechanism of improved clutch performance achieved through the utilization of opposed clutch faces having linearly parallel and circular or spiral surface treatment is not fully understood. As previously stated, it is believed, however, that the constantly changing relationship between the opposed ground and turned marks, collectively referred to as the surface irregularities 96 and 98, which alternates between perpendicularity, parallelism and back to perpendicularity twice per revolution provides a constant pumping and redistributing energy to the clutch fluid thereby ensuring uniform fluid distribution and enhancing clutch and brake performance. In order to confirm the benefits and performance of the present invention, four different combinations of clutch face surface treatments were tested at several different operating temperatures. Due to the time and expense attendant a full scale testing program, it was determined to perform such tests on a low velocity friction apparatus (LVFA). The low velocity friction apparatus has been used extensively for the study of automatic transmission clutch and fluid characteristics because the results obtained through such tests have been found to correlate well with the performance of such materials in actual transmission service. A low velocity friction apparatus in which the data presented in Figures 7 to 10 was gathered is illustrated in Figure 6. Referring now to Figure 6, a low velocity friction apparatus 100 is illustrated and includes a shaft 102 which is driven by a variable speed apparatus (not illustrated) capable of sweeping from 0 R.P.M. up to a maximum selected rotational speed and back to 0 R.P.M. At the end of the shaft 102 is a demountable cap 104 of one of the clutch materials under test having a terminal surface 106 which may be finished as desired to duplicate the clutch surface to be tested. The shaft 102 and cap 104 are generally received within a cup 108. The cup 108 receives a sample plate 110 having an annulus 112 of the second material under test which likewise defines a face 114 opposite the face 106 of the cap 104. The cup 108 receives a small charge of a typical friction clutch lubricant 109 such as automatic transmission fluid or other similar representative clutch lubricant. The cup 108 is supported upon a hemispherical bearing 116 which is in return received in a complementally configured blind opening 118 of a support piston 120. The piston 120 is supported by an air bearing to minimize external, particularly lateral, forces and friction. The cup 108 also includes a radial torque arm 122 which is terminated in a tangential knife edge 129. The knife edge 129 abuts and contacts a strain gauge 126 or similar transducer which senses the force transmitted through the interface between the surfaces 106 and 114 and drives suitable display and recording instrumentation (not illustrated). The following Figures represent the performance of various materials utilized for the surface 106 of the demountable cap 104 and the surface 114 of the annulus 112. The coefficient of dynamic friction is presented as the Y axis or ordinate distance. The X axis or abscissa presents the speed in meters per second which is the nominal linear speed of the frictional interface at an average radius of the annulus 112. The result plotted is an average of a first accelerating speed sweep beginning at 0 R.P.M. and increasing to the maximum plotted speed of .5 metres per second and a second decelerating speed sweep back to 0 R.P.M. over a span of approximately 20 seconds. Several traces are presented in each Figure which represent the performance under the stated temperature condition. Turning now to Figure 7, the five traces presented show the test results obtained by the foregoing test procedure with one surface that is either the surface 106 or the surface 114 of 1018 steel ground to a surface roughness of Ra 30 and the other surface of 1117 turned steel. For such surface treatments, the coefficient of dynamic friction is about .121. Turning to Figure 8, the clutch surface treatments are a first surface of 1117 steel ground to a surface roughness of Ra 0.4065 micrometers (16 micro inches) and a 1018 steel plate also having a ground surface. This combination of surface treatment results in a coefficient of dynamic friction of approximately .074. In Figure 9, a 1117 steel face has been turned with a roughness of Ra 0.762 micrometers (30 micro inches) and is engaged against a 1035 tumbled steel plate. Finally in Figure 10, a 1117 steel clutch surface, turned to have a surface roughness of about Ra 0.762 micrometers (30 micro inches), engages a 1035 ground steel surface. This combination produces a coefficient of dynamic friction of approximately .117. It should be appreciated that the foregoing test results are representative of performance of various clutch surface treatment combinations. They clearly show the improved coefficient of dynamic friction achieved with opposing ground and turned surfaces, presented in Figures 7 and 10, relative to other combinations of clutch surface treatments illustrated in Figures 8 and 9. Once again, it should be understood that the specific disposition of the ground or turned surface on either the rotor or armature, or other clutch components, is per se of no consequence. That is, it is the combination of ground and turned surfaces not the specific components that carry them that is significant. The invention thus resides in clutch or brake components where one of the surfaces is turned to have concentric or spherical surface treatment interruptions or irregularities whereas the opposing clutch component is ground such that its surface defines linearly parallel surface interruptions or irregularities.

Claims: A selectively activated energy transfer device comprising a first member (56; 64) disposed for rotation about an axis and having a first friction surface (58; 66) disposed perpendicularly to the axis, a second member (64; 56) having a second friction surface (66; 58) disposed in opposition to the first friction surface (58; 66), and means (50) for selectively translating one of the first and said second members (56, 64) and the respective friction surface (58, 66) into contact with the other of the members (64, 56) and its respective friction surface (66, 58), one of the first and said second friction surfaces (58, 66) having generally circular surface irregularities (98), characterized in that the other of the first and second friction surfaces (66, 58) has linearly parallel surface irregularities (96). A device as claimed in claim 1, wherein the second member (64; 56) is fixed and the device functions as a brake. A device as claimed in claim 1, wherein the second member (64; 56) rotates about the rotational axis of the first member (56; 64) and the device functions as a clutch. A device as claimed in any of the preceding claims, wherein the selectively translating means (50) includes an electromagnetic coil (52). A device as claimed in any of the preceding claims, wherein the linearly parallel surface irregularities (96) are produced by grinding and have a roughness of between Ra 0.0762 and 7.62 micrometers (3 and 300 microinches). A device as claimed in any of the preceding claims, wherein the generally circular surface irregularities (98) are produced by turning and have a roughness of between Ra 0.0762 and 7.62 micrometres (3 and 300 microinches). A device as claimed in any of claims 1 to 4, wherein the linearly parallel surface irregularities (96) are produced by grinding and the generally circular surface irregularities (98) are produced by turning. A device as claimed in any of claims 1 to 4, wherein the linearly parallel and the generally circular surface irregularities (96, 98) have a roughness of from between Ra 0.0762 and 7.62 micrometres (3 and 300 microinches).

Inventors: YESNIK MARC A, YESNIK, MARC A.
Applicants: BORG WARNER AUTOMOTIVE, BORG-WARNER AUTOMOTIVE TRANSMISSION AND ENGINE COMPONENTS CORPORATION
Application year: 1991
Main classification: F16D27
IPC codes: F16D27, F16D55, F16D65, B60K17, B60T1, F16D69