Abstract:
A belt for a continuously variable transmission (CVT) includes at least one continuous band supported against a contact face in a slot formed in each of a plurality of transverse elements of the CVT. The band has a surface positioned against the contact face. This surface is coated with chromium nitride by physical vapor deposition to reduce wear.

Description:
TECHNICAL FIELD 
     The present invention relates to a CVT belt having a band with a chromium nitride coating to improve wear resistance. 
     BACKGROUND OF THE INVENTION 
     Continuously variable transmissions (CVTs) are a result of the continuing pursuit of more fuel-efficient motor vehicles. The CVT differs from the conventional automatic transmission in that automatic transmissions use planetary gear sets to accomplish speed ratio changes, whereas CVTs use pulleys and a belt to change speed ratio. A conventional automatic transmission usually offers four, five or six fixed ratios or speeds, whereas a CVT offers an infinite number of ratios which can be achieved by changing the relative radius of travel of the driving belt on the driving and driven pulleys. 
     The variator of the CVT includes two steel pulleys and a steel belt. Each pulley is comprised of two opposing angled sheaves, one moveable and one fixed. The pulleys can be opened and closed to allow the belt to travel at different radii by axial movement of the moveable sheave with respect to the fixed sheave. When the driving pulley is fully open (small radius of belt travel) and the driven pulley is fully closed (large radius of belt travel) very high speed reduction ratios are achieved. Conversely, when the driving pulley is fully closed (large radius of belt travel) and the driven pulley is fully open (small radius of belt travel) increases in output speed over input speed are achieved. 
     CVTs have become increasingly popular in recent years because they may provide improved fuel economy versus conventional step gear automatic transmissions, the ability to operate the engine at lower rpms over a wider range of the fuel economy schedule, smooth shifting, more efficient vehicle front end packaging, as well as manual transmission interchangeability and all-wheel drive compatibility. The application of CVTs into light duty vehicles, especially for future use in conjunction with vehicles having higher horse power engines, requires CVTs to have higher torque capacity and excellent durability or wear resistance of components. 
     A problem which can occur with current designs results from internal belt vibrations and can manifest itself as “belt shudder” or “scratch.” Belt shudder is an objectionable vibration which has been found in certain CVT arrangements. This type of vibration begins to occur at relatively low mileages and can be commercially unacceptable. Belt shudder usually occurs during light throttle parking lot maneuvers, i.e., conditions under which less than ten percent throttle is applied, when the speed is in the range of 1400 to 2200 rpm. It usually occurs at a temperature below 80° C. Belt shudder is caused by a stick-slip phenomenon that occurs between the inner band and the element shoulder of the CVT belt. Belt shudder usually occurs when the crosshatched surface of the inner band becomes polished down as a result of wear. The belt shudder occurs at the transition point at which the belt changes from a pull to a push torque transfer mechanism. 
     The two primary methods for reducing the incidence of belt shudder are calibration strategy and fluid development. Calibration helps avoid regions or conditions under which belt shudder occurs. Proper calibration reduces pressure during transition from push to pull to “ratio-around” the problem. Fluid with properly designed friction characteristics can help prevent stick-slip phenomenon. The desired friction characteristics include positively sloped friction coefficient versus sliding speed, or higher dynamic friction than static friction. This represents a significant technical challenge when trying to balance against the needs of other transmission requirements. 
     SUMMARY OF THE INVENTION 
     The present invention provides a CVT belt having improved wear resistance by providing a chromium nitride coating onto a contact surface of the continuous band of the belt which contacts the transverse element, thereby reducing or eliminating belt shudder. 
     More specifically, the invention provides a belt for a continuously variable transmission (CVT) including at least one continuous band supported against a contact face in a slot formed in each of a plurality of transverse elements. The continuous band has a surface positioned against the contact face. This surface is coated with chromium nitride to reduce wear. Preferably, the coating is achieved by physical vapor deposition, such as reactive gas sputtering or arc evaporation. The nitride coating is preferably between approximately 0.5 and 2.5 micrometers in thickness. More specifically, the nitride coating is approximately 1.3 to 1.8 micrometers in thickness, and preferably approximately 1.7 micrometers in thickness. 
     The continuous band is formed of a maraging steel with a nitride surface heat treatment. After the nitride coating is applied, the surface has a Vickers hardness of approximately 1500 to 3000 Hv. 
     A method of manufacturing the band, including applying a chromium nitride coating, may also include ultrasonically degreasing the band, ion cleaning the band in an argon plasma atmosphere, and applying the coating by reactive gas sputtering in a closed field unbalanced magnetron sputtering system. 
     The above features and advantages, and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a partial perspective view of a CVT belt for use with the present invention; 
         FIG. 2  shows a schematic cross-sectional view of a CVT belt in accordance with the invention; and 
         FIG. 3  shows a chart illustrating Rpk decrease in percentage for different test components under different test loads. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a partial perspective view of a CVT belt  10  for use with the present invention. As shown, the CVT belt  10  includes a plurality of transverse elements  12  having opposing slots  14 ,  16  for receiving steel rings  18 ,  20 . As shown, the ring  20  includes  12  continuous steel bands  22 ,  24 ,  26 ,  28 ,  30 ,  32 ,  34 ,  36 ,  38 ,  40 ,  42 ,  44 . The continuous steel bands  22 - 44  are formed from a maraging steel with a nitride surface heat treatment. As viewed in  FIG. 1 , the top surface of each band  22 - 44  is machined smooth, and the lower surface is machined with a cross-hatch pattern. Each maraging steel band  22 - 44  has a chemical composition (weight percentage) of: Ni: 18.5%; Co: 8.9%; Mo: 4.8%; Ti: 0.39%; and Fe: balance. 
       FIG. 2  shows a schematic vertical cross-sectional view of the CVT belt  10  of  FIG. 1 , with only three of the bands  22 - 44  shown for simplicity. The bands are represented by reference numbers  22 ,  32 ,  44 ,  22 ′  32 ′,  44 ′. The transverse element  12  shown in  FIG. 2  is a low carbon steel component having a base  46 , a neck  48  and a head  50 . The base  46  has side faces  52 ,  54  positioned to contact the pulleys  56 ,  58 , respectively. Each transverse element  12  also includes an alignment feature  60 , such as a protusion, to facilitate alignment with adjacent transverse elements  12 . 
     Open slots  62 ,  64  are formed between the head  50  and shoulders  66 ,  68 , respectively, of the element  12 . The slots  62 ,  64  receive the rings  20 ,  18 , respectively. The shoulder  66  has a contact face  70  which engages the lower surface  74  of the continuous band  44 , and the shoulder  68  has a contact face  72  which engages the lower surface  76  of the continuous band  44 ′. 
     Rubbing of the contact face  70  against the lower surface  74  and the contact face  72  against the lower surface  76  may lead to wear of the lower surfaces  74 ,  76 , which may result in belt shudder, as described previously. Accordingly, the lower surfaces  74 ,  76  of the respective bands  44 ,  44 ′ are coated with chromium nitride via physical vapor deposition to improve wear resistance. 
     The bands  44 ,  44 ′ are cleaned ultrasonically before being loaded into a physical vapor deposition machine, such as an Unbalanced Magnetron Sputtering system (available from Teer Ltd. of England or Hauzer of Holland), or by arc evaporation. The thickness of the coating is preferably 1.7 micrometers, or between 0.5 and 2.5 micrometers. The deposition temperature is below 180° C. Arc evaporation PVD machines are available from Ionbond of Madison Heights, Mich. 
     Prior to physical vapor deposition, the bands are ultrasonically degreased in a 5% solution of an industrial degreaser, such as Blue Gold, and a 5% solution of an industrial cleaner, such as Contrad 70, while heated to 55° C. The bands are rinsed in de-ionized water after each 30 minute cleaning step. Final cleansing is achieved with a rinse of copious methanol. 
     After chemical cleaning, the chromium nitride deposition is achieved by first ion cleaning the surface of the bands for 30 minutes in an argon plasma atmosphere and a substrate bias of −400 V pulsed at a frequency of 250 kHz and 500 nsec pulse width. A 100 nm chromium interlayer is deposited in about four minutes as a bond coating at a similarly pulsed bias voltage of 100 V to enhance adhesion. The chromium target of 99.99% in a nitrogen containing argon environment is used. The base pressure of the system is about 1.3 exp (−4) Pa and the pressure during deposition is about 0.13 Pa. The gas is a mixture of 99.999% pure argon and 99.99% pure nitrogen. The nitrogen flow rate is varied between 10 and 75 standard cubic centimeters per minute (sccm) to provide different Cr/N ratio thin films. The CrN thin films are deposited onto the bands by reactive gas sputtering in the Closed Field Unbalanced Magnetron sputtering system. The CrN depositions are completed in 40 to 120 minutes while being similarly pulsed biased at 150 V. Hundreds or thousands of bands could be treated together simultaneously in such a process in a mass production environment. 
     In a test subject, the band surface roughness was approximately 675 nm Ra at 10× magnification. Accordingly, the roughness average did not change significantly as a result of the physical vapor deposition. 
     The coating hardness of the end product is in the range of 1500 to 3000 Hv (Vickers hardness), which is much harder than the original maraging steel band with the nitride surface heat treatment. Because the coating is very thin and deposited via sputtering or arc evaporation, it has a much stronger bonding to the substrate than any other types of coatings, such as plating or thermal spray. In addition, a chromium nitride coating has a better adhesion to the substrate than other coating materials such as titanium nitride and chromium carbide. The coating is not easy to debond from the substrate even in harsh temperature cyclical conditions. 
     Testing of chromium nitride coated bands has shown that the coated bands have little or no wear in comparison to uncoated bands, which experience substantial polishing wear. This result is illustrated in  FIG. 3 .  FIG. 3  illustrates the decrease in percentage Rpk after a test cycle is performed to induce wear. Rpk is the measure of peak heights above the nominal/core roughness. As shown in  FIG. 3 , with a test load of 100 Newtons, a band having no coating experienced a 67% decrease in Rpk while a band with a thin (0.7 micrometer) chromium nitride coating experienced only a 49% decrease in Rpk percentage. Surprisingly, a band having a thick (1.7 micrometer) chromium nitride coating experienced only a 10% decrease in Rpk percentage. Similarly, under a 175 Newton test load, the uncoated band experienced a 56% decrease in Rpk, the band with the 0.7 micrometer chromium nitride coating experienced a 52% decrease in Rpk, and the band having 1.7 micrometer chromium nitride coating experienced only a 14% decrease in Rpk. These differences are substantial, and translate directly into a reduction or elimination of belt shudder since roughness of the surface of the belt which contacts the element is maintained due to the chromium nitride coating. This roughness stability eliminates the stick-slip phenomenon. 
     Supporting disclosure related to CVT belts may be found in U.S. Pat. Nos. 6,464,606; 6,497,633; 6,254,503, 6,337,309; and 6,451,745, each of which is hereby incorporated by reference in its entirety. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.