Abstract:
A method of manufacturing a vehicle suspension component, such as a helical coil spring, includes the step of rotating a first impeller to project ceramic peening media in a first direction toward the vehicle suspension component to peen a surface section of the vehicle suspension component. In one example, the vehicle suspension component is peened with metal peening media before peening with the ceramic peening media.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to shot peening and, more particularly, to shot peening a vehicle suspension component using ceramic peening media to increase fatigue properties of the suspension component. 
         [0002]    Suspension components, such as coil springs, stabilizer bars, torsion bars, and the like, have considerable fatigue resistance to withstand repeated cycles of mechanical stress. For example, coil springs are manufactured from steel rods by heating and forming the rods into the desired coil shape. The coil springs are then heat treated and shot peened with steel particles to increase the fatigue resistance. The steel particles impact the surface of the coil spring, thereby compressing the surface and creating a residual compressive surface stress that offsets mechanical tensile stresses to resist fatigue. 
         [0003]    Although using steel particles is effective for increasing resistance to fatigue, there are opportunities for improvement. For example, one problem related to the use of steel particles is that the steel particles wear the peening equipment at a rather quick rate. Depending on the frequency of use, portions of the peening equipment may require replacement over relatively short time intervals, which increase operating expenses. 
         [0004]    Additionally, the level of fatigue resistance that is attainable using steel particles is limited. For example, using larger diameter steel particles would produce a greater amount of residual compressive surface stress. However, the gain in fatigue resistance from the greater residual compressive surface stress is offset by an increase in surface roughness due to impact with the larger diameter steel particles. Thus, steel particles have limited effectiveness for improving fatigue resistance. 
         [0005]    Therefore, there is a need for a peening method that provides less wear on the peening equipment and produces suspension components having enhanced fatigue resistance. The disclosed examples address this need while avoiding the shortcomings and drawbacks of the prior art. 
       SUMMARY OF THE INVENTION 
       [0006]    An example method of manufacturing a vehicle suspension component, such as a helical coil spring, includes the step of rotating a first impeller to project ceramic peening media in a first direction toward the suspension component to peen a surface section of the suspension component and thereby increase a fatigue resistance of the suspension component. 
         [0007]    In one example, the vehicle suspension component is peened with metal or ceramic peening media having a first average size, followed by peening with ceramic peening media having a second average size that is smaller than the first average size. The first peening media compresses the surfaces of the automotive suspension component to provide deep residual compressive surface stress, and the second ceramic peening media smoothes those surfaces to provide a desirable surface roughness while also increasing residual surface stress. The ceramic peening media also produces less wear on the peening equipment. The combination of the first stage peening to obtain deep residual compressive stress and the second stage ceramic peening to obtain low surface roughness and high residual surface stress provides an increase in the fatigue resistance. 
         [0008]    The disclosed examples thereby provide less wear and suspension components having enhanced fatigue resistance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows. 
           [0010]      FIG. 1  is a perspective view of a vehicle suspension system. 
           [0011]      FIG. 2  is a schematic view of an example peening device utilizing one or more blast wheels. 
           [0012]      FIG. 3  is a schematic view of another peening device in which the blast wheels are laterally angled. 
           [0013]      FIG. 4  is a schematic view of a duplex peening process. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]      FIG. 1  illustrates selected portions of an example vehicle suspension system  10 , such as a suspension system of an automobile. The suspension system  10  includes a frame  12  that supports lower control arms  14  and upper control arms  16 . A knuckle  18  is secured between each respective lower control arm  14  and upper control arm  16 , and each knuckle  18  supports a wheel end  20 . Although a four-bar suspension arrangement is shown, it is to be understood that the present invention may be utilized with any suspension arrangement for various types of vehicles. 
         [0015]    A stabilizer bar  22  is arranged laterally between the lower control arms  14 . The stabilizer bar  22  includes a lateral bar portion  24  supported on the frame  12  by brackets  26 . The stabilizer bar  22  also includes arms  28  that are secured to the lower control arms  14  by stabilizer bar links  30 . The stabilizer bar links  30  transmit vertical inputs from the lower control arm  14  and the upper control arm  16  to the stabilizer bar  22  to provide vehicle stability during roll conditions. A coil spring  32  is located between the lower control arm  14  and the frame  12  on each side of the vehicle suspension system  10  for absorbing vibration and impact transferred through the wheel ends  20 . Given this description, one of ordinary skill in the art will recognize that other types of coil springs having different designs, coil thicknesses and coil diameters than the coil springs  32  may alternatively be used. 
         [0016]    The coil springs  32  are manufactured from a steel rod using known forming and heat treatment processes, for example. The coil springs  32  of the disclosed embodiment are peened using a peening process to increase a fatigue resistance of the coil springs  32 . The peened coil springs  32  have a surface hardness of about 46-57 on the Rockwell Hardness C-scale (HRC). The hardness provides the benefit of resisting wear and abrasion, while maintaining a desirable level of toughness. 
         [0017]      FIG. 2  illustrates an example of the peening process. In this example, the peening process utilizes a peening device  42  that includes a blast wheel  44  having impellers  46 . The impellers  46  are driven to rotate at a preset rotational velocity and thereby project ceramic peening media  48  at a corresponding projection velocity and projection rate through an opening  50  in a nominal direction  52 . For example, the nominal direction  52  refers to an average or preset direction, and a portion of the ceramic peening media  48  may deviate within a tolerance of the nominal direction  52 . In one example, the projection velocity is greater than 50 m/s (meters per second) and the projection rate is between 40 kg/min (kilograms per minute) and 200 kg/min to produce a desirable level of fatigue resistance. In a further example, the projection velocity is between 60 m/s and 80 m/s to produce a desirable level of fatigue resistance. 
         [0018]    A supply arrangement  54  supplies the ceramic peening media  48 , such as beads or particles, from a storage reservoir  56  to the blast wheel  44 . The ceramic peening media  48  is manufactured from a known ceramic material, such as zirconium silicate, zirconium dioxide, silicon oxide, silicon carbide, aluminum oxide, other known inorganic non-metallic material, or a combination thereof In a further example, the ceramic peening media  48  is ZIRSHOT® ceramic beads, available from Saint-Gobain. 
         [0019]    A conveyer  58  transports the coil springs  32  (shown schematically) through the peening device  42  such that the projected ceramic peening media  48  impacts surfaces of the coil springs  32  to provide a residual compressive stress at surfaces of the coil springs  32 . In the illustrated example, the conveyor  58  rotates the coil springs  32  about longitudinal axis  60  to provide uniform peening of the surfaces of the coil springs  32 . Although the disclosed examples pertain to the coil springs  32 , it is to be understood that other components within the vehicle suspension system  10  may likewise be peened to increase fatigue resistance. 
         [0020]    Using the ceramic peening media  48  provides the benefit of increased fatigue resistance compared to previous peening processes that do not utilize ceramic media. For example, the ceramic peening media  48  has an average particle size of about 150-230 micrometers. Preferably, the average particle size is about 210 micrometers. The relatively small size compared to metal peening media provides a surface roughness on the coil springs after peening that is less than about 0.025 micrometers. In one example, the surface roughness of the coil springs  32  is about 0.015-0.021 micrometers. The term “about” as used in this description to describe roughness refers to normal variability associated with measuring roughness. 
         [0021]    Optionally, a second blast wheel  44 ′ that is similar to the first blast wheel  44  is used to project ceramic peening media  48 ′ in nominal direction  62 , which is transverse to direction  52 , for example. The second blast wheel  44 ′ may be located before or after the first blast wheel  44  relative to the movement of the conveyor  58 . Each of the blast wheels  44 ,  44 ′ may be vertically oriented directly above the conveyor  58  as in the illustrated example, or angled laterally as illustrated in  FIG. 3 . Combinations of vertical and lateral orientations are also contemplated. 
         [0022]    Using the blast wheels  44 ,  44 ′ and different directions  52 ,  62  provides the benefit of uniformly peening all of the surfaces of the coil springs  32 . Given that peening is a “line of sight” process, using the different directions  52 ,  62  permits the ceramic peening media  48  to access all of the surface portions of a given coil spring  32 . Given this description, one of ordinary skill in the art will recognize that one or more additional blast wheels may be used in conjunction with blast wheels  44 ,  44 ′, depending on the design needs of a particular coil spring  32 . 
         [0023]      FIG. 4  illustrates an example duplex peening process that includes two peening devices  72   a ,  72   b  that are similar to the peening device  42  described above. The first peening device  72   a  corresponds to a first peening stage and the second peening device corresponds to a second peening stage. In this example, the first peening device  72   a  includes three blast wheels  74   a ,  74   b , and  74   c  (shown schematically) that project metal peening media  76 , such as steel media, in corresponding nominal directions  78   a ,  78   b , and  78   c . A conveyer  58 ′ transports the coil springs  32  (shown schematically) through the first peening device  72   a  such that projected metal peening media  76  impacts the coil springs  32  to provide a residual compressive stress at surfaces of the coil springs  32 . The conveyor  58 ′ rotates the coil springs  32  as described above. 
         [0024]    The metal peening media  76  has an average particle size of about 560-600 micrometers. Preferably, the average particle size is about 584 micrometers. The relatively large size of the metal peening media  76  compresses the surfaces of the coil springs  32  to provide a residual compressive surface stress of S 1  and a surface roughness after peening that is greater than about 0.025 micrometers, up to 0.031 micrometers, for example. Given this description, one of ordinary skill in the art will recognize that other sizes of the metal peening media  76 , or other peening media, may alternatively be used to provide different residual compressive surface stresses and different surface roughnesses in the first stage of peening. 
         [0025]    After peening the coil springs  32  using the first peening device  72   a , the conveyor  58 ′ transports the coil springs  32  to the second peening device  72   b . The second peening device  72   b  includes three blast wheels  82   a ,  82   b , and  82   c  (shown schematically) that project ceramic peening media  48  in corresponding nominal directions  78   a ,  78   b , and  78   c  such that the projected ceramic peening media  48  impacts the coil springs  32 . The conveyor  58 ′ rotates the coil springs  32  as described above. Optionally, the metal peening media  76  of the first stage and the ceramic peening media  48  of the second stage are collected after a given peening cycle, filtered to remove fines, and reused in a subsequent peening cycle. 
         [0026]    As described above, the ceramic peening media  48  has an average particle size of about 150-230 micrometers, and preferably about 210 micrometers. In this example, the metal peening media  76  of the first stage has already compressed the surfaces of the coil springs  32 . In the second stage, the ceramic peening media  48  provides additional compression. In addition, the smaller, ceramic peening media  48  smoothes the surfaces of the coil springs  32  and provides a surface roughness that is about 0.015-0.021 micrometers. 
         [0027]    In the disclosed example, using a combination of the metal peening media  76  in the first stage to obtain deep residual compressive stress and the ceramic peening media  48  in the second stage to obtain high residual surface stress and low surface roughness provides the synergistic benefit of significantly increasing the fatigue resistance of the coil springs  32 . In one example, the synergistic benefit is achieved by using approximately equal projection velocities in the first and second stages. That is, the ceramic peening media  48  is projected with a velocity that is within about ±20 m/s of the velocity of the metal peening media  78 . In a further example, the projection velocities are between 60 m/s and 80 m/s to produce a desirable level of fatigue resistance. 
         [0028]    Additionally, using the ceramic peening media  48  provides the benefit of reducing abrasive wear on the peening equipment compared to using metal peening media. The ceramic peening media  48  is smaller and less dense than a metal media, and therefore impacts the blast wheels  44 ,  82   a ,  82   b , and  82   c , conveyor  58 ′, and other components with less energy for a given projection velocity. This reduces the rate of abrasive wear and extends the useful life of the impellers, conveyor  58 ′, and other components. Further, the ceramic peening media  48  extends the life and reduces surface discontinuities and damage on the blast wheels  44 ,  82   a ,  82   b , and  82   c  (wear due to abrasion), resulting in improved blasting performance over the time an individual impeller is used. In use, cast metal impellers stay much smoother, resulting in less “splaying” of the ceramic peening media  48  off the impellers in a random fashion as the blast wheels  44 ,  82   a ,  82   b , and  82   c  rotate. Thus, a greater amount of the ceramic peening media  48  will leave the tip of the impeller in the desired/theoretical projection direction and with the desired velocity, which results in improved peening. 
         [0029]    Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
         [0030]    The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.