Patent Application
20090036031
The present disclosure relates generally to a surface finishing process, and more particularly, to a coating assisted surface finishing process.
On a microscopic scale, surfaces of most components are not perfectly smooth. Instead, these surfaces are characterized by numerous microscopic hills and valleys (termed “asperities”). When surfaces of two components contact and undergo relative motion with respect to each other, the asperities of one surface score and scratch the other surface. The surfaces of such interacting components are subject to surface failures, such as scuffing and wear, due to the interaction of these asperities. These contacting components that undergo relative motion often require premature component replacement due to wear related failures. In this disclosure, such components that contact and undergo relative motion with respect to each other are referred to as “tribological components.” Non-limiting examples of tribological components include mating gears, a shaft sliding on a bearing, piston sliding on a cylinder, ball bearing components, etc.
Typical manufacturing processes for tribological components include processes used to change the shape of the material (such as, for example, casting, forging, machining, pressing, etc.), and processes used to finish the component to a desired dimension (for example, grinding, shaping, milling, etc.). Some tribological components may also be subjected to a final polishing process, such as honing, lapping etc., to obtain a desired surface finish along the working surfaces of the components. In this disclosure “working surfaces” refer to surfaces of tribological components that contact each other while undergoing relative motion with respect to each other. For example, the contacting surfaces of the teeth of a pair of meshed gears, surfaces of a piston and a cylinder that contact each other during the sliding of the piston, etc. are all referred to as working surfaces. The term “polishing” is also used generally to refer to any process that improves the surface finish of a working surface.
One polishing process used to improve the surface finish of working surfaces of gears is described in U.S. Pre Patent Publication No: 2004/0088861 A1 (the '861 publication) issued to Vinayak et al. on May 13, 2004. In the polishing process of the '861 publication, the gear is immersed in a slurry containing ceramic elements and vibrated using a high frequency shaking apparatus. During the vibration process, the ceramic elements impact the exposed surfaces of the gear in a random manner resulting in an smooth gear surface. In the '861 publication, the gear geometry before polishing is designed to account for excess material removal from regions that are more exposed to the ceramic elements than regions that are protected by the geometry of the gear. For example, due to the geometry of the gear, the top surface of the gear may be more prone to impact by the ceramic elements than the root of the gear. Therefore, material removal from the top surface of the gear may be more than the root of the gear. To account for the uneven material removal during polishing, the top surface of the gear may be designed to include excess material that may be removed during polishing.
Although the process of the '861 publication may produce a tribological component with a smooth surface finish, the process may have drawbacks. For instance, the process of the '861 publication may also remove material from non-working surfaces of the component, and therefore, change other dimensions of the component. The process of the '861 publication may also be expensive. Also, having to account for uneven material removal by providing excess material at exposed regions of the gear may complicate the design process. Since the process of the '861 publication immerses and shakes the tribological component in a slurry, the size and motor rating of the shaking apparatus may have to be increase with the size of the component, thereby increasing the cost of the polishing operation.
The present disclosure is directed at overcoming one or more of the shortcomings set forth above.
In one aspect, a method of polishing a component is disclosed. The method includes providing a first component with a first working surface and depositing a coating on a second working surface of a second component. The first component and the second component are tribological components. The method also includes mating the first component to the second component. The mating includes contacting the first working surface with the second working surface. The method further includes operating at least one of the first component and the second component, where the operating includes the first working surface and the second working surface undergoing relative displacement while contacting each other, and stopping the operation after polishing the first working surface.
In another aspect, a method of polishing a first component by a second component of a tribological pair is disclosed. The method includes mating the first component to the second component in a tribological configuration. The first component and the second component are moved such that a first surface of the first component and a second surface of the second component contact and slide against each other, so that the second component polishes the first component. The method further includes discontinuing moving of the first component and the second component when the first surface obtains a desired surface finish.
In yet another aspect, a component polishing system is disclosed. The system includes a first coupling mechanism configured to attach a first component. The first component includes a first working surface. The system also includes a second coupling mechanism configured to attach a second component and position the second component to contact the first working surface. The second component includes a second working surface with an abrasive coating thereon. The first component and the second component are tribological parts that are configured to slide on each other in an application. The system further includes a mechanical power source coupled to one of the first coupling mechanism or the second coupling mechanism. The mechanical power source is configured to induce relative displacement between the first component and the second component.
Gear 12 may also include a pitch circle through a point on flank 28 where gear 12 contacts pinion 14 in the meshed configuration (depicted in
As shown in
The outer surface 52 of abrasive pinion 50 may include a coating 54. In some embodiments, outer surface 52 may include the entire external surface of abrasive pinion 50, while in other embodiments, outer surface 52 may only include selected surfaces. These selected surfaces may include the working surfaces of abrasive pinion 50. During the transmission of motion between abrasive pinion 50 and gear 12, outer surface 52 of abrasive pinion 50 may rub against outer surface 42 of gear 12. When the outer surfaces 42 and 52 slide against each other, coating 54 on outer surface 52 may abrade outer surface 42 of gear 12. Rotation of gear 12 and abrasive pinion 50 may cause the entire working surface of gear 12 to be abraded by abrasive pinion 50. Repeated abrasion, resulting from continued rotation of gear 12 and abrasive pinion 50 may reduce the height of asperities 44 on outer surface 42 and polish outer surface 42. In some embodiments, such as, for example, in a pair of gears undergoing rolling contact, the two mating gears may be mated in a slightly off-pitch configuration to enhance the polishing action.
It is contemplated that, in some embodiments, a separate surface polishing system 500 may be eliminated, and the polishing of gear 12 may be carried out in machine 900. For instance, when polishing of gear 12 is desired, pinion 14 (see
Coating 54 may be deposited on pinion 50 by a vapor deposition technique, such as chemical vapor deposition or physical vapor deposition. Any conventional CVD or PVD process known in the art may be used to apply coating 54. Coating 54 may form a conformal coating over outer surface 52. In this disclosure, “conformal coating” refers to a coating that substantially conforms to the shape of the underlying surface (that is, outer surface 52). A conformal coating may generally resemble the shape of the surface it is applied on. However, it is contemplated that a conformal coating may not be present on sharp discontinuities of the surface, including crevices, points, pores, cracks, sharp edges, and internal surfaces.
Coating 54 on outer surface 52 may have a thickness 56. Thickness 56 may have any value to suit an application. At very low values of thickness 56, coating 54 may wear off quickly and may need to be periodically refurbished. Very high values of thickness 56 may cause coating 54 to develop large internal stresses, and thereby, crack or peel. In general, the precise values of thickness 56 beyond which these issues may arise depends upon the application. For the gear polishing application illustrated herein, a preferred range of thickness 56 may be between about 1 micron and about 20 microns. However, it is contemplated that for other applications, thickness 56 may have different values. In some embodiments, thickness 56 of coating 54 may be substantially uniform. That is, thickness 56 of coating 54 at different regions of outer surface 52 may be substantially the same. However, it is contemplated that in some embodiments, thickness 56 of coating 54 may vary over the outer surface 52. In these embodiments, thickness 54 may vary between about 1 micron and about 20 microns. That is, a minimum coating thickness may be about 1 micron and a maximum coating thickness may be about 20 microns.
Although the description above illustrates polishing of gear tooth using CASFP, the working surface of any tribological component may be polished using CASFP. In general, to polish the working surface of a first component of a tribological component pair, a coating may be applied to the working surface of a second component. The coated second component is then allowed to contact and undergo relative motion with respect to the first component of the tribological component pair. While undergoing relative motion, the coating on the second component abrades and polishes the working surface of the first component.
For example, to polish a rough external surface (working surface) of a shaft that slides on a cavity of a bearing, the coating may be applied to the cavity surface (working surface of the bearing), and the shaft allowed to slide on the bearing with the coated working surface. Repeated sliding of the shaft on the bearing abrades and polishes the working surface of the shaft. The coated bearing may then be removed and replaced with a standard (uncoated) bearing. In embodiments, where the coated bearing is part of a machine dedicated to polishing shafts (polishing machine), the polished shaft may be removed from the polishing machine after polishing. A rough bearing surface may likewise be polished by applying the coating on the shaft.
The first component with the first surface is prepared (step 610) using any operation. Preparation of the first component may include manufacturing or remanufacturing the component. Manufacturing the component may include any component fabrication process, such as casting and machining. Remanufacturing refers to the process of cleaning and repairing a used component for reuse. The first working surface may be a rough surface. The rough surface may be manifested by a high value of one or more of maximum roughness Rz, peak to valley height Hz, arithmetic mean roughness Ra, and RMS roughness Rq.
A coating may then be deposited on the second working surface of the second component (step 620). The second component may be first cleaned to remove contaminants from its surfaces. This cleaning may include removing rust, debris, or other organic contaminants from the surfaces of the component. The cleaning may include mechanical cleaning, chemical cleaning, or a chemical assisted mechanical cleaning. Mechanical cleaning may include scrubbing the surfaces of the component. A chemical solvent may be used along with mechanical scrubbing. The component may also be rinsed in water and dried.
The deposition step (step 620) may also include depositing a coating on the surfaces of the second component. In some embodiments, the coating may be selectively applied to some surfaces, for instance, the second surface. In these embodiments, the surfaces of second component where the coating is not applied may be covered before depositing the coating. Covering these surfaces may include applying a mask on these surfaces. Any mask application process known in the art may be used to apply the mask.
Any conventional coating process, such as PVD or CVD techniques, may be used to apply the coating on second component. PVD techniques may include deposition processes such as, for instance, cathodic arc deposition, evaporative deposition, electron beam physical vapor deposition, pulsed laser deposition, and sputter deposition. CVD processes may include any conventional CVD techniques such as, for example, atmospheric pressure CVD, low pressure CVD, ultra low vacuum CVD, plasma enhanced CVD, etc. The coating on the second component may be applied to the desired thickness.
After coating, the first component and the second component are mated together (step 630). The term “mating” refers to locating the first and second component in the same structural orientation as they would be in actual application. In this orientation, the first surface of the first component may contact the second surface of the second component. Mating the components may also include coupling the first component to a power source. The power source may be configured to impart relative motion between the first component and the second component. In some embodiments, the second component may be coupled to a resistance source. In these embodiments, the resistance source may apply mechanical resistance to the motion of the second component. It is also contemplated that, in some embodiments, the second component may be coupled to the power source and the first component may be coupled to the resistance source.
For instance, in an embodiment where the first component is gear 12 with rough surface 42 and the second component is abrasive pinion 50 with coating 54, mating the two components refers to the process of attaching the two components to their respective shafts (16A and 18A, see
In some embodiments, a lubricant may be applied to the contacting surfaces. The lubricant may include any conventional lubricant used in the art. It is also contemplated that, in some embodiments, an abrasive slurry may also be applied to the contacting surfaces.
After the components are mated, they may then be operated (step 640). Operating the component may include activating the power source to impart relative motion between the two components. In embodiments which include a resistance source, operating the components may also include activating the resistance source. Relative motion between the components may induce the contacting first surface (rough surface) and the second surface with the coating to slide/rub against each other. Repeated rubbing/sliding of the first component and the second component against each other may abrade the rough surface, thereby polishing it.
In the embodiment depicted in
The first surface may then be inspected to determine if a desired surface finish has been attained (step 650). This inspection may include visual inspection of the first surface or may include measurements to determine one or more of the maximum roughness Rz, peak to valley height Hz, arithmetic mean roughness Ra, and RMS roughness Rq. Any means capable of measuring surface roughness, such as surface profilometer, atomic force microscope (AFM), etc., may be used to perform the roughness measurements. The roughness measurements may be compared to acceptable values of surface roughness (step 660). Acceptable values of surface roughness may be roughness values that an operator determines is suitable for the first component. If the surface finish of the rough surface is not within acceptable limits, the sliding/rubbing of the components against each other may be continued. If the surface finish is within acceptable limits, the sliding/rubbing motion may be discontinued and the first component removed (step 670).
In some embodiments, the inspection and the comparison steps (step 650 and 660) may be a visual operation where an operator looks at the first surface to judge if the surface finish looks acceptable to him. In some embodiments, the inspection and comparison steps may be automated. In these embodiments, the operation of the components (step 640) may be periodically stopped and roughness measurements of first surface taken. The measured values may then be compared to preset acceptable roughness values. If the measured values of roughness are less than or equal to the acceptable values, the operation of the components may be discontinued. It is also contemplated that, in some embodiments, the inspection and the comparison steps (step 650 and 660) may be eliminated. In these embodiments, operation of the components (step 640) may be continued for a predetermined time. Predetermined time may be a operating time which the operator knows will polish the first surface. In some embodiments, predetermined time may be determined based on a measured roughness of first surface. For instance, predetermined time may be read off a table that list operating times for different values of surface roughness.
The disclosed embodiments relate to a surface finishing process. The process can be used to polish a rough surface of a component that contacts a mating component and undergoes relative displacement with respect to each other. A coating of a hard material is applied to contacting surfaces of the mating component before the components are contacted and allowed to undergo relative displacements with respect to each other. As the components rub against each other, the hard coating on the mating component abrades the rough surface to polish the surface. To illustrate the application of the disclosed surface finishing process, an exemplary embodiment of the CASFP, where the outer surface of a spur gear is polished with a coated pinion will now be described.
Surface profilometer measurements of a selected region of an outer surface 42 of spur gear 12 may indicate a mean roughness Ra of about 0.213 microns. A coating assisted surface finishing process may be used to polish outer surface 42. A PVD process may be used to deposit an approximately 3 micron thick layer of tungsten containing diamond like carbon coating 50 (W-DLC) on an outer surface 52 of another gear (abrasive pinion 50). Since PVD deposition of W-DLC coating is well known the art, a description of the deposition process is not included herein. The abrasive pinion 50 may then coupled to shaft 18A (
In the CASFP, a component is polished using the surface of another component that it will be in contact with during operation. Therefore, material removal and polishing will be confined to locations where the components actually contact each other, thereby minimizing material removal. Since material is only removed from selected regions, the component may not be weakened due to the polishing process. Moreover, since the overall dimensions of the component may not change due to polishing, the design process may be simplified by not having to account for dimensional changes at other surfaces of the component. Also, since specialized machinery may not be required to polish these components, the associated cost may be minimized.
It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed surface finishing process. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed surface finishing process. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.
