Patent Application
20090173301
The present invention is generally directed to a surface treated rocker arm shaft for an internal combustion engine and is more specifically directed to a surface hardened rocker arm shaft having a case hardened and/or a highly polished surface that is capable of improved wear resistance and deterrence of crack initiation and propagation.
Internal combustion engines, such as multi-cylinder diesel engines, typically include a crankshaft, a camshaft and a rocker arm shaft. The crankshaft is connected with a plurality of piston rods, which in turn are connected with a plurality of corresponding pistons. Reciprocating movement of the pistons within corresponding combustion cylinders causes rotation of the crankshaft. The crankshaft is typically interconnected with the camshaft via a gear set and thereby rotatably drives the camshaft during operation. The camshaft includes a plurality of cams, with each cam being associated with an inlet valve, and an exhaust valve or a fuel injector valve. More particularly, the rocker arm shaft carries a plurality of rocker arms, with each rocker arm having a roller follower that engages a corresponding cam on the camshaft. Rotation of the camshaft causes oscillatory pivotal movement of the rocker arms around the rocker arm shaft. The rocker arm shaft is subject to cyclic bending moments as a result of forces applied thereto by the roller follower and cam.
Typically, rocker arm shafts are manufactured from a high strength through hardened steel such as AISI E52100. Through hardening of the steel imparts a high hardness throughout the entire shaft. The expense of some through hardened steels make them impractical for use as rocker arm shafts in typical internal combustion engines.
In addition, the above mentioned cyclic bending moments can cause fatigue failure of the rocker arm shaft. Although the use of the through hardened steel can improve wear resistance, fatigue generated surface cracks can propagate inwardly through the core portion. Such propagation of the cracks through the core portion has resulted in catastrophic failure of the rocker arm shaft.
According to one aspect of the present invention, a shaft for a rocker arm assembly for an internal combustion engine includes a substantially cylindrical outer surface at least a portion of which defines a bearing surface thereon and an interior core portion located radially inward of the outer surface. At least a portion of the outer surface has a hardness greater than that of the core portion, for providing wear resistance and deterring crack initiation and propagation.
Rocker arm shafts having a bearing surface that has a hardness greater than that of the core portion can improve resistance to the initiation and propagation of surface cracks inwardly through the core portion.
In one aspect of the present invention, the bearing surface has a Rockwell C scale hardness of at least 59. In another aspect of the present invention, the bearing surface, has a concentration of carbon and/or nitrogen at the outer surface. This concentration extends radially inward from the outer surface to a depth of about 0.063 inches.
In yet another aspect of the present invention, the bearing surface has an arithmetic mean roughness of less than about 2.5 micro inches.
Referring to
In one embodiment, the rocker arm shaft 24 is manufactured from through hardened ASM 52100 steel and is further case hardened by carburization for increased wear resistance and deterrence of crack initiation and propagation. The carburization case hardening process includes one of gas diffusion, pack diffusion and liquid diffusion. In the carburization process the rocker arm shaft 24, in particular the bearing surface 28 is exposed to a carbon rich atmosphere (e.g., carbon monoxide, carbon powder, or a molten carbon rich bath) for a predetermined period of time. During carburization the carbon rich atmosphere is at a temperature between approximately 1550° F. to 1750° F. The temperature and time are selected based on a desired surface hardness and penetration depth of the carbon. After carburization, the rocker arm shaft 24 is cooled to a temperature of approximately 70° F. to achieve a desired surface hardness. Cooling can be accomplished by quenching in a liquid and/or by air cooling. The carburization process causes the bearing surface 28 to have a Rockwell hardness, C scale, of at least 59.
As illustrated in
While the carburization process is described for hardening the bearing surface 28, the present invention is not limited in this regard as the present invention is adaptable to other hardening processes including, but not limited to, nitriding wherein nitrogen is diffused into the bearing surface, carbonitriding wherein carbon and nitrogen are diffused into the bearing surface, flame hardening, induction hardening, laser beam hardening and electron beam hardening.
In addition, other surface treatment processes to provide wear and impact resistance and deter crack initiation and propagation can be used. Such a surface treatment process includes lapping-like scratching of the surface under extremely high compression of the surface to reduce slip planes, increase surface hardness, increase impact resistance, and increase surface compressive stresses by about twenty percent to a depth of about 0.012 inches. Surface roughness is reduced to less than 1 micro inch. For example, Mikronite Technologies, Inc. of Eatontown, N.J. has a Mikronite® brand surface treatment processes which can be employed.
Another process that can deter crack initiation and propagation and increase impact, wear and corrosion resistance is a process using abrasive or non-abrasive media with or without chemical solutions, applied by vibratory methods. Such a process can provide: 1) a superfinished surface, defined as having an ISO Standard No. 4287 Arithmetic Mean Roughness of less than or equal to 2.5 micro inches; 2) an isotropic surface, defined as a surface having no orientation to its surface irregularities; and 3) a specular brightness, defined as a surface in which a clear reflection of an object can be seen. For example, REM Chemicals, Inc. of Southington, Conn. has an Isotropic Superfinish (ISF®) process that can be employed.
In one embodiment, the case hardened bearing surface 28 of
The rocker arm assembly of
Portions of the outer surface 126 which do not require hardening are coated with a mask 134 prior to initiation of the case hardening process. The mask 134 is made up of a substance impermeable to carbon, for example copper, to preclude diffusion of carbon into the portions of the outer surface 126 which do not require hardening. In one embodiment, the mask 134 is deposited on the portions of the outer surface 126 which do not require hardening by an electro-chemical plating process. After case hardening, for example, carburizing, the mask 134 is removed. Although the mask 134 is described as being copper, the present invention is not limited in this regard as other coatings are also suitable including but not limited to water soluble coatings.
In another embodiment, the portions of the outer surface 126 in the contact region 129, illustrated in
The rocker arm assembly of
Referring again to
While that above rocker arm shafts 28, 128 and 228 are illustrated as solid substantially cylindrical shafts, the present invention is not limited in this regard as other shaft configurations are adaptable for use in the present invention, including but not limited to case hardening and/or surface treatment of any portion of: hollow rocker arm shafts, rocker arm shafts with grooves for receiving and/or guiding the rocker arms and stepped rocker arm shafts having a plurality of different diameters.
Although the present invention has been disclosed and described with reference to certain embodiments thereof, it should be noted that other variations and modifications may be made, and it is intended that the following claims cover the variations and modifications within the true scope of the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61019936 | Jan 2008 | US |
