Patent Application
20070039173
The present invention relates to methods and apparatus for securing a shaft in a bore by expanding the outer end of the shaft after insertion into the bore, a process known in the art and referred to herein as “staking”; and more particularly, to an improved method and apparatus for mounting a roller and hardened shaft to a body of a roller cam follower device such as a roller hydraulic valve lifter (RHVL) or a roller finger follower (RFF) for an internal combustion engine.
Hydraulic valve lifters (HVL) for internal combustion engines are well known.
One form of hydraulic valve lifter (RHVL) includes a roller for following the surface of a cam lobe with low frictional loss and low wear in a cam-in-block engine. The roller is disposed between first and second spaced-apart lifter body elements and is rotatably mounted on a hardened steel shaft extending through bores in the body. The shaft has a Rockwell C hardness typically of about 60-62.
In the prior art, the shaft is secured to the body by orbital riveting or staking with up to 5000 pounds of mechanical force. Deforming the ends of the shaft by staking is possible only because the ends of the shaft have been retempered to reduce the hardness to an R15N value of about 74-76.
A problem exists in the prior art, in that the ends of the hardened shafts are pre-softened by a supplier to permit mechanical staking thereof and are prone to nicking and damage due to handling prior to assembly into a lifter or follower. Damaged shafts are rejected for lack of fit into a bearing or into the housing bores. Further, the high pressure assembly process can cause the shafts to be driven out of round, also resulting in rejects. Further, the retempering procedure makes the prior art shafts relatively expensive.
This problem exists also in the manufacture of other roller cam follower devices such as the roller finger follower (RFF) in cam-in-head or overhead cam engine and is inherent in all applications requiring staking of the end of a hardened shaft to secure it in a bore.
What is needed in the art is a method and apparatus for staking a hardened steel roller shaft in a roller hydraulic valve lifter or a roller finger follower.
It is a principal object of the present invention to reduce the cost of manufacturing a roller cam follower device.
Briefly described, in a method and apparatus for staking a shaft in bores in a shaft-supportive element, for example, a hydraulic lifter body, the shaft is inserted into the bores such that the shaft ends are substantially flush with the bore exits. Preferably, the bore exits are slightly chamfered to receive staked material from the shaft. Mechanical rams that also define a first electrode are advanced axially of the shaft into contact with the shaft ends. A second electrode is brought into contact with the lifter body surrounding the bores. While the first electrode is applying a mechanical load to the ends of the shaft, electric current is passed between the first and second electrodes, causing resistive heating of the shaft ends and the body. The time and applied amperage are sufficient to cause slight softening of the ends of the shaft, at which time the current is switched off, while the mechanical axial load from the first electrode is maintained momentarily to controllably deform the ends of the pin to complete the radial expansion of the pin. The remainder of the shaft is not softened nor retempered by the process.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Referring to
Referring to
When electric current is passed through lifter body 22 and the first and second electrodes, body 22 and shaft ends 130a, 130b are heated by resistive heating to cause the shaft ends to become softened at elevated temperature, at which point the current is turned off. Rams 126a, 126b then are axially actuated against ends 130a, 130b of shaft 112 at a relatively low pressure, which may include first withdrawing the rams from contact therewith, or maintaining a sustained pressure on the ends, resulting in an annular radial protrusion 134 of material displaced from shaft 112 into chamfers 24, similar to the staking shown in detail in
While the hardness of the pre-softened shaft of the example disclosed is in the range of 60-62 on the Rockwell C scale, it is understood that any hardness as known in the art above that suitable for staking, as a pre-staked hardness, would fall within the scope of the invention.
While the invention as described incorporates the first electrodes 136a, 136b into the mechanical rams 126a, 126b, it is understood the electrode elements and ram elements may be separate members.
The intensity and duration of the electric current varies with the mass and material of the shaft and the body, and the pre-staked hardness of the shaft, and hence must be determined for each individual application. Such determination may be made without undue experimentation and is well within the scope of one skilled in the art.
While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.
