Camshaft and camshaft manufacturing method

A cam lobe of a camshaft has a base circle portion and a lifting lobe portion. A camshaft journal of the camshaft has first and second bearing portions that bear reaction forces from the base circle portion and the lifting lobe portion, respectively. The first bearing portion has an axial width smaller than that of the second bearing portion with at least a portion of an axial end surface of the first bearing portion facing the cam lobe being disposed away from the cam lobe relative to an axial end surface of the second bearing portion. The base circle portion has an axial width smaller than that of the lifting lobe portion with at least a portion of an axial end surface of the base circle portion facing the camshaft journal being disposed away from the camshaft journal relative to an axial end surface of the lifting lobe portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2008-003743, filed on Jan. 10, 2008. The entire disclosure of Japanese Patent Application No. 2008-003743 is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a camshaft and a camshaft manufacturing method.

2. Background Information

Japanese Laid-Open Patent Publication No. 2001-82111 discloses a conventional camshaft in which a width of a sliding contact surface of a base circle portion of a cam lobe is smaller than a width of a sliding surface of a nose (lobe) portion of the cam lobe. With this conventional camshaft, a side surface of the base circle portion (where a surface pressure is smaller than at the lobe portion) is cut away by an amount according to a surface pressure imparted thereon, thereby enabling the weight of the camshaft to be reduced in an efficient manner.

In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved camshaft and camshaft manufacturing method. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.

SUMMARY OF THE INVENTION

With the conventional camshaft, a side surface of the cam lobe is merely cut away by an amount according to the surface pressure imparted on the cam lobe. Consequently, although the durability of the camshaft may be maintained, there is no mention of improving the durability of the camshaft in the above identified reference. Therefore, there exists a need to improve durability while reducing weight of the camshaft.

Accordingly, one objective of a camshaft and a camshaft manufacturing method is to improve the durability of the camshaft while reducing its weight.

In order to achieve the above object, a camshaft is adapted to be rotatably coupled to a shaft bearing part of an engine. The camshaft includes a cam lobe and a camshaft journal. The cam lobe has a base circle portion and a lifting lobe portion, and configured and arranged to operate one of an intake valve and an exhaust valve. The camshaft journal has a first bearing portion configured and arranged to bear a reaction force from the base circle portion of the cam lobe and a second bearing portion configured and arranged to bear a reaction force from the lifting lobe portion of the cam lobe. The first bearing portion of the camshaft journal has an axial width that is smaller than an axial width of the second bearing portion with at least a portion of an axial end surface of the first bearing portion that faces the cam lobe being disposed further away from the cam lobe with respect to an axial end surface of the second bearing portion that faces the cam lobe by a first prescribed distance. The base circle portion of the cam lobe has an axial width that is smaller than an axial width of the lifting lobe portion with at least a portion of an axial end surface of the base circle portion that faces the camshaft journal being disposed further away from the camshaft journal with respect to an axial end surface of the lifting lobe portion that faces the camshaft journal by a second prescribed distance. The cam lobe is disposed adjacent to the camshaft journal such that a minimum axial spacing between axially opposing surfaces of the cam lobe and the camshaft journal is equal to or greater than a prescribed axial spacing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially toFIGS. 1 to 3, an engine1provided with an exhaust camshaft6and an intake camshaft7is illustrated in accordance with an illustrated embodiment.FIG. 1is a schematic front elevational view of the engine1.FIG. 2is an exploded perspective view of a cylinder head3and the exhaust and intake camshafts6and7of the engine1.FIG. 3is a schematic top plan view of the cylinder head3with the exhaust and intake camshafts6and7installed therein.

As shown inFIG. 1, the engine1has a cylinder block2on top of which the cylinder head3is fastened and a cylinder head cover4that covers the top face of the cylinder head3. An oil pan5for storing oil is provided on a bottom face of the cylinder block2as shown inFIG. 1.

The exhaust camshaft6and the intake camshaft7are arranged in parallel on the top side of the cylinder head3. As shown inFIGS. 1 and 2, a cam sprocket6ais attached to an axial end of the exhaust camshaft6. Also, a cam sprocket7ais attached to an axial end of the intake camshaft7. A variable valve timing mechanism10is provided on the distal end of the cam sprocket7aas shown inFIGS. 1 and 2.

As shown inFIG. 1, the engine1also includes a crankshaft8that protrudes from the inside of the cylinder block2. A crankshaft sprocket8ais attached to the protruding end of the crankshaft8. A timing chain9is arranged around the cam sprocket6a, the cam sprocket7a, and the crankshaft sprocket8aas shown inFIG. 1such that rotation of the crankshaft8causes the exhaust camshaft6and the intake camshaft7to be rotationally driven. As shown inFIG. 1, rotation of the crankshaft8also rotates an oil pump15by using a chain16.

As shown inFIG. 2, the exhaust camshaft6and the intake camshaft7are rotatably arranged on a plurality of shaft bearing sections3aformed in the upper surface of the cylinder head3. A plurality of cam brackets12each having a shaft bearing section12ais fastened to the shaft bearing sections3afrom above using a plurality of bolts B. The exhaust camshaft6includes a plurality of camshaft journals61rotatably supported between the shaft bearing sections3aof the cylinder head3and the shaft bearing sections12aof the cam brackets12. Likewise, the intake camshaft7includes a plurality of camshaft journals71rotatably supported between the shaft bearing sections3aof the cylinder head3and the shaft bearing sections12aof the cam brackets12. Thus, the shaft bearing sections3aof the cylinder head3and the shaft bearing sections12aof the cam brackets12collectively form a shaft bearing part of the engine1.

A plurality of exhaust valves13is provided on the side of the cylinder head3where the exhaust camshaft6is arranged, and a plurality of intake valves14is provided on the side of the cylinder head3where the intake camshaft7is arranged.

As shown inFIGS. 2 and 3, the exhaust camshaft6includes a plurality of cam lobes62disposed on both axially facing sides of and closely adjacent to each of the camshaft journals61of the exhaust camshaft6. The intake camshaft7includes a plurality of cam lobes72disposed on both axially facing sides of and closely adjacent to each of the camshaft journals71formed on the intake camshaft7.

The cam lobes62of the exhaust camshaft6are configured and arranged to operate (i.e., open and close) the exhaust valves13as the exhaust camshaft6rotates. The cam lobes72of the intake camshaft7are configured and arranged to operate (i.e., open and close) the intake valves14as the intake camshaft7rotates.

FIG. 4is an enlarged schematic side view of one of the camshaft journals71and a pair of the cam lobes72of the intake camshaft7.FIG. 5is a cross sectional view of the intake camshaft7as taken along a section line5-5inFIG. 4.

Each of the cam lobes72is slidably coupled to a valve lifter (lifter member)14a, which is operatively coupled to one of the intake valves14. The cam lobes72of the intake camshaft7are configured and arranged to operate the intake valves14by converting rotation of the intake camshaft7into linear motion of the lifters14aof the intake valves14. As shown inFIGS. 4 and 5, each of the cam lobes72has a base circle portion72aand a lifting lobe portion72b. The base circle portion72ais configured and arranged not to operate or actuate the corresponding intake valve14(e.g., the intake valve is closed). The lifting lobe portion72bis configured and arranged to operate or actuate the intake valve14(e.g., the intake valve is opened) by pushing the lifter14aas the intake camshaft7rotates.

As shown inFIG. 4, each of the camshaft journals71includes a first bearing portion71aand a second bearing portion71b. The first bearing portion71ais configured to bear a reaction force from the base circle portion72aof the cam lobe72via the shaft bearing sections3aand12a. The second bearing portion71bis configured and arranged to bear a reaction force from the lifting lobe portion72bof the cam lobe72via the shaft bearing sections3aand12a. The reaction forces occur when the intake camshaft7rotates.

Each of the camshaft journals71is disposed on the intake camshaft7with respect to each of the cam lobes72arranged on both axially facing sides of the camshaft journal71so that a minimum axial spacing between axially opposing surfaces of the camshaft journal71and the cam lobe72is equal to or greater than a prescribed axial spacing. This prescribed axial spacing is determined based on casting requirements associated with cast forming the intake camshaft7(e.g., a requirement for removing the core sand after casting) of and performance requirements of the intake camshaft7.

In the illustrated embodiment shown inFIG. 4, the first bearing portion71aof each of the camshaft journals71has an axial width w1that is smaller than an axial width w2of the second bearing portion71b. The axial width w1in the first bearing portion71ais made smaller than the axial width w2in the second bearing portion71bpreferably by forming a pair of recess portions71c(removed material portions) as shown inFIG. 4(e.g., material corresponding to a first prescribed width b1(first prescribed distance) is removed from both axially facing sides of the first bearing portion71aas compared to a camshaft journal in which the recess portion71cis not formed). In other words, an axial end surface of the first bearing portion71athat faces the cam lobe72is disposed further away from the cam lobe72with respect to an axial end surface of the second bearing portion71bthat faces the cam lobe72by the first prescribed width b1.

On the other hand, the base circle portion72aof each of the cam lobes72has an axial width w3that is smaller than an axial width w4of the lifting lobe portion72b.The axial width w3in the base circle portion72ais made smaller than the axial width w4in the lifting lobe portion72bpreferably by forming a pair of recess portions72c(removed material portions) as shown inFIG. 4(e.g., material corresponding to a second prescribed width b2(second prescribed distance) is removed from both axially facing sides of the base circle portion72aas compared to a conventional cam lobe in which the recess portion72cis not formed). In other words, an axial end surface of the base circle portion72athat faces the camshaft journal71is disposed further away from the camshaft journal71with respect to an axial end surface of the lifting lobe portion72bthat faces the camshaft journal71by the second prescribed width b2.

The first prescribed width b1of the recess portions71cof the camshaft journal71is set to such a dimension that a surface pressure imparted on a bearing surface of the first bearing portion71awill be substantially equal to a maximum surface pressure imparted on a bearing surface of the second bearing portion71b. In the illustrated embodiment, material corresponding to the first prescribed width b1is removed uniformly in the axial and radial directions from the first bearing portion71aso that axial end surfaces (bottom surfaces of the recess portions71c) of the first bearing portion71aextend substantially perpendicular to the center axis of the intake camshaft7. Similarly, the second prescribed width b2of the recess portions72cof the cam lobes72is set to such a dimension that a surface pressure imparted on a sliding surface of the base circle portion72awill be substantially equal to a maximum surface pressure imparted on a sliding surface of the lifting lobe portion72b. In the illustrated embodiment, material corresponding to the second prescribed width b2is removed uniformly in the axial and radial direction from the base circle portion72aso that axial end surfaces (bottom surfaces of the recess portions72c) of the base circle portion72aextend substantially perpendicular to the center axis of the intake camshaft7. The first prescribed width b1of the recess portions71cof the camshaft journal71and the second prescribed width b2of the recess portions72cof the cam lobe72can be set to the same value, or can be set to different values.

By forming the recess portions71con the first bearing portion71aof the camshaft journal71and the recess portions72con the base circle portions72aof the cam lobes72, the cam lobes72on both sides of the camshaft journal71can each be shifted toward the camshaft journal71by an amount corresponding to the dimension of the removed material (the first and second prescribed widths b1and b2). Therefore, the distance from the camshaft journal71to the cam lobes72is decreased. As a result, the bending strength, i.e., the durability, of the intake camshaft7can be improved.

Additionally, by removing material to the dimensions described above (e.g., the first and second prescribed widths), the weight of the camshaft7can be reduced without lowering the durability of the camshaft journals71and the cam lobes72.

As the intake camshaft7rotates, the cam lobes72operate the intake valves14by pushing against the lifters14a. In the illustrated embodiment, each of the cam lobes72is preferably arranged with respect to the corresponding lifter14asuch that a widthwise (axial) center C of the cam lobe72is closer to the camshaft journal71than an axial center P of the corresponding lifter14aas shown inFIG. 4. In other words, a distance between the axial center C of the cam lobe72and the camshaft journal71is preferably set smaller than a distance between the axial center P of the lifter14aand the camshaft journal71. Thus, since the torque of the cam lobe72acts at a position offset from the axial center P of the lifter14a, the lifter14acan be rotated about its axial center P and uneven wearing of the lifter14acan be suppressed.

FIG. 6is an enlarged schematic side view of the intake camshaft7illustrating a manufacturing method of the constituent portions of the intake camshaft7in accordance with the illustrated embodiment. The portions indicated with virtual lines (long dash-dot-dot lines) show how the bearing portions would be shaped if the recess portions71cand72cwere not formed, and the portions indicated with solid lines show how the camshaft journal71and the cam lobe72are shaped when the recess portions71cand72care formed according to the illustrated embodiment.

By forming the recess portions71con the first bearing portion71aof the camshaft journal71(e.g., removing material corresponding to the first prescribed width b1as compared to the shape shown in the virtual lines) and forming the recess portions72con the base circle portion72aof the cam lobe72(e.g., removing material corresponding to the second prescribed width b2as compared to the shape shown in the virtual lines), the cam lobes72can each be shifted toward the corresponding camshaft journal71while maintaining the prescribed axial spacing. More specifically, as shown inFIG. 6, a spacing a1is formed between axially opposing surfaces of the first bearing portion71aof the camshaft journal71and the lifting lobe portion72bof the cam lobe72, and a spacing a2is formed between axially opposing surfaces of the second bearing portion71bof the camshaft journal71and the lifting lobe portion72aof the cam lobe72. The camshaft journal71and the cam lobe72are positioned with respect to each other so that the smaller one of the spacing al and the spacing a2(i.e., a minimum axial spacing) is equal to or greater than the prescribed axial spacing, which is determined based on casting requirements associated with cast forming the intake camshaft7and performance requirements of the intake camshaft7. When the camshaft journal71is arranged with respect to the cam lobe72so that the spacing a1is equal to the spacing a2, the spacing a1and the spacing a2are set to be equal to or greater than the prescribed axial spacing. Therefore, a rear end portion of the intake camshaft7can be shortened by an amount (width b3inFIG. 6) corresponding to a dimension by which the cam lobes72are shifted toward the camshaft journals71while the prescribed axial spacing between the camshaft journal71and the cam lobe72being ensured. As a result, the longitudinal dimension of the intake camshaft7can be shortened and the weight of the intake camshaft7can be reduced.

The prescribed spacing a shown inFIG. 6is determined based on casting requirements associated with cast forming the intake camshaft7and performance requirements of the intake camshaft7.

Although the illustrated embodiment presents an example in which the recess portions72care provided on both axially facing end surfaces of the base circle portion72aof each of the cam lobes72, it is also acceptable to provide the recess portion72conly on the side that faces the camshaft journal71. In the latter case, too, the cam lobes72can be shifted toward the camshaft journals71and a rearward end portion of the intake camshaft7can be shortened by an amount corresponding to the amount by which the cam lobes72are shifted. Therefore, the weight of the intake camshaft7can be reduced.

The recess portions can be formed on the exhaust camshaft6based on similar design conditions as the intake camshaft7as explained above such that the cam lobes62can be shifted toward the camshaft journals61by an amount corresponding to the dimension of the removed material in the recess portions. Therefore, the distance from the camshaft journals61to the corresponding cam lobes62can be shortened and the durability of the exhaust camshaft6can be improved. Additionally, a rearward end portion of the exhaust camshaft6can be shortened by an amount corresponding to the amount by which the cam lobes62are shifted toward the camshaft journals61such that the weight of the exhaust camshaft6is reduced.

Although in the illustrated embodiment described above the first prescribed width b1of the recess portions71cof the camshaft journal71is set to such a dimension that a surface pressure imparted on a bearing surface of the first bearing portion71awill be substantially equal to a maximum surface pressure imparted on a bearing surface of the second bearing portion71b, it is acceptable to set the first prescribed width b1of the recess portions71cof the camshaft journal71to any width so long as the surface pressure imparted on the bearing surface of the first bearing portion71awill be equal to or smaller than the maximum surface pressure imparted on the bearing surface of the second bearing portion71b.

Similarly, although in the illustrated embodiment described above the second prescribed width b2of the recess portions72cof the cam lobe72is set to such a dimension that a surface pressure imparted on a sliding surface of the base circle portion72awill be substantially equal to a maximum surface pressure imparted on a sliding surface of the lifting lobe portion72b, it is acceptable to set the second prescribed width b2of the recess portions72cof the cam lobe72to any width so long as the surface pressure imparted on the sliding surface of the base circle portion72awill be equal to or smaller than the maximum surface pressure imparted on the sliding surface of the lifting lobe portion72b.

Although in the illustrated embodiment described above the recess portions71cof the camshaft journal71are formed by removing material uniformly in the axial and radial directions from the first bearing portion71aof the camshaft journal71, it is acceptable to remove material from the first bearing portion71aof the camshaft journal71so that the axial width of the recess portion71cvaries (tapers) along the radial direction of the first bearing portion71aso long as the surface pressure imparted on the bearing surface of the first bearing portion71awill be equal to or smaller than the maximum surface pressure imparted on the bearing surface of the second bearing portion71b.

Similarly, the recess portions72cof the cam lobe72are formed by removing material uniformly in the axial and radial directions from the base circle portion72aof the cam lobe72, it is acceptable to remove material from the base circle portion72aof the cam lobe72so that the axial width of the recess portion72cvaries (tapers) along the radial direction of the base circle portion72aso long as the surface pressure imparted on the sliding surface of the base circle portion72awill be equal to or smaller than the maximum surface pressure imparted on the sliding surface of the lifting lobe portion72b.

Although in the illustrated embodiment described above the recess portions71care only provided on the first bearing portion71aof each of the camshaft journals71, it is acceptable to form the recess portion by removing material from the second bearing portion71b, too, except for a portion where the maximum surface pressure occurs. For example,FIG. 7shows a modified embodiment in which a recess (or tapered) portion71c′ is formed in the second bearing portion71bby removing material in accordance with the surface pressure imparted on the bearing surface of the second bearing portion71bsuch that the surface pressure does not exceed the maximum surface pressure.

Similarly, in the illustrated embodiment described above the recess portions72care only provided on the base circle portion72aof each of the cam lobes72, it is acceptable to form the recess portion by removing material from the lifting lobe portion72b, too, except for a portion where the maximum surface pressure occurs. For example,FIG. 7shows the modified embodiment in which a recess (or tapered) portion72c′ is formed in the lifting lobe portion72bby removing material in accordance with the surface pressure imparted on the sliding surface of the lifting lobe portion72bsuch that the surface pressure does not exceed the maximum surface pressure.

General Interpretation of Terms