Machine element with at least a fitting member pressure-fitted on a shaft and method of making the same

A machine element has a shaft and at least a fitting member such as a cam or gear mounted on the shaft by pressure-fit. The fitting member has a bore for receiving the shaft. The shaft is formed with protrusions at least an area of the shaft on which the fitting member is fitted. The protrusions preferably extend continuously or discretely, peripherally or helically on the outer surface of the shaft and have a diameter larger than than the remaining areas of the shaft. The bore of the fitting member is formed with at east an inner chordal surface having a perpendicular from the center of the bore smaller than the radius of the protrusions. The fitting member is configured to be forced onto the protrusions of the shaft with at least one inner chordal surface forming a corresponding outer surface on the protrusions in a shaving and/or deforming operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a machine element of the type having at 
least a fitting member pressure-fitted on a shaft, such as a camshaft for 
use in internal combustion engines, a shaft-supported gear, and a 
shaft-supported rotor. The fitting member has a bore for receiving the 
shaft, which is eccentric to the center of the fitting member for use in 
the camshaft or the like and concentric in the shaft-supported gear or the 
like. At least one portion of the area of the shaft in which the fitting 
member is to be provided has a diameter which is greater than the 
remaining areas of the shaft. The bore has at least a narrow portion 
defining a spacing diametrically smaller than the increased diameter 
portion of the shaft. The fitting member is forced onto the increased 
diameter portion of the shaft with the narrow portion of the bore cutting 
and/or deforming the increased diameter portion. 
The present invention a also relates to a method of manufacturing the 
above-described machine element. 
2. Description of the Prior Art 
Machine elements are known in which the fitting members and the shafts, 
separately manufactured, are assembled by a press fit accompanying a 
chip-removing operation. U.S. Pat. No. 4,903,543 or U.S. Pat. No. 
4,947,547 describes a camshaft which includes a shaft and at least one cam 
slid onto and connected to the shaft in an area of the shaft. The cam 
defines a bore for receiving the shaft. The bore has at least one radially 
inwardly directed projection. At least one portion of the area off the 
shaft has a diameter larger than the remaining areas of the shaft. The 
increased diameter portion of the shaft is a bead-like material 
displacement which extends circumferentially on the shaft and is 
manufactured by rolling. The cam is forced onto the increased diameter 
portion of the shaft with a projection in the bore of the cam for forming 
in a chip-removing operation, a groove in the increased diameter portion. 
The projection is so finished as a cutting edge with an angular 
cross-section to form the groove in the outer surface off the shaft. It is 
also required for the fitting member to be harder than the shaft, since 
the fitting member must cut the groove off on the shaft with its 
projection. However, it is not easy to provide and finish the axially 
extending angular projection as a cutting edge within the bore of the 
fitting member. The cost of manufacturing the camshaft considerably rises 
when the fitting member is made from a harder material for coupling with 
the relatively hard shaft. 
The object of the present invention is to provide a machine element in 
which at least one fitting member is pressure-fitted onto the increased 
diameter portion of a shaft with at least an inner wall portion of the 
fitting member shaving and/or deforming the outer surface of the shaft for 
a tight connection therebetween, while the fitting member is manufactured 
with ease at a relatively low cost. Another object of the present 
invention is to provide an assembling method to manufacture the 
aforementioned machine element. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the shaft has protrusions on the 
outer surface thereof in the area of the shaft in which the fitting member 
is to be mounted. The protrusions have a diameter larger than the 
remaining areas of the shaft. The protrusions extend continuously or 
discretely and axially, peripherally or helically on the shaft and are 
formed by means of the known manner as padding, cutting, rolling and 
knurling. The fitting member has a bore, concentric or eccentric to the 
center of the fitting member, for receiving the shaft. The bore is formed 
with at least a chordal inner surface as an inner wall portion defining a 
space diametrically smaller than the protrusions. The chordal surface has 
such a perpendicular chord from the center of the bore that is adequately 
smaller than the radius of the protrusions. The bore in the fitting member 
consists of alternately connected arcuate and chordal surfaces in which 
the inner diameter of the arcuate surface is slightly larger than the 
outer diameter of the protrusions. It is also composed of a plurality of 
chordal surfaces in the form of a polygon. The fitting member is not 
always required to be harder than the shaft or the protrusions thereof. It 
can be similar in hardness to the shaft or the protrusions thereof. 
The fitting member is forced onto the protrusions of the shaft with the 
inner chordal surface of the fitting member forming in a shaving and/or 
deforming operation an outer chordal surface on the protrusions for a 
tight connection between the fitting member and the shaft. In the 
procedure of pressure-fitting the shaft into the bore of the fitting 
member, the chordal surface of the bore has the central portion thereof 
for shaving engagement with the protrusions of the shaft when the fitting 
member is harder than the shaft or for plastically deforming engagement 
with the protrusions of the shaft when the fitting member is similar in 
hardness to the shaft. Both side portions adjacent to central portion of 
the bore force the protrusions to deform partly plastically, partly 
elastically, so that the inner chordal surface of the bore is secured to a 
corresponding outer surface formed in the protrusions by a shaving and/or 
deforming operation. This connection is substantially similar to an 
angular joint. It ensures that the machine element thus assembled has no 
relative motion arising between the shaft and the fitting member under 
practical working conditions. 
It is easy to form and finish the flat chordal surface within the bore as 
tile cutting edge for shaving and/or deforming the protrusions of the 
shaft as compared with the known projection in the form of an angular 
cutting edge for forming a groove in the outer surface of the shaft. 
Another advantage of the present invention is that the fitting member is 
not required to be harder than the shaft. Therefore, the machine element 
according to the present invention can be manufactured with ease at a 
lower cost than the prior art.

DETAILED DESCRIPTION OF THE INVENTION 
As seen in FIGS. 1 and 2, the shaft 10 has a fitting area A on which the 
cam piece 20 is fitted. The area is provided on the outer periphery 
thereof with a plurality of peripherally extending protrusions 12 in the 
form of an annular-groove knurling. The protrusions are previously formed 
by rolling for fitting engagement with the chordal inner surface 22 which 
is formed in the bore 25 of the cam piece 20 to assemble the cam piece and 
the shaft into a camshaft. The peripheral or helical protrusions in the 
form of an annular or helical-groove knurling can be formed by means of 
pressure deformation, for example, by rolling. The protrusions extend 
continuously, peripherally in the form of an annular-groove knurling as 
seen in FIG. 1. The protrusions extend continuously, helically in the form 
of a helical-groove knurling as seen in FIG. 3. The protrusions extend 
discretely, peripherally or helically in the form of a criss-cross or 
diamond knurling as seen in FIG. 4. The protrusions in the form of a 
coarse criss-cross or diamond knurling are shaped by knurling. The knurled 
and rolled protrusions are preferable considering an efficiency in 
production. 
As seen in FIG. 2, the protrusions 12 have an outer diameter D1 larger than 
the outer diameter D of the shaft 10 except the fitting area A in which 
the individual root between the two adjacent protrusions 12 has a diameter 
smaller than D. As seen in FIG. 2, the bore in the cam piece 20 consists 
of inner arcuate surfaces 21 and inner chordal surfaces 22 which are 
alternately, circumferentially connected. The arcuate surface 21 has a 
diameter D2 larger than the diameter D1 of the protrusions 12. The chordal 
surface 22 has a perpendicular chord R from the center of the bore 25 that 
is shorter than the radius of the protrusions 12 but longer than the 
radius of the root between the two adjacent protrusions. If the inner 
diameter D2 were smaller than or equal to the other diameter D1, a highly 
tight fitting would be obtained. However, it is preferable for an 
efficient assembling of the cam piece and the shaft that the inner 
diameter D2 of the inner arcuate surface is larger than the outer diameter 
D1 of the protrusions. The bore 25 of FIG. 2 consists of four similar 
arcuate surfaces 21 and four similar chordal surfaces 22. It can consists 
of any number of arcuate or chordal surfaces. Both or either of arcuate 
and chordal surfaces may be in number more or less than 4. For example, an 
arcuate surface and a chordal surface can constitute the bore. 
Furthermore, the bore in the shape of a polygon is formed only by the 
chordal surfaces. The polygonal bore makes a tight fitting with the shaft 
so that there occurs no relative motion between the fitting member and the 
shaft. The chordal surface is easy to manufacture and finish since it is 
simple and flat. This results to remarkably reduce the cost of 
manufacturing the camshaft as compared with the conventional camshaft. 
As seen by the arrow B in FIG. 5, the cam piece 20 is axially slid onto the 
fitting area A from an end of the shaft 10. Then, tile chordal surface 22 
within the bore 25 of the cam piece 20 has its central portion shaving the 
protrusions 12 partly, both side portions adjacent to the central portion 
deforming the same plastically, and both further side portions deforming 
the same elastically, so that the cam piece 20 is tightly fitted on the 
protrusions. 
As seen in FIG. 6, the cam piece 20 is chamferred at the forward end of the 
bore 25 and formed with a conical surface 23. When the cam piece 20 is 
slid onto the fitting area A of the shaft in the direction shown by the 
arrow B, the conical surface 23 facilitates the inner chordal surface 22 
to form the corresponding outer surface on the protrusions 12 of the shaft 
10 in a shaving and/or deforming operation. The conical surface 23 also 
serves as a stopper to determine the axial position of the cam piece on 
the shaft. 
There is shown a shaft-supported rotor for use in a rotary compressor in 
FIG. 7, in which the rotor body 20 in the form of a disk is tile fitting 
member. The bore 25 is eccentric to the center of the rotor body 20. The 
shaft 10 has the protrusions 12 formed on the outer surface in the fitting 
area of the shaft by means of pressure deformation. The bore 25 is 
composed of four inner arcuate surfaces 21 and four inner chordal surfaces 
22, alternately connected to each other. The diameter D1 of the 
protrusions 12 is smaller than the diameter D2 of the arcuate surface 21, 
while the radius of the protrusions 12 is larger than the perpendicular 
chord to the chordal surface 22 from the center of the bore 25. The 
shaft-supported rotor has the same longitudinal section as shown in FIG. 
1. 
A shaft-supported gear is shown in FIG. 8, in which the spur gear 20 is the 
fitting member. The bore 25 is concentric to the center of the gear 20. 
There is the same relation among the diameter D1 of the protrusions 12, 
the diameter D2 of the inner arcuate surfaces 21, and the perpendicular 
chord to the inner chordal surface 22 from the center of the bore 25 as 
shown in FIG. 
Three sorts of protrusions were formed on the outer surface of the shafts 
and tested by measuring the pressure-fitting load when inserted into the 
hexagonal bore of the fitting members and the bonding strength between the 
shaft and the fitting member after the protrusions being inserted into the 
bore. The fitting members were made from a Fe-8% Cr alloy and had an axial 
length of 11 mm. The shafts were made from a steel similar to SAE 1050. 
The first protrusions extended continuously, axially in the form of 
axial-groove knurling. The second protrusions extended discretely, 
peripherally or helically in the form of a criss-cross or diamond 
knurling. The third protrusions extended continuously, peripherally in the 
form of an annular-groove knurling. The hexagonal bore had an inscribed 
circle which was radially smaller by 0.8 mm than the protrusions, so that 
there was in maximum a radial interference of 0.8 mm between the fitting 
members and the shafts. The test results are shown in FIG. 9, which 
illustrates that the axial-groove knurling gives the relatively small 
bonding strength or applicable torque to the machine element, although it 
needs the relatively small pressure-fitting load, that the criss-cross and 
annular-groove knurlings advantageously give the relatively large bonding 
strength or applicable torque to the machine element, and that the 
criss-cross knurling needs the pressure-fitting load less than the 
annular-groove knurling. 
Regular hexagonal, octagonal, decagonal and dodecagonal bores in the 
fitting members were tested by measuring the bonding strength between the 
fitting members and the shafts after each bore being forced onto the 
protrusions in the form of the criss-cross knurling on the shaft. To be 
exact, the polygonal bores are composed of relatively long chordal sides 
and relatively short arcuate sides. The polygonal bores had the same 
circumcircle which was radially larger by 0.8 mm than the protrusions, so 
that there was in maximum a radial interference between the fitting member 
and the protrusions. The fitting members and the shafts were made from the 
same materials as those illustrated in FIG. 9. The test results are shown 
in FIG. 10 in which the graph illustrates the relation between the angular 
number of the regular polygonal bores and the bonding strength. From FIG. 
10 it is known that the polygonal bores with an angular number of 8 to 10 
are superior in bonding strength or applicable torque to the hexagonal and 
dodecagonal bores. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.