Method for assembling an impeller onto a turboshaft

An impeller is assembled onto a turboshaft such that the impeller is deformed at its end walls so as to deform the inner wall of the hole of the impeller against the shaft such that the clearance between the shaft and the hole of the impeller is reduced to zero, thereby, preventing bending or offsetting of the shaft relative to the impeller. For causing the deformation of the shaft, washers with a circular projection are disposed on both sides of the impeller, with the projection facing the end walls of the impeller. The washers and the impeller are clamped by a nut which is screwed to the threaded end of the shaft, resulting in the projection being forced into the end wall of the impeller, thereby deforming the impeller.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a turbocharger used in an internal 
combustion engine, especially to a method for assembling an impeller onto 
a turboshaft. 
2. Description of the Prior Art 
Turbochargers are being increasingly used in internal combustion engines to 
obtain higher power. Turbochargers, which deliver a supercharge of air to 
the engine, are used at very high speeds which may exceed 100,000 rpm. 
Therefore, it is very important for them to be balanced in their rotating 
mass. The rotating mass of turbochargers comprises the turbine spun by the 
exhaust gas, the impeller or compressor rotor spun by the turbine through 
the shaft, and other elements such as spacers, thrust collars, and nuts 
mounted on the shaft. To obtain a well balanced rotating mass, the above 
components are machined to a high precision. The turbine and the impeller 
are especially important due to their functions, sizes, and masses. 
These components, however, have to be assembled into a unit and even if 
each component is precision machined, it is still difficult to obtain a 
well balanced assembly. It is not so difficult to balance the turbine 
relative to the shaft, as the turbine is integrated together with the 
shaft and machined as a unit, but it is very difficult to assemble the 
impeller onto the shaft. The shaft integrated with the turbine is known as 
a turboshaft. 
Conventionally, the shaft is first inserted into an axial hole in the 
impeller, than the impeller is clamped by a nut with a force great enough 
to enable torque to be transmitted from the shaft to the impeller and to 
fix the axial position of the impeller. The great force by which the nut 
is clamped in this operation tends to bend the part of the shaft holding 
the impeller. Further, the shaft may be clamped offset from the impeller 
center axis within the clearance between the shaft outer wall and the 
inner surface of the impeller hole. 
To prevent the above-mentioned bending and offseting of the shaft, it is 
desirable to minimize the clearance between the shaft outer wall and the 
inner wall of the hole. This, however, would reduce the efficiency of 
assembly work. In practice, also, zero clearance cannot be achieved due to 
the need for dimensional tolerances even with precision machined 
components. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for assembling 
an impeller onto a turboshaft which can obviate the above problems. 
Another object of the present invention is to provide a turbocharger which 
can be more easily assembled and is excellent in rotating balance. 
One object of the present invention is attained by a method for assembling 
an impeller onto a turboshaft which is integrated with a turbine at one 
end thereof, the impeller having flat end walls in the central region 
thereof between which end walls an axial hole extends, said flat end walls 
being parallel to each other and perpendicular to the impeller center 
axis, said method comprising the steps of: inserting the turboshaft 
through the axial hole in the impeller; and deforming the impeller by 
applying an axial force to said end walls of the impeller such that the 
clearance between the outer wall of the turboshaft and the inner wall of 
said axial hole of the impeller is reduced to zero at the resultant 
deformed areas. 
Another object of the present invention is attained by a turbocharger 
comprising an impeller and a turboshaft which is integrated with a turbine 
at one end thereof, the impeller being fitted to the turboshaft via an 
axial hole provided in the impeller, end walls of the impeller being 
deformed concentrically around said axial hole such that the clearance 
between the outer wall of the turboshaft and the inner wall of said axial 
hole of the impeller is reduced to zero at the resultant deformed areas.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIGS. 1 and 2 show several main components of a conventional turbocharger 
housed in a casing 1. As is well known, the turbocharger comprises a 
turbine 2, which is spun by the exhaust gas from the engine, and an 
impeller 3 or a compressor rotor which is spun by the turbine 2 via a 
turboshaft 4 and compresses the intake air to the engine. The impeller 3 
has a hole 3' into which the shaft 4 is to be inserted. The turboshaft 4 
is rotatably supported by bearings 5. The impeller 3 is rigidly secured 
onto the shaft 4 at one end portion and the turbine 2 is integrated with 
the shaft 4 at the opposite end portion. To maintain the axial position of 
the shaft 4, a thrust collar 6 and a spacer 7 are disposed between the 
impeller 3 and the bearing 5. The thrust collar 6 is rotatably secured to 
the peripheral casing element (not shown) and abuts on one side a shoulder 
4' of the shaft 4. The shoulder 4' is formed by a change in the outer 
diameter of the shaft 4. The outer diameter is smaller toward the end 
portion onto which the impeller 3 is to be secured. The impeller 3 is 
clamped by a nut 8 which is screwed to the threaded end portion 4" of the 
shaft 4. 
The nut 8 urges the impeller 3 against the spacer 7 and the thrust collar 
6. The thrust collar 6 is thus frictionally supported against the shoulder 
4' of the shaft 4. Thus, the clamping force of the nut 8 must be great 
enough to transmit the torque from the shaft 4 to the impeller 3 as well 
as to hold the axial position of the impeller 3 on the shaft 4. 
To facilitate the assembly work, it is advantageous to provide a clearance 
t in the order, for example, of a micron between the outer wall of the 
shaft 4 and the inner wall of the hole 3' of the impeller 3 thereby 
allowing the shaft 4 to be smoothly inserted into the hole 3'. Provision 
of a clearance t, however, would mean that the smaller diameter section of 
the shaft 4 from the shoulder 4' would tend to bend within the clearance t 
of the hole 3' if the end wall of the spacer 6 or the nut 8 were not 
exactly normal to the center axis of the shaft 4, as shown in an 
exaggerated fashion in FIG. 3a, or would tend to be fitted offset from the 
center axis of the impeller 3, as shown in an exaggerated fashion in FIG. 
3b. 
FIG. 4 shows a preferred embodiment of a turbocharger according to the 
present invention, in which the corresponding parts are represented by the 
same numerals as in FIGS. 1 and 2. This embodiment differs from that of 
FIGS. 1 and 2 in having washer 9 arranged on both sides of the impeller 3. 
Each washer 9 has a circular projection 10 or 11 (FIGS. 5a and 5b) about 
its center axis. The circular projection is concentrically formed around 
the hole into which the shaft is inserted and consists of a continuous 
circle in FIG. 5a and of a partially discontinuous circle in FIG. 5b. Each 
washer 9 is arranged with its projecting side facing the end walls of the 
impeller 3, said end walls extending parallel to each other and 
perpendicular to the center axis of the impeller 3 in the central region 
thereof for abuting the other elements, such as the nut 8 and the spacer 
7. 
The impeller 3 and the shaft 4 are usually made of separate materials, for 
example, the impeller 3 of a light alloy and the shaft 4 of steel. This 
means that the impeller 3 is deformed easier than the shaft 4. With the 
above-mentioned arrangement, as shown in FIG. 4, the nut 8 is screwed to 
the threaded shaft end 4" to clamp the impeller 3 onto the shaft 4. As the 
clamping force is increased, the projections 10 or 11 are forced into the 
end walls of the impeller 3, causing the end walls of the impeller 3 to 
deform. The deformation extends toward the shaft 4 if space or clearance 
exists between the inner wall of the hole 3' of the impeller 3 and the 
outer wall of the shaft 4, so that the inner wall forming the hole 3' 
deforms adjacent to the end walls of the impeller 3 as shown by the 
numeral 12 in FIG. 4, reducing the clearance to zero at the resultant 
deformed areas. The features of the deformation are described below. 
FIGS. 6a, 6b, and 6c show the steps of the deformation when the impeller 3 
and the shaft 4 are assembled coaxially before deformation. The circular 
projection 10 or 11 of the washer 9 is located about the same center axis 
O, as shown in FIGS. 6a and 6b, thus the deformation arises uniformly 
around the shaft 4, as shown in FIG. 6c, reducing the clearance to zero 
with the axes coinciding. 
FIGS. 7a, 7b and 7c show the steps of the deformation when the impeller 3 
and the shaft 4 are assembled offset at a distance t, corresponding to the 
clearance before deformation, as is liable to occur in many cases when 
clearance exists. In conventional turbochargers, this probably would have 
resulted in the shaft being assembled bent or offset, as shown in FIGS. 3a 
and 3b. According to the present invention, however, the washers 9 and the 
shaft 4 are initially located offset from the center axis O of the 
impeller 3, as shown in FIGS. 7a and 7b, but the distance t of the offset 
is decreased by the deformation, as shown in FIG. 7c. The details of the 
deformation are as follows: The deformation does not arise instantly, but 
rather progressively over a certain time during the nut screwing. The 
shaft 4 is urged by the deformed wall portion 12' which is located close 
to the shaft 4 when the clearance still remains, thereby reducing the 
distance t' between the axes of the impeller 3 and the shaft 4. Therefore, 
the deformation, according to the present invention, has the effect of 
centering the shaft 4 relative to the impeller 3. 
Further, the deformation, according to the present invention, has the 
effect of preventing the shaft from being bent within the hole 3' of the 
impeller 3 such as shown in FIG. 3a, because there is zero clearance at 
both end walls of the impeller 3 where the deformation arises. 
FIG. 8 shows another turbocharger shaft assembly, in which the washer 9 
located on the turbine side is omitted. Instead, a circular projection 10' 
is provided on the spacer 7' facing the impeller 3. The circular 
projection 10' is similar to those illustrated in FIG. 5a or 5b. This 
arrangement causes deformation in a similar way as described above. 
FIGS. 9a and 9b show another embodiment of the present invention, in which 
the impeller 3 is deformed prior to fitting the impeller 3 onto the turbo 
shaft 4. For this purpose, a device comprising a pair of press means 13, 
14 and a guide shaft 15 is prepared. Each of the press means is provided 
with a projection 10 or 11 similar to those illustrated in FIGS. 5a and 
5b. The outer diameter of the guide shaft 15 is equal to that of the turbo 
shaft to be assembled. After inserting the impeller 3 into the device, as 
shown in FIG. 9a, this device causes similar deformation 12 by applying 
the force F to the press means 13, 14 in the inward direction shown by the 
arrow in FIG. 9b. The deformed impeller 3 is assembled into the 
turbocharger after it is drawn from the device. While it is not easy to 
draw the impeller 3 from the device and to fit it to the shaft because of 
the zero clearance, it is easier to fit the impeller 3 onto the shaft 4 
compared to the case where the impeller and the shaft are machined for a 
tight fit. This is because the zero clearance according to the invention 
is restricted to a small distance.