A rotor-shaft assembly includes a ceramic solid hubbed turbine rotor having an integral stub shaft interference fit within one end of a generally cylindrically shaped sleeve member. The sleeve member defines a hub portion adapted to mate with a piston ring seal and a coaxial bore therethrough. The interference fit, which is defined by the change in the diameter of the bore, schedules the compressive forces acting on the stub shaft. A metal shaft is brazed within the other end of the sleeve member in a torque transmitting relationship.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to rotor-shaft assemblies of the type used in 
exhaust gas driven turbochargers, and more particularly to the attachment 
of a ceramic rotor to a metal shaft assembly. 
One means of improving the response time of a turbocharger is to reduce the 
moment of inertia of the rotating parts by constructing the parts of 
lighter material. Yet the material chosen must be able to withstand the 
harsh operating environment of the turbocharger. Since the compressor 
impeller does not see high temperatures in comparison to the turbine 
wheel, designers began to make the impellers of low weight aluminum alloy 
which can harmoniously exist in the turbocharger environment. 
In the never ending quest for a lighter, economically feasible alternative 
to the relatively heavy tubine wheel, and one which could survive the high 
temperatures and gaseous environment of the turbine, the industry's focus 
turned to using ceramics as a substitute. Once the decision is made to use 
the lighter, ceramic turbine wheel, there are two alternatives design 
choices which must be considered; either construct only a ceramic turbine 
wheel or an integral ceramic shaft and turbine wheel. However, there are 
problems associated with either solution; the first requires a ceramic to 
metal joint and the second revolves around economics and durability of a 
ceramic shaft as compared to a metal shaft and the problems associated 
with attachment of the compressor impeller thereto. 
Thereafter, most efforts have been focused on solving the problem of the 
ceramic to metal joint as evidenced by U.S. Pat. Nos. 4,063,850; 4,125,344 
and 4,424,003 and German Pat. No. 2,734,747. However, none of these 
efforts have resulted in a reliable joint as evidenced by the fact that 
there is no commercially available or production model turbine wheel on 
the market, whether it be in turbochargers or any other turbomachinery. 
Several of these structures focus on shrink fitting the ceramic stub shaft 
of the turbine wheel within a metallic sleeve while others have 
concentrated on the use of an adhesive in order to bond the two materials 
together. 
Utilization of the shrink fit method of attachment gives rise to a further 
problem: the reduction in the imposition of the high tensile stresses upon 
the ceramic stub shaft by the sudden discontinuity of contact between the 
sleeve member and ceramic rotor. This problem leads to the design feature 
of scheduling the compressive forces exerted by the sleeve onto the shaft 
by substantially tapering the thickness of the sleeve. This reduction in 
the thickness of the sleeve results in a reduction in the compressive 
stresses acting on the rotor and the tensile stresses imposed on the 
ceramic rotor at the point where contact between the sleeve and rotor 
ends. It is the tensile stresses which cause the propagation cracks in the 
ceramic material and can eventually lead to joint failure. 
The high temperature, thermal cycling atmosphere of the turbocharger leads 
to the degradation and failure of the ceramic rotor-metal shaft joint. 
Failures occur because of several reasons; the metal sleeve radially 
expands by a greater degree than the ceramic rotor because it has a 
coefficient of expansion greater than a ceramic thereby loosening the 
joint; thermal cycling causes "ratcheting", i.e. the easing out of the 
ceramic stub from the sleeve; and in the case of adhesives, the breakdown 
of the adhesive in the high temperature environment. 
In addition to the above problem of joint integrity, there exists a 
secondary problem of oil containment if the ceramic rotor or the ceramic 
to metal joint fails. Heretofore, none of the existing metal to ceramic 
joints have incorporated any means of preventing oil leakage into the 
turbine housing in the event of a joint failure. 
According to the present invention, a ceramic rotor is attached to a metal 
shaft to form a rotor-shaft assembly. The rotor-shaft assembly includes a 
metal sleeve member having a generally coaxial bore formed therethrough. 
One end of the sleeve extends generally radially outward to form a hub 
portion which defines an annular surface area generally coaxial to the 
shaft. The sleeve hub portion includes an annular groove which is sized to 
generally mate with a piston ring located within the center housing near 
the turbine end of the turbocharger. The ceramic rotor includes a hub and 
plurality of blades periodically spaced about the circumference of the 
hub. The rotor further includes a stub shaft integral with and generally 
symmetrical about the axis of the hub. The stub shaft is cold press fitted 
within the end of the sleeve which defines the sleeve hub portion. In 
addition, the stub shaft has an annular groove therearound. Once the stub 
has been cold pressed into the sleeve member, a crimped groove, 
corresponding in location to the groove in the stub, is rolled into the 
sleeve member. The other end of the sleeve is then interference fitted or 
brazed onto the shaft in order to place the shaft in torque receiving 
relationship with the rotor. 
It is an object of the present invention to provide a ceramic to metal 
joint for use within a turbocharger. 
It is another object of the present invention to provide a sleeve member 
for joining a ceramic rotor to a metal shaft and including a portion of a 
seal between the center housing and the turbine housing. 
It is another object of the present invention to provide a means for 
preventing lubricant from entering the turbine housing in the event of a 
joint failure or ceramic rotor failure. 
It is a further object to provide a method of assemblying a ceramic rotor 
to a metal shaft.

DETAILED DESCRIPTION OF THE INVENTION 
A turbocharged engine system 10 is shown in FIGS. 1 and 2, and generally 
comprises a combustion engine 12, such as a gasoline or diesel powered 
internal combustion engine having a plurality of combustion cylinders (not 
shown), for rotatably driving an engine crankshaft 14. The engine includes 
an air intake conduit or manifold 16 through which air is supplied by 
means of a compressor 18 of the turbocharger 20. In operation the 
compressor 18 draws in ambient air through an air inlet 22 into a 
compressor housing 24 and compresses the air with a rotatable compressor 
impeller 26 to form so called charge air for supply to the engine for 
combustion purposes. 
Exhaust products are discharged from the engine through an exhaust conduit 
or manifold 28 for supply to a turbine 30 of the turbocharger 20. The high 
temperature (up to 1000.degree. C.) exhaust gas rotatably drives a turbine 
wheel 32 within the turbine housing 34 at a relatively high rotational 
speed (up to 190K RPM) to correspondingly drive the compressor impeller 26 
within the compressor housing 24. In this regard, the turbine wheel and 
compressor impeller are carried for simultaneous rotation on a common 
shaft 36 supported within a center housing 38. After driving communication 
with the turbine wheel 32, the exhaust gases are discharged from the 
turbocharger 20 to an exhaust outlet 40 which may conveniently include 
pollution or noise abatement equipment as desired. 
The turbocharger, as is shown in FIG. 2, comprises the compressor impeller 
26 carried on a rotatable shaft 36 within the compressor housing 24. The 
shaft 36 extends from the impeller 26 through a center housing 38 and an 
opening 42 formed through the center housing wall 44 for connection to the 
turbine wheel 32 carried within the turbine housing 34. A compressor back 
plate 54 separates the center housing 38 and the impeller 26. 
The center housing 38 includes a pair of bearing bosses 46 which are 
axially spaced from one another. The bearing bosses 46 form bearing bores 
48 for reception of suitable journal bearings 50 for rotatably receiving 
and supporting the shaft 36. A thrust bearing assembly 52 is also carried 
about the shaft for preventing axial excursions of the shaft. 
Lubricant such as engine oil or the like is supplied via the center housing 
38 to the journal bearings 50 and to the thrust bearing assembly 52. A 
lubricant inlet port 56 is formed in the center housing 38 and is adapted 
for connection to a suitable source of lubricant such as filtered engine 
oil. The port 56 communicates with a network of internal supply passages 
58 which are suitably formed in the center housing 38 to direct the 
lubricant to the appropriate bearings. The lubricant circulated to the 
bearings is collected in a suitable sump or drain for passage to 
appropriate filtering, cooling and recirculation equipment, all in a known 
manner. To provide against leakage of the lubricant from the center 
housing into the turbine housing a seal or piston ring 60 is received 
within an annular groove in the surface of the side wall which defines the 
shaft opening 42. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The rotor-shaft assembly of the present invention is shown in FIGS. 3 and 6 
in its preferred form. The assembly 62 includes a ceramic rotor and a 
metal sleeve member. The ceramic rotor or ceramic turbine wheel 32 
includes a hub 66 and a plurality of blades 68 (see FIG. 2) periodically 
spaced about the circumference of the hub. The rotor 32 further includes a 
stub shaft 70 integral with and generally symmetrical about the axis of 
the hub 66. In addition the stub shaft includes an annular groove 71 
therearound having a depth of approximately 0.010 inches and located a 
desired axial distance from the back face of the rotor blades. 
The metal sleeve member 72 is generally cylindrically shaped and includes a 
coaxial bore 74 therethrough which may be cast, machined or otherwise 
formed therein. The bore 74 defines three distinct sections therein (see 
FIG. 7): a first and a second section, 76 and 77 respectively, of constant 
diameter and a tapered section 79 therebetween. The first section 76 is 
sized to be interference fit with the stub shaft of rotor 32. The second 
section 77 has a diameter greater than that of the diameter of the first 
section 76 and is also sized such that it is slightly larger than the 
diameter of the stub shaft 70. The tapered section 79 acts as a transition 
between the first and second sections and accordingly is tapered radially 
outward in the direction of rotor 32 (left to right in FIGS. 3 to 6) by a 
magnitude A. 
The taper reduces the clamping forces on the stub shaft by reducing the 
amount of interference fit. Thus, the tapering of the sleeve member 72 
results in a scheduling of the compression forces acting on the stub shaft 
from some maximum value at the inboard end (compressor end) of the tapered 
section 79 to a minimum value at the outboard end (turbine end) of this 
same section. This feature in turn minimizes the tensile stresses exerted 
upon the stub shaft due to the sudden discontinuities in the degree of the 
compressive forces, yet permits torque transmission. Furthermore, the 
tapering assists in piloting the stub shaft into the proper coaxial 
relationship with the sleeve member during assembly. 
By way of example, where the finished outside diameter of the sleeve is 
approximately 0.360 inches and the diameter of the stub shaft is 
approximately 0.310 inches, the magnitude of angle A is approximately 
1.degree. and the total change in the diameter of cavity 74 is 
approximately 0.0016 to 0.0020 inches. Since there is only a slight 
difference (ideally zero clearance) between the diameters of the stub 
shaft and section 77, the amount of interference fit equals the change in 
the cavity diameter. Whenever upscaling or downscaling the rotor-shaft 
assembly of the present invention, the interference fit is adjusted in 
order to give the same degree of compressive stresses as occurs in the 
above example. Typically, the angle A can have a magnitude of 
approximately 0.5 to 2.0.degree.. 
At the outboard end of the sleeve 72 is a generally radially outwardly 
extending hub portion 78 which defines a annular surface area 80 coaxial 
to the sleeve member 72. The annular surface 80 includes an annular piston 
ring groove 82 therein which is sized to operably mate with the piston 
ring 60 located within the center housing 38 of the turbocharger 20. In 
the assembled state, the hub portion extends radially away from the stub 
shaft thereby creating an annular gap 84 of increasing diameter in the 
direction of the rotor blades. 
This features ensures that if failure of the joint or breakage of the 
ceramic rotor occurs the seal between the center housing 38 and the 
turbine housing 34 remains intact. Furthermore, the method of attaching 
shaft 36 to sleeve 72 eliminates any potential flow path from the center 
housing to the turbine housing in those cases where the ceramic rotor stub 
shaft 70 rachets itself from the sleeve 72. Hence, sealing is a 
combination of piston ring 60 and the braze joint between the metal shaft 
36 and the sleeve 72. Together they prevent oil leakage on failure of the 
metal-to-ceramic joint or failure of the ceramic. Additionally, seal 60 
provides normal function of sealing during normal operation. 
An alternative rotor-shaft assembly is shown in FIG. 4. The assembly of 
FIG. 4 shows the metal shaft 36 including an enlarged end which defines a 
sleeve portion 86 having a coaxial bore 87 therein. The outer surface 88 
of the outboard end of the enlarged sleeve portion 86 is reduced in 
thickness in order to schedule the compressive forces acting on the stub 
shaft 70. This method of scheduling the compressive forces is different 
from the method used in the preferred embodiment, since it is a reduction 
in the amount of interference fit rather than a reduction in sleeve 
material thickness. At least one passage 90 extending radially outward 
from the bore 87 flow connects the bore to the ambient. Passage 90 acts as 
a pressure relief during insertion of the stub shaft 70 within the bore 87 
to avoid the buildup of compressed air therein. In addition, FIG. 4 shows 
that one important aspect of the present invention can be used in 
conjunction with the ceramic to metal sleeves used in the past. As shown, 
the stub shaft 70 includes the groove 71 therein. After insertion of the 
stub shaft into the sleeve, a crimp is rolled into the sleeve member as 
will be explained below. 
A second alternative rotor-shaft assembly is shown in FIG. 5 and includes 
an enlarged shaft end which defines a sleeve portion 86 and includes a 
central aperture, or cavity or bore 87 which is generally symmetrically 
distributed about axis of the shaft. As with the preferred embodiment, the 
surface area defining the bore defines the three distinct sections; 76, 77 
and 79, as explained above. The tapered section 79 reduces the 
interference fit and therefore the compressive forces on the stub shaft 
which in turn acts to minimize the tensile stresses in the stub shaft 70 
as explained above. 
Adjacent the inboard end of the bore is at least one radially extending 
aperture 90 which extends through the sleeve portion and flow connects the 
bore and the ambient. This aperture acts as a pressure relief during the 
press fitting of the ceramic stub shaft within the sleeve to prevent the 
buildup of compressed air within the cavity. In addition, the sleeve 
member 86 has been roll crimped into the groove 71 on the stub shaft 70 in 
order to prevent axial movement between the stub shaft and the sleeve. 
In all embodiments, the sleeve is located within the bearing 50 nearest the 
turbine end of the turbocharger. This placement assists in lessening the 
degree of thermal cycling experienced by the joint. Hence, loosening of 
the joint by thermal expansion of the sleeve and/or ratcheting is 
minimized. Additionally, all embodiments have been shown to include the 
groove 71 in the stub shaft 70 and a rolled crimp 92 in the sleeve member. 
This feature is optional, and is not necessary for a working ceramic to 
metal joint. 
As is common with the preferred embodiment shown in FIGS. 2, 3 and 6, the 
alternative embodiment of FIG. 5 includes an improved shaft seal 
arrangement used in association with a ceramic rotor-metal shaft joint for 
the turbine end of the turbocharger. More specifically, the improvement 
comprises an arrangement for preventing lubricant supplied to the bearings 
50 from leaking along the shaft and into the turbine housing 34 in the 
event the ceramic rotor or the rotor-shaft joint fails, as explained 
above. 
FIGS. 4, 5 and 6 also show the progression of the problems encountered and 
the solutions arrived at during the development of a working ceramic 
rotor-shaft assembly. In the embodiment of FIG. 4, the sleeve portion is 
to be constructed of the same material as the shaft, more particularly 
Incoloy 903, 907 or 909. It is well known in the art that this alloy 
possesses a low coefficient of thermal expansion and great strength, yet 
it is costly in comparison to a steel alloy. 
Therefore, there was an attempt to reduce the amount of Incoloy by making 
just the sleeve (FIG. 5) of this alloy. This then lead to the problem of 
attaching the Incoloy sleeve to a steel shaft as well as the sleeve to a 
ceramic rotor. Two alternative methods of attachment were arrived at; 
either have a closed inboard sleeve end as shown in FIG. 5 or open the 
sleeve end as in FIG. 6. In the case of the closed end of FIG. 5, the end 
of the shaft would have to be friction or inertia welded to the back side 
of the sleeve member. This method of attachment led to the problem of how 
to grip the sleeve without crushing it and/or the stub shaft during the 
welding process. This problem then led to the development of the present 
open end design as shown in FIG. 6 and the method of sleeve to shaft 
attachment as described below. 
In addition to the above, the alternate embodiments of FIGS. 4 and 5 impose 
the additional burden of having to machine a blind hole. Machining a blind 
hole is more costly than machining an open ended sleeve design as shown in 
FIG. 6. 
According to the present invention the rotor-shaft assembly is constructed 
by first cold press fitting the ceramic stub shaft 70 within the sleeve 
hub portion 76. During this operation the stub shaft 70 is piloted into 
the sleeve bore 74 assisted by the tapered section 79. After insertion of 
the stub shaft 70, the groove 71 is located within section 76 which is 
inboard of tapered section 79. Thereafter the outer surface of the sleeve 
member is roll crimped at 92 in order to force sleeve material into the 
annular groove 71 of the stub shaft 70. The roll crimping of the sleeve 
into the groove 71 is accomplished by using a tube cutter having a 
radiused cutting edge which deforms the metal rather than cutting it. The 
deformation of sleeve material into the groove 71 acts as a mechanical 
lock preventing the tendency of the ceramic stub shaft to ratchet itself 
out of the sleeve due to differential thermal expansion of the two 
materials during thermal cycling of the joint while operating the 
turbocharger. It should be noted that while this operation results in 
sleeve material being forced within the groove 71 in the stub shaft, the 
crimp in the outer surface of the sleeve has been shown to be exaggerated 
in the drawings. In reality, this crimp in the outer surface of the sleeve 
cannot be felt or seen by the naked eye after final machining. 
Furthermore,, the groove 71 is not completely filled with sleeve material 
nor is it necessary that it is. All that need be accomplished is that 
enough sleeve material is deformed so as to act as a mechanical detent at 
each end of groove 71 to prevent axial movement between the sleeve and the 
stub shaft. 
Once the rotor 32 and sleeve member 72 are attached, the inboard end of the 
sleeve is brazed or interference fitted onto the shaft 36 in the case of 
the sleeve member 72 as shown in preferred embodiment. Brazing is 
accomplished by depositing braze material into the sleeve base adjacent 
the end of the stub shaft. The end of shaft 36 is then inserted into the 
sleeve until it abuts the braze material. Thereafter, heat is applied 
using induction coils until the braze material wicks out of the sleeve, 
signalling that the gap between the inner surface of the sleeve and the 
metal shaft has been completely filled. 
Cold press fitting of the stub shaft within the sleeve is an improvement in 
the art, since it allows the sleeve to retain its mechanical properties. 
On the other hand, the shrink fitted method of attachment between the 
metal sleeve and ceramic rotor weakens the sleeve material in the sense 
that the material has been annealed or tempered by the heat shrinking 
process and therefore loosens more easily when subjected to the thermal 
cycling atmosphere of a turbocharger. 
Various modifications to the depicted and described apparatus will be 
apparent to those skilled in the art. Accordingly, the foregoing detailed 
description of the preferred embodiment of the invention should be 
considered exemplary in nature, and not as limiting to the scope and 
spirit of the invention as set forth in the appended claims.