Patent Application
20110033282
The present invention relates to a charging device, more preferably an exhaust gas turbocharger for a motor vehicle with the characteristics of the preamble of claim 1
From DE 196 53 210 C2 a turbocharger for combustion engines is known. There, in the case of temperature fluctuations on the compressor side, thermal expansion of both the shaft and of a component such as for example a compressor wheel arranged on the shaft, as well as heat expansion of part regions of the surrounding housing is taken into account. By at least forming a wall element arranged on the compressor housing and facing the compressor housing of material that is softer than the material of the compressor wheel, the compressor wheel in the event of heat expansion upon contact with the wall element can rasp said wall element down so far until contact between compressor wheel and wall element just ceases to exist. Because of this, the gap between the housing inner wall and the compressor wheel is additionally minimized and compressor efficiency improved.
Components, too, which are arranged on the shaft on the compressor side, like the shaft proper, are subject to heat expansion in the event of a temperature increase. At present it is usual to produce at least some components arranged on the shaft from the same steel type. Usually, for the compressor wheel an aluminium or titanium material can be employed for weight reduction. In order to prevent the formation of unbalance in the compressor wheel the components arranged on the shaft are fixed on the shaft through a fixing device with a corresponding initial clamping force. In the event of a temperature increase however the compressor wheel for example of aluminium or titanium material is subject to greater expansion than the shaft on which the compressor wheel is arranged because of the higher expansion coefficient. Because of this, the clamping force with which the components are fixed on the shaft is additionally increased by an expansion force caused through the greater heat expansion of the compressor wheel. Accordingly, this expansion force that occurs through the thermal expansion has to be taken into account with the initial clamping force otherwise the permissible surface pressure of the clamped components can be exceeded and in the worst case tearing of the shaft can occur. However, it is conceivable that the maximum permissible initial clamping force is not always adequate for all application range of the components arranged on the shaft. Because of insufficient initial clamping force which in the event of a falling temperature for example in winter can be additionally reduced through the high expansion coefficient of the compressor wheel, slipping, twisting or tilting of the compressor wheel can occur. This in turn can result in a negative unbalance change of the compressor wheel which could be objected to by the customer as acoustic interference.
The present invention deals with the problem of stating an improved or at least a different embodiment for a charging device, which is more preferably characterized by a possible increase of the initial clamping force with which the components are fixed on the shaft, without the permissible surface pressure of the component clamped onto the shaft being exceeded throughout the application range.
According to the invention, this problem is solved through the subject of the independent claim. Advantageous embodiments are the subject of the dependent claims.
The invention is based on the general idea of carrying out a material pairing of components arranged on the shaft on the compressor side and of the shaft proper so that in a predetermined temperature range an expansion coefficient of at least one component is smaller than an expansion coefficient of the compressor wheel. Through such a material pairing a temperature-related expansion of the compressor wheel can be partially offset through a lower expansion of the at least one component, so that the expansion force resulting from the expansion of the compressor wheel and acting on the shaft is reduced. Since the compressor wheel in combination with the at least one component in percentage terms expands less greatly relative to the shaft than the compressor wheel alone, the clamping force resulting from this is also reduced. This makes possible increasing the initial clamping force with which the components are positioned on the shaft through a fixing device, since because of the reduced expansion force that occurs during a temperature increase the maximum surface pressure of the components arranged on the shaft and positioned through the fixing device is not exceeded. Because of a correspondingly increased initial clamping force adequately stable positioning of the compressor wheel at least with respect to its unbalance on the shaft is ensured.
In a preferred embodiment the expansion coefficient in of the at least one component in the predetermined temperature range is not only smaller than the expansion coefficient of the turbine wheel, but even smaller than the expansion coefficient of the shaft. Because of this, the effect described above can be increased and the initial clamping force can also be further increased accordingly.
Further important features and advantages of the invention are obtained from the subclaims, from the drawing and from the corresponding FIGURE description by means of the drawing.
It is to be understood that the features mentioned above and still to be explained in the following can not only be used in the respective combination stated but also in other combinations or by themselves without leaving the scope of the present invention.
A preferred exemplary embodiment of the invention is shown in the drawing and is explained in more detail in the following description.
It shows, schematically, the sole
As shown in
Usually the compressor wheel 2 and additional components 5,6,7 are fixed on the shaft through clamping by the fixing device 4. Here, an initial clamping force has to be selected so that in the event of heat expansion of all components occurring through a temperature increase by the resultant expansion force acting on the shaft the permissible surface pressure of the clamped components 2,4,6,5,7 is not exceeded. If both the fixing device 4 as well as the components 5,6,7 were produced of a same material as the shaft 3, only the heat expansion of the compressor wheel 2 would lead to an expansion force occurring in addition to the clamping force and acting on the shaft 3 in the event of a temperature increase. This happens since the expansion coefficient α2 of the shaft because of the usual material pairing is smaller than the expansion coefficient α1 of the compressor wheel 2. In the case of a temperature increase the expansion of the compressor wheel 2 that occurs inter alia along the shaft is thus greater than the longitudinal expansion of the shaft 3. Since the axial space of the compressor wheel 2 on the shaft 3 is restricted by the fixing device 4 an expansion force effect on the shaft 3 results because of the heat expansion. For this reason, the initial clamping force with which the components 2,5,6,7 are positioned on the shaft has to be selected so that the maximum permissible surface pressure of the components 2,4,5,6,7 clamped to each other is not exceeded even in the case of heat expansion.
It is now possible to increase this initial clamping force if at least one of the components 5,6,7 is equipped with an expansion coefficient α3 which is at least smaller than the expansion coefficient α1 of the compressor wheel 2. By such a measure the expansion force that results because of the temperature increase can be reduced, as a result of which it becomes possible to increase the initial clamping force. This in turn has the advantage that throughout the entire predetermined temperature range under consideration or the operating range of the charging device 1, the clamping force acting on the components 2,4,6,5,7 remains in a range in which neither the permissible surface pressure of the components 2,4,5,6,7 is exceeded nor the clamping force acting on the components 2,5,6,7 dropped so far that slipping, twisting or tilting of at least the compressor wheel 2 occurs for example because of a lowering of the temperature.
Here, heat expansion follows the following formula
l
x=(l0+Δl)x=l0x(1+α·ΔT)x
w:
with E Modulus of elasticity
Consequently, because of the longitudinal expansion Δl of the compressor wheel 2 which can only develop incompletely because of the lower expansion coefficient α2 of the shaft 3, expansion of the shaft 3 and compression of the components 2,4,5,6,7 arranged on the shaft 3 occurs.
Preferably the expansion coefficient α3 can be selected in a predetermined temperature range of the at least one component 5,6,7 so that it is even smaller than the expansion coefficient α2 of the shaft 3. In this case, the shaft 3 would expand more greatly relative to the at least one component 5,6,7 than the at least one component 5,6,7. Because of this, a space gain occurs in the event of heat expansion upon a temperature increase which is available to the heat expansion of the compressor wheel 2. Through this material pairing the heat expansion of the compressor wheel 2 which accompanies a temperature increase can be offset to a greater extent or, if suitably designed, almost entirely. This makes possible a further reduction of the expansion force that occurs through temperature increase. In principle, this can be carried out with all charging devices 1 wherein for example the shaft is made of steel or a steel alloy and the compressor wheel is produced of aluminium or titanium or an aluminium alloy and/or a titanium alloy. In this case the expansion coefficient α2 of the shaft 3 is always smaller than the expansion coefficient α1 of the compressor wheel 2.
Preferably, for at least one such component 5,6,7, an alloy is used which because of the Invar effect of the alloy has an expansion coefficients α3 which is anomalously low. Such alloys, which have such invariance of expansion with respect to a temperature change (Invar effect) can even have negative heat expansion coefficients in certain temperature ranges. Because of this, expansion coefficients α3 of at least one component 5,6,7 can be designed which are almost zero or negative.
The principle of offsetting the heat expansion of the compressor wheel 2 by at least one component 5,6,7 with a lower expansion coefficient can be employed for any component arranged on the shaft together with the compressor wheel 2 and positioned or clamped through the fixing device 4. The components 5,6,7 can for example be designed with an expansion coefficient α3 that is lower compared with the expansion coefficient α1 of the compressor wheel 2. It is likewise possible to design at least one fixing element of the fixing device 4 so that the at least one fixing element has an expansion coefficient αf that is smaller than the expansion coefficient α1 of the compressor wheel 2. Likewise at least one such fixing element can be equipped with an expansion coefficient αf that is also smaller than the expansion coefficient α2 of the shaft section. As with the components 5,6,7 this can be achieved through an alloy having an Invar effect. Because of this, the expansion coefficient αf of the alloy of such a fixing element is anomalously low and the offsetting of the heat expansion of the compressor wheel 2 described above can be accomplished through such a design of the expansion coefficient αf. Here it is possible that such a fixing element is designed for example as a shaft nut.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102009035629.0 | Jul 2009 | DE | national |
