A rotary anode stem tube is secured to a bearing by a coupling arrangement including a nested body and shaft, the shaft being thermally conductively coupled to the stem tube and the body being thermally conductively coupled to the bearing. The body and shaft have a relatively large interface region in facing spacing relation. Different positions of the interface region engage in thermally conductive relation in accordance with the temperature of the stem tube to selectively increase the thermal conductivity of the shaft to the body and selectively decrease the cooling time of the stem tube without an unacceptable increase in the bearing temperature.

FIELD OF THE INVENTION 
The invention relates to a rotary anode X-ray tube comprising means for 
varying the heat resistance of the heat dissipation path dissipating the 
heat from the rotary anode stem tube through the bearing. 
BACKGROUND OF THE INVENTION 
In such an arrangement known from DE-PS 591 625, in the stationary 
condition an anode part is brought into good thermally conducting contact 
with a heat-dissipating body. 
When the X-ray tube is switched on, the striking electron beam produces in 
the rotary anode stem tube a high heat dissipation, which can lead there 
to a temperature of, for example 1500.degree. C. Before the tube is 
switched on again, a cooling to, for example, 150.degree. C. must take 
place in order that, when the tube is subsequently switched on again, the 
occurrence of too high temperatures is avoided. 
It is attempted to keep the required cooling time (in dependence upon the 
application, for example, approximately 20 minutes) as short as possible. 
At high temperature, the dissipated heat is conducted away from the anode 
stem tube mainly by radiation. At low temperatures, on the contrary, 
essentially only the heat transport through the material of the rotary 
anode and via the bearing to a bearing support dissipating heat to the 
environment is left. More particularly with the use of sliding bearings, 
this path can contribute essentially to shortening of the required cooling 
time of the rotary anode stem tube. 
A substantial reduction of the heat resistance of the heat dissipation 
path, though permanent or, above known case described in the only in the 
stationary condition, can lead to unacceptable high bearing temperatures, 
however. 
SUMMARY OF THE INVENTION 
The invention has for its object to construct a rotary anode X-ray tube of 
the kind mentioned in the opening paragraph in such a manner that the 
cooling time of the rotary anode stem tube is shortened without the risk 
of the bearing temperature assuming unacceptable high values. 
This object is achieved in that a device is provided, by which the 
variation of the heat resistance is obtained in dependence upon the 
temperature variation of a part following the rotation occurring after the 
electron beam has been switched off. 
The heat resistance is first reduced with a given time delay. The 
temperature of the rotary anode stem tube has then already considerably 
decreased due to heat emission to a value which, in spite of the 
subsequently reduced heat resistance, can no longer lead to high bearing 
temperatures. 
According to a preferred particularly reliable embodiment, the variation of 
the heat resistance is obtained by the temperature variation in a part in 
good thermally conducting connection with the rotary anode stem tube. As 
control criterion, use is made of a temperature varying uniformly with the 
temperature of the rotary anode stem tube and primarily ensuring the 
heating of the bearing. However, it is also possible to use the 
temperature of a part in thermally conducing connection with the bearing 
as criterion for the variation of the heat resistance. 
A control device provided in accordance with the invention consists in 
general form of a sensor sensing the temperature and of a drive for moving 
at least one part having a contact surface. 
Preferably, the control device can comprise an element expanded in 
dependence upon temperature. A particularly simple embodiment that can be 
obtained thereby is characterized in that the variable heat resistance 
consists of two parts, which have corresponding adjacent contact surfaces, 
which can be moved with respect to each other by thermal expansion of at 
least one of the elements. A single part constructed in a simple manner 
then fulfils simultaneously the functons of the sensor and of the drive. 
An advantageous constructive embodiment is characterized in that the 
variable resistance is arranged in the interior of a tubular shaft 
connecting the rotary anode stem tube with a rotor body, in that the shaft 
has at least one contact surface, and in that at least one opposite 
contact surface is arranged at a projection extending within the shaft and 
being in good thermally conducting contact with the rotor body. 
According to an advantageous further embodiment of the invention, it is 
ensured that the shaft has such an axial length and such a small wall 
thickness that its heat resistance from the rotary anode stem tube to the 
rotor body is higher than 30% of the heat resistance obtained due to the 
bearing. The heat resistances of the shaft and the variable heat 
resistance precede in parallel arrangement the heat resistance of the 
bearing. Comparatively large arrangements of the resulting heat resistance 
are obtained if the ratio of the heat resistances of the shaft and of the 
bearing is as large as possible. 
In order that the heat transport through the contact surfaces is not 
impeded by the formation of thermally isolating foreign layers, it is 
ensured that the quality of the contact surfaces is improved. 
The heat contact resistance over the contact surfaces is inversely 
proportional to the size of the contact surface and to the pressure force. 
An enlargement of the effective contact surface can be attained in that 
the contact surfaces have associated depressed parts and embossed parts, 
respectively. 
In order that the invention may be readily carried out, it will now be 
described more fully, by way of example, with reference to the 
accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
To a radially external region of a rotary anode stem tube 2 is directed a 
concentrated electron beam 1, which originates from a cathode (not shown) 
and produces an X-ray radiation. A high heat dissipation leading to 
temperature of up to 1500.degree. C. is then obtained in the rotary anode 
stem tube 2. 
The rotary anode stem tube 2 is soldered through the shaft 3 to the rotor 
body 4 so as to be locked against rotation. The rotor body 4 is journalled 
on the stationary and preferably cooled bearing support 6 through an 
indicated sliding bearing 5 (as described in principle in EP-A 14 14 76). 
The rotor body 4 acts as a shortcircuit rotor, on which an asynchronous 
torque is exerted by means of a rotary field formed by a motor stator (not 
shown). The motor stator is outside a mainly metallic and gas-tight 
housing (not shown) surrounding the elements shown in the FIGURE, as is 
well known. 
Due to the high temperatures of the rotary anode stem tube 2 of 
approximately 1500.degree. C., the shaft 3 is heated to about 800.degree. 
C., the rotor body 4 is heated to about 400.degree. C. and the bearing 
support 6 is heated to about 200.degree. C., temperatures averaged over 
the volume areas being indicated, which, when the thermal parallel path is 
interrupted, are adjusted through the projection 7 of the rotor body 4. 
The projection 7 is arranged within the hollow cylindrical shaft 3 in 
spaced relation as shown over a first portion of their respective facing 
structures, for example, the nested portion of projection 7 in shaft 3. 
The projection 7 has contact surfaces 8 and 9, to which correspond contact 
surfaces 10 and 11, respectively, of the shaft 3. The contact surfaces 8 
and 10 are flat surfaces over a second portion, for example, of their 
respective structures, in the form of circular rings. The contact surfaces 
9 and 11 on the contrary are over a third portion and are uneven and are 
provided with annular embossed indentations, respectively, which 
indentations have an approximately triangular cross-section. As a result, 
the effective heat contact surface is enlarged at surfaces 9 and 11. 
In the condition shown in the FIGURE, which is obtained at high temperature 
values of the rotary anode stem tube 2, the contact surfaces 8 and 10 and 
9 and 11, respectively, are located opposite to each other at small 
relative distances. These distances are shown on an exaggerated large 
scale in the drawing. (Apart from heat radiation) no heat is conducted 
over the separation fold and the vacuum. Since the shaft 3 is constructed 
over a long axial path with a very small wall thickness 12, its heat 
resistance to the rotor body 4 is high. For example, the heat resistance 
due to the axial length and wall thickness of shaft 3 may be higher by 30% 
of the heat resistance of the bearing body 5. Consequently, only a small 
part of the temperature of the rotary anode stem tube 2 can act upon the 
bearing body 5. 
At a temperature of the rotary anode stem tube 2 decreasing particularly 
due to heat emission, the temperature of the shaft 3 also decreases, which 
then shrinks axially. At a temperature of the rotary anode stem tube 2 of 
about 20% of its maximum temperature, the contact surfaces 8 and 10 and 9 
and 11, respectively, adjoin each other. Heat is then transmitted through 
the contact surfaces. The cooling of the shaft 3 is then accelerated and 
on the other hand the projection 7 is heated. Consequently, a large 
elastic pressure force is then rapidly produced between the contact 
surfaces, which results in a very low heat resistance from the shaft 3 
through the contact surfaces 8 and 10 and 9 and 11, respectively, to the 
rotor body 4. The further cooling of the rotary anode stem tube 12 to 
about 10% of its maximum temperature is considerably acclerated. At the 
now low temperature level there is no risk of the temperature of the 
bearing 5 assuming unacceptable values. 
The distances between the respective contact surfaces 8 and 10 and 9 and 11 
can be dimensioned so differently that contacts are obtained at different 
times and at different temperatures of the rotary anode stem tube 2. As a 
result, a further reduction of the overall cooling time of the rotary 
anode stem tube 2 can be attained. 
It is further possible to utilize radial temperature expansions for 
bridging cylindrical gaps between the stem 3 and the projection 7.