Bearing arrangement in a tunable magnetron

A bearing arrangement in a tunable magnetron, in which a sleeve-shaped body (12) is journalled on a central column (10) by means of two bearings (14, 15). The distance between the inner rings (18, 19) and outer rings (22, 23) of the bearings is determined by distance means (20, 21) which generally can be sleeve-shaped. According to the invention, at least one distance means, suitably both distance means, is temperature compensated by being composed of an odd number of partial elements (30-32; 37-39). The elements all extend in the longitudinal direction of the distance means, and each distance means includes two end elements (30, 32; 37, 39) which bear against the respective bearing part (18, 19; 22, 23) and an intermediate element (31; 38). The elements in each distance means are in such force-transmitting connection with each other that, with temperature variations, two adjacent elements impart to the distance means length variations in opposite direction. The total length of all elements acting in one direction is so selected in relation to the total length of all elements acting in the opposite direction, and in relation to the linear expansion coefficients for the different elements, that a predetermined variation of the total length of the distance means with the temperature is obtained.

BACKGROUND OF THE INVENTION 
The invention relates to an arrangement in a tunable magnetron comprising a 
sleeve-shaped body which by means of two bearings is rotatably journalled 
on a stationary column or pillar and which at one end supports a tuning 
body projecting into the resonance cavities of the magnetron. An inner 
bearing part of a bearing has a fixed position relative to the column and 
an outer bearing part of a bearing has a fixed position relative to the 
sleeve-shaped body, while the distance between the bearings is determined 
by distance means. 
A magnetron of this general construction is described in Swedish Pat. No. 
191,373 (corresponding to U.S. Pat. No. 3,343,031), for example. The 
tuning body here has portions of different electric conductivity, formed, 
for example, by means of circumferentially distributed teeth or apertures 
in the body, and projects through a gap made in the rear part of the anode 
plates defining the resonance cavities. In order to achieve a high 
efficiency the gap is made very small, as large gaps between the tuning 
body and the anode plates will reduce the efficiency. 
Small gaps will result in high requirements on the bearing, in particular 
as regards freedom from play. Due to the small dimensions of the gaps 
there is already a very small play, and consequent inclination of the 
sleeve-shaped body will result in an appreciable influence on the electric 
HF signal generated by the magnetron, in particular the frequency of the 
signal. Play in the bearings can furthermore result in vibrations so that 
the operating life of bearings and thereby of the whole magnetron will be 
reduced. Very high requirements are therefore imposed upon the bearing for 
both electrical and mechanical reasons. 
To avoid play in the bearings it is important that both bearings are 
loaded, or in other words that they are biased. The biasing force can be 
achieved in different ways depending upon how the contact lines through 
the contact points in the bearings are oriented. In principle the contact 
lines can be parallel or intersecting. The latter lines can intersect each 
other either between or beyond the bearings. These different types of 
biasing forces are often called: "tandem", "face--to-face" and 
"back-to-back". The parallelism and symmetry can be more or less exact, 
dependent on practical circumstances. 
Besides freedom from play it is of great importance that the bearings are 
not too heavily loaded. The biasing involves, as a rule, a certain 
increase of the friction in the bearing and this friction must be kept low 
and accurately limited. 
These requirements should be fulfilled even in several operating 
conditions, i.e. involving operation of the bearings in a vacuum and under 
varying temperature conditions. The temperature will vary from the 
surrounding temperature at the start to varying high temperatures during 
operation, depending upon frequent variations of the electric power 
applied to the magnetron and variations of the microwave power delivered 
by the magnetron. Due to the effective thermal insulation between the 
different parts in the radial direction of the bearing arrangement there 
is furthermore, in the steady state, a high temperature gradient in the 
radial direction. In contrast to this, the temperature gradient in the 
axial direction is small because both the central column supporting the 
whole bearing arrangement and the rotatably-journalled sleeve are usually 
made of materials having good heat conductivity. The bearings must operate 
without play and with a low friction within the whole temperature range. 
In such magnetrons it is usual that the sleeve-shaped body is influenced by 
a continuous axial magnetic force. By means of this force a biasing of the 
tandem type can be achieved. It is then important that both bearings are 
loaded and in such a way that they have the same loading and each take up 
half the force. 
Many solutions of the bearing problem in tunable magnetrons of the 
above-described kind have been proposed. A bearing arrangement is 
described in European Pat. No. 0009903 (corresponding to U.S. Pat. No. 
4,281,273), for example in which both the inner rings and the outer rings 
of the bearings are displaceably arranged on a fixed column in the 
rotating sleeve body. The outer rings of the bearings together with a 
distance sleeve arranged between them are pressed against a fixed stop on 
the rotating sleeve. The inner rings are, on the one hand, influenced by a 
spring pressing the whole assembly of inner rings and intermediate 
distance elements against a stop on the column and on the other hand, by a 
spring included in the distance element and pressing the two inner rings 
away from each other. The stop on the column is furthermore adjustable in 
the axial direction. This adjustment of the stop on the column is then 
carried out in such manner that the load is distributed in the desired 
manner between the bearings. During the adjustment, as well as during 
thermally-induced variations during operation, the inner rings of the two 
bearings will be displaced on the column. The adjustment for achieving the 
desired distribution of the load between the bearings is very critical. If 
the spring characteristic of the springs should vary with time adjustment 
will be erroneous. Another drawback of this arrangement is that the inner 
and outer bearing rings must have loose tolerances against the column and 
the sleeve body, respectively, which in itself involves play and can give 
rise to vibrations. 
SUMMARY OF THE INVENTION 
An object of the invention is to make a bearing arrangement in a tunable 
magnetron of the kind described in which freedom from play is obtained in 
both bearings within the whole temperature range and without the necessity 
of complicated and critical adjustment operations and without 
deterioration of the properties of the bearings by means of a loose fit 
with play at several places. 
According to the invention this is achieved by means of an arrangement of 
the kind described, which is characterized in that, for the purpose of 
temperature compensation, at least one distance means comprises at least 
three elements which partly overlap each other in the direction of length 
of the column. These elements are made of at least two materials having 
different linear expansion coefficients. They comprise two end elements 
abutting at one end the respective bearing part, and at least one 
intermediate element. Adjacent elements adjoin each other at their ends so 
that, with temperature variations, two adjacent elements will impart to 
the distance means length variations in opposite directions. The total 
length of all elements producing a length variation in one direction is so 
selected relative to the total length of all elements producing a length 
variation in the opposite direction, and relative to the linear expansion 
coefficients of the materials of the different elements, that a desired 
variation of the total length of the distance means with temperature is 
obtained. 
The number of elements will always be an odd number. If the elements are 
numbered consecutively from one bearing to the other, those elements which 
cooperate in one direction will be the elements having odd numbers, while 
the elements which cooperate mutually and counteract the first elements 
will all be elements having even numbers. 
By means of the invention, the longitudinal expansion of a distance means 
due to temperature variations can in principle be adapted accurately to 
the expansion of the other parts of the bearing arrangement so that a 
ratio between the load of the two bearings, initially set during 
manufacture due to fixed stops, will be maintained within the whole 
temperature range. The invention also makes practical the manufacture of 
magnetrons having all kinds of biasing of the bearing without deviating 
from the requirement for low and accurately-determined friction. Biasing 
of the "back-to-back" type gives, for example, a more stable and thereby a 
more accurate construction than the "tandem" or "face-to-face" types. 
Preferably, both distance means are provided with the same temperature 
compensation as the one described, whereby no relative motion between the 
inner and outer parts of the two bearings due to temperature variations 
will occur within the whole temperature range. This will contribute to a 
more accurate biasing with freedom of play and low friction. 
A further improvement can be achieved if the column, and suitably also the 
sleeve-shaped body, are provided with the same temperature compensation as 
the distance means with respect to that part of the column or the 
sleeve-shaped body, respectively, which is situated between the bearings. 
Then, no relative motion will take place as a result of temperature 
variations and both bearings could in principle be mounted without a loose 
fit on the column and in the sleeve-shaped body. 
The elements are suitably shaped as sleeves arranged within each other. 
In a preferred embodiment the lengths are so selected relative to the 
linear expansion coefficients that the total longitudinal variation with 
variations in the temperature will be substantially equal to zero within 
the operational temperature range of the magnetron. 
Suitably all elements having an odd number can be made of one material and 
all elememts having an even number can be made of another material, the 
ratio between the total length of the first elements and the total length 
of the last elements being inversely proportional to the ratio between the 
expansion coefficients of the materials of the elements. In a combination 
of materials which has proved to give good results the material of the 
elements having odd numbers including the two end elements is molybdenum 
and the material of the elements having even numbers is stainless steel.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 reference numeral 10 designates a fixed centrally located column, 
which at one end is terminated by a magnetic pole shoe 11, while 12 
designates a sleeve-shaped rotor which at one end supports a sleeve-shaped 
tuning body 13 and which is rotatably journalled on the column 10 by means 
of two ball bearings 14, 15. The tuning body projects at its free end into 
the resonance cavities via grooves cut in the rear edge of the anode 
plates (not shown) and is provided in this region with apertures 16 
distributed around its circumference for producing a tuning variation upon 
rotation of the body 13 about the axis 17. Between the inner rings 18, 19 
of the ball bearings there is a distance sleeve 20 and a similar distance 
sleeve 21 is arranged between the outer rings 22, 23 of the bearings. The 
assembly consisting of the inner rings 18, 19 and the distance sleeve 20 
is pressed against a stop 24 on the column by means of a spring washer 25 
and the assembly consisting of the outer rings 22, 23 and the distance 
sleeve 21 is pressed against a stop 26 on the rotor body 12 by means of a 
screw-threaded ring 27. The sleeve-shaped rotor 12 is furthermore 
continuously subjected to an axial force F in the direction of the arrow, 
for example, a force produced magnetically. 
The inner bearing rings can suitably be arranged on the column with a press 
fit. As a result of increased temperature in operation this press fit will 
change to a sliding fit without introducing play. 
According to the invention at least one distance sleeve is provided with 
temperature compensation. In FIG. 1 temperature compensation is introduced 
in both distance sleeves and also in the sleeve-shaped rotor. Only the 
inner distance sleeve will be described in detail. 
The inner distance sleeve, as shown in FIG. 1, is composed of three partial 
sleeves 30, 31, 32 of which the outer and the inner sleeves 30, 32 are 
made of one material while the intermediate partial sleeve 31 is made of 
another material. The outer partial sleeve 30 bears at one end against a 
shoulder 33 on the intermediate partial sleeve 31 and the intermediate 
sleeve 31 bears at one end against a shoulder 34 on the inner partial 
sleeve 32. The partial sleeves are free to move relative to each other. 
The outer partial sleeve 30 bears at its other end 35 against the inner 
ring 18 of the bearing 14, while the inner partial sleeve 32 bears at its 
other end 36 against the inner ring 19 of the other bearing 15. 
The total length of the inner distance sleeve 20, which is decisive for the 
distance between the inner rings of the ball bearings, is determined by 
the length of the individual partial sleeves, measured between the 
abutment places. For the total length L the following relationship is 
valid: 
EQU L=l.sub.1 -l.sub.2 +l.sub.3 
where l.sub.1, l.sub.2, l.sub.3 are the lengths of the partial sleeves 
according to FIG. 1. 
At temperature variations the intermediate partial sleeve 31, which is made 
of one material, will counteract the other two partial sleeves which are 
made of another material. The resulting length variation .DELTA.L for a 
temperature variation .DELTA.t will be: 
EQU .DELTA.L=l.sub.1 .DELTA..sub.1 .DELTA.t-l.sub.2 .DELTA..sub.2 
.DELTA.t+l.sub.3 .alpha..sub.1 .DELTA.t 
where .alpha..sub.1 is the linear expansion coefficient of the material of 
the partial sleeves 31, 32 and .alpha..sub.2 is the linear expansion 
coefficient of the material of the partial sleeve 31. If the resulting 
length variation is to be equal to zero the following is valid: 
EQU l.sub.1 .alpha..sub.1 .DELTA.t-l.sub.2 .alpha..sub.2 .DELTA.t+l.sub.3 
.alpha..sub.1 .DELTA.t=0 
or 
EQU (l.sub.1 +l.sub.3)/l.sub.2 =.alpha..sub.2 /.alpha..sub.1. 
In order to ensure that the distance sleeve does not change its length due 
to temperature variations, in this example the ratio between the total 
length of the outer and inner partial sleeves of the first material and 
the length of the intermediate sleeve of the section material should be 
inversely proportional to the ratio between the linear expansion 
coefficients. 
In the present example it is assumed that the partial sleeves 30, 32 are 
made of molybdenum having the expansion coefficient .alpha..sub.Mo 
.apprxeq.5.10.sup.-6 mm/.degree.C. while the sleeve 31 is made of 
austenitic stainless steel having the expansion coefficient .alpha..sub.St 
.apprxeq.17.10.sup.-6 mm/.degree.C. The total length of the sleeves 30, 32 
will thus be approximately 3.4 times the length of the sleeve 32. 
In a manner similar to the inner distance sleeve the outer distance sleeve 
is composed of partial sleeves 37, 38 and 39. The sleeve-shaped rotor also 
is temperature compensated in the example shown and is composed of the 
three partial sleeves 40, 41 and 42. 
FIG. 2 shows how the central column can be constructed to have a 
corresponding temperature compensation. The illustrated section of the 
column consists of three parts, namely an inner cylindrical part 43, a 
sleeve-shaped intermediate part 44 and a sleeve-shaped outer part 45. By 
means of a screw-thread 46 the intermediate part 44 is screwed onto the 
inner part 43 until a shoulder on the intermediate part abuts a shoulder 
on the inner part at 47, and by means of a screw-thread 48 the outer part 
45 is screwed onto the intermediate part until a shoulder on the outer 
part abuts a shoulder on the intermediate part at 49. The support surfaces 
for the inner bearing rings are indicated by the dot-dash lines 50 and 51 
and the center lines of the ball races are designated 52, 53. 
In this case a first distance a.sub.1 is defined as the distance between 
the center line 52 and the stop surface 47, while a second distance 
a.sub.2 is defined as the distance between the stop surfaces 47 and 49 and 
a third distance a.sub.3 is defined as the distance between the stop 
surface 49 and the center line 53. In order to ensure a constant distance 
between the center lines 52 and 53 independently of the temperature, in 
this case the following relationship should be fulfilled: 
EQU (a.sub.1 +a.sub.3)/a.sub.2 =.alpha..sub.2 /.alpha..sub.1. 
In a preferred embodiment, temperature compensation of the kind described 
is introduced in the central column as well as in the two distance sleeves 
and in the rotor. 
As previously stated, the sleeve-shaped rotor is continuously subjected to 
an axial force F, which is taken up by the bearings. In the example shown 
the bearings are so biased that the force vectors in the two bearings have 
the same direction, a so-called tandem arrangement, and furthermore that 
the bearings each take up half the force. Due to the described temperature 
compensation of the central column, the distance sleeves and the rotor, 
this initially-set condition will be maintained in the whole temperature 
range, whereby both bearings will operate without play within the whole 
temperature range. 
A number of modifications of the described arrangement are possible within 
the scope of the invention. Thus, the partial elements of the distance 
means need not be shaped as sleeves but can, for example, be shaped as 
rods, a number of such distance means composed of rods distributed around 
the circumference. The number of individual parts in each distance means 
need not be three, but can be an arbitrary odd number. It is not necessary 
that the resulting length variation with temperature is zero, but the 
temperature compensation can be such that a controlled length variation 
with temperature is achieved. Such a controlled-length variation would be 
adapted to a known length variation of another part of the arrangement, 
which may in turn be without temperature compensation or may possibly be 
provided with corresponding temperature compensation. This will permit 
arrangements with other types of biasing, for example, "back-to-back" or 
"face-to-face", and the use of different types of ball bearings.