DESCRIPTION OF THE BACKGROUND ART Components that shall be used at microwave frequencies must be manufactured with very great precision, for instance with respect to the geometrical dimensions of the components.

These high precision requirements are difficult to fulfil in one single manufacturing step. Furthermore, it is beneficial to be able to adjust the frequency relationship of such components even after manufacture. Consequently, the component structure is machined with less precision in a first manufacturing stage and is then trimmed to the desired frequency.

In the case of a cavity filter, trimming can be effected with the aid of one or more trimming screws which are inserted to a given depth in the filter, wherewith the filter frequency can be finely adjusted by adjusting the depth of penetration of the screw in the filter. This frequency adjustment must be effected accurately and also so that the filter will remain trimmed when later used in a system in operation. One simple method of endeavouring to stabilise the position of the trimming screw, and thus the set frequency, has been to lock the screw in its desired position by means of a nut. One drawback with this simple solution, however, is that there is

always a certain amount of clearance between the threads and screw of a typical screw joint. This clearance can be avoided by tightening the lock nut, although this will result, at the same time, in deviations from the set values.

There is therefore a need for a screw device which will enable microwave components to be trimmed and which requires no auxiliary devices, such as lock nuts, or that does not require the screw to be secured and locked in a desired position.

A trimming device that does not use lock nuts is described, for instance, in U. S. Re. 30 406. Trimming is effected by means of a screw which comprises two threaded segments of mutually the same dimensions and a plain segment, i. e. an unthreaded segment, which includes two radial slots and which is placed between said threaded segments. This plain and resilient element integral with the screw is intended to take up the clearance between the screw and the thread. The trimming device also includes a threaded sleeve. The screw is fitted by first compressing the screw axially so as to offset the two threaded segments relative to each other and so as to obtain a resilient screw joint. The screw is then screwed into the threaded sleeve. In a first position, the threads on the screw will be somewhat oblique due to the spring effect, but are straightened with the aid of the internal threads of the sleeve as the screw is screwed into said sleeve.

U. S. 2,542,377 A describes a self-locking screw which comprises two threaded segments of mutually the same diameter and the same pitch, and a plain, unthreaded, resilient segment disposed between said two threaded segments. The threads of the threaded segments are arranged with a negative axial offset. The plain segment is made resilient by providing circular slots symmetrically around the segment and its resiliency is further amplified by providing the screw

with an axial bore at the underside of the screw, with the slots penetrating into said bore. The screw hole that fits the screw is provided with a unitary and through-passing thread (c. f. column 3, lines 5-58).

SUMMARY OF THE INVENTION The present invention relates to a screw joint in association with components that require tuning, preferably a screw joint for trimming microwave components, which will enable accurate frequency settings to be achieved.

One object of the invention is to provide a screw joint with which a spring action can be obtained in the screw so as to eliminate clearance between screw and thread, and with which no further devices are required to achieve this function.

Another object of the invention is to provide a screw joint which can be used to trim microwave components, e. g. cavity filters, and, when used with a waveguide, the bias of which can be controlled far out towards the waveguide.

These objects are achieved in accordance with the invention with a screw joint that includes a threaded screw hole and a screw which comprises two threaded segments, preferably of two different diameters, and a plain, unthreaded, and resilient segment disposed between said threaded segments, wherewith either the in lead of the two threaded segments of the screw or the in lead of the corresponding threads in the screw hole are offset relative to each other through a given angle.

The invention is based on the realisation that screw torque is influenced by the occurring thread tolerances, wherewith a poor thread fit will result in a lower torque. The screw torque can be raised by providing the screw or the threaded

hole with two different threads, so that the offset between the in leads of respective thread systems will be proportional to the torque that can be exerted on the screw.

The characteristic features of the inventive screw arrangement will be more apparent from the accompanying Claims.

One advantage afforded by the inventive screw joint is that when used as a waveguide trimming screw, the pre-stress in the screw can be controlled towards the waveguide.

Another advantage afforded by the invention is that accurate trimming can be achieved when used as a microwave component, wherewith the position of the screw can be fixed in the absence of any clearance or play. Trimming is effected solely by means of the screw without requiring the use of other devices such as a threaded sleeve or a lock nut. This enables a trimming screw to be fitted in a space saving fashion while facilitating fitting of the screw, since the screw need only be manipulated one more time for compression of the screw.

Another advantage afforded by the inventive device is that it can be applied at two different locations, by providing the specially configured threads either on the screw or in the screw hole in the material in which the screw shall be fitted.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings.

Figures la-lc illustrate the preferred embodiment of the inventive trimming screw in two views, and also show corresponding thread arrangements in the screw hole.

Figures 2a-2c illustrate an alternative embodiment of the inventive trimming screw in two views, and also show corresponding thread arrangements in the screw hole.

Figure 3 illustrates an inventive screw joint in a contemplated component, for instance a cavity filter.

DESCRIPTION OF PREFERRED EMBODIMENTS The thread arrangement of an inventive screw joint can be applied either in the screw hole or, correspondingly, on the screw. There is achieved in both instances a screw joint in which the play or clearance between the threads of the screw hole and of the screw is eliminated so that the pre-stress of the screw can be controlled towards the waveguide.

Figures la-lc illustrate the preferred embodiment of an inventive screw joint. The screw 10 is comprised of two threaded screw segments 11 and 12 of mutually the same pitch.

In this embodiment, the diameter of the first screw segment 11 is slightly larger than the diameter of the second screw segment 12. More specifically, the smallest inner diameter of the first threaded screw segment 11 must be larger than the maximum outer diameter of the second threaded screw segment 12. Between the first screw segment 11 and the second screw segment 12 is a plain, i. e. unthreaded, resilient segment 13 whose diameter corresponds to the outer diameter of the second threaded screw segment 12. The resilient segment 13 of the illustrated embodiment includes two radially extending slots 14a and 14b which are situated on mutually opposite sides of said segment. In order to obtain a beneficial spring effect, the slots 14a and 14b are disposed one above the

other and are narrow in relation to the length of the resilient segment and slightly deeper than half the diameter of the screw in the vicinity of the resilient segment.

Provided at the top of the screw is an appropriate screwdriver slot 15 for use in screwing the screw 10 into a screw hole provided in the component 16 concerned. In the illustrated preferred embodiment, the screw hole is comprised of a first and a second threaded hole-segment 17 and 18 whose diameters are adapted to the respective diameters of the first and second screw segments 11 and 12.

In a first alternative, the thread arrangement of an inventive screw joint can preferably be applied in the screw hole into which the screw 10 shall be fitted. As mentioned in the aforegoing, the diameter of the first hole-segment 17 and the first screw segment 11 must be greater than the diameter of the second hole-segment 18 and the second screw segment 12 in the case of this application. An essential characteristic in this respect is that the points at which the lead turns of the threads of the hole-segments begin at respective upper edges of the hole-segment are offset relative to one another through a given angle. In this case, the screw segments 11 and 12 are formed with conventional threads with no axial mismatch.

In another alternative, the special thread arrangement of an inventive screw joint can be applied on the screw 10. The screw 10 is characterised by how the threads are arranged on the first and the second threaded screw segments 11 and 12.

An important characteristic feature is that the points at which the lead turns of the threads of said screw segments begin at respective bottom edges of the screw segment are offset relative to one another through a given angle. In this case, the hole segments 17 and 18 are provided with standard threads with no axial mismatch.

When turning the screw 10, the resilient segment 13 of the screw is either axially extended or axially compressed in both application alternatives.

The modus operandi of the screw is determined essentially by the geometric dimensions of the screw joint. More specifically, it is determined by the order in which the screw segments are screwed into their corresponding hole segments. In a first conceivable alternative, the narrower screw segment 12 is screwed-in first. This is achieved with a screw joint in which the combined length of the second screw segment 12 and the plain segment 13 is greater than the length of the first hole-segment 17., When the first screw segment 11 shall then be screwed in, it is necessary to first turn the screw through a certain angle before screwing of the screw into the hole segment can be commenced, whereas the other screw segment 12 can be screwed into its hole segment in a normal fashion. As a result, the slots 14a and 14b are drawn apart wherewith a pulling effect is obtained in the resilient segment 13, so that the joint will be resilient.

In a second alternative, the thicker screw segment 11 is screwed-in first. This is achieved with a screw joint in which the total length of the second screw segment 12 and the plain segment 13 is smaller than the length of the first hole segment 17. When the second screw segment 12 shall then be screwed-in, it is necessary to first turn the screw through a certain angle before screwing of the screw segment into its hole segment can be commenced, whereas the first screw segment 11 is screwed into its hole segment in a normal fashion. As a result, the slots 14a and 14b are drawn together, therewith compressing the resilient segment 13.

Figure 3 illustrates the use of the preferred embodiment illustrated in Figure 1 with a cavity filter 36 that includes a cavity 39. In the case of this example, the narrower

segments 32 and 33 of the screw are longer than the first hole segment 37, so as to obtain a spring action. When tightening the screw, the second threaded screw segment 32 is inserted in the first hole-segment 37 in a first step, whereafter screwing of this screw segment 37 in the second hole segment 38 is commenced. Screwing of the first screw segment 31 into the first hole segment 37 is commenced in a second step. The threads at either the two hole segments 37 and 38 or on the two screw segments 31 and 32 have been formed so that the points at which their respective lead turns begin are offset relative to one another through a certain angle. Consequently, it is necessary to turn the screw through this angle before screwing of the first screw segment 31 into the first hole segment 37 can be commenced.

During this further rotation of the screw, the radial slots in the plain segment 33 are drawn apart, so as to automatically obtain the desired spring effect or resiliency in the screw. The length of the second hole segment 38 must be at least sufficiently large to obtain a mechanically durable and electrically tight joint.

A screw that includes the inventive arrangement can be varied in accordance with different alternative embodiments. In the preferred embodiment, shown in Figure 1, the smallest internal diameter of the first threaded screw segment 11 is greater than the maximum external diameter of the second threaded screw segment 12. The diameter of the resilient segment 13 corresponds to the outer diameter of the second screw segment 12. On advantage with this preferred embodiment is that the lead turns of the special thread arrangement according to the invention can be applied either in respect of the threads of the screw segments or in respect of the threads of the screw hole. In accordance with Figures 2a-2c, the screw 20 may alternatively have a construction in which the first and the second threaded screw-segments 21 and 22 have mutually the same diameter. In this case, the plain

resilient segment 23 will have a diameter that corresponds to the inner diameter of the screw segments and will include two radial slots 24a and 24b as described in Figure 1. In the case of this embodiment, the inventive thread arrangement is provided solely on the screw segments 21 and 22. The corresponding screw hole in the component 26 is comprised of one single segment 27 provided with a conventional thread.

The resiliency in the screw is achieved by mismatching the points at which the lead turns of the screw segment threads begin at the bottom edge of respective screw segments through a given angle.

The screw 10 can be used beneficially for microwave component applications, e. g. with cavity filters or other equipment where trimming is required. In such applications, the screw is made of a material that has good electric conductivity and is formed as a solid body without cavities at the bottom surface of the screw, such cavities otherwise resulting in the leakage of field lines into the screw.

The spring effect obtained with the screw, this spring effect being needed in order to obtain an optimal fit between screw and thread, is achieved by providing a segment with slots that extend radially and that have a given depth. On one hand, the resilient segment must be dimensioned so that its resilience is able to take up the clearance between the threads on the screw and the threads in the screw hole, any such clearance resulting in a lower torque. On the other hand, however, excessive resilience can also result in lower torque. In view of this, the resilient segment of the preferred embodiment is provided with two slots.

The tension in the screw can be controlled by varying the offset between the lead turns of respective thread systems on the screw or in the threaded hole. This difference of degree must at least be sufficiently large to take-up the tolerance difference between screw and the threaded hole. The

difference in degree must, however, also be adapted to the resiliency of the resilient segment, so that this segment can be fully compressed by the rotation achieved through the difference in degree.

It will be understood that the invention is not restricted to the aforedescribed and illustrated embodiments thereof and that modifications can be made within the scope of the accompanying Claims.