In a similar device, which is known from British patent GB-827,271, the second element is rotated, as a result of which the first and the third element are moved in opposite axial directions. The second element is provided with internal and external screw threads, which have been formed in a tube by means of for example a metal-removing technique.

One drawback of such a method of manufacturing and internally and externally threaded tube is that it is a relatively labour- intensive process, whereby furthermore a relatively great deal of material must be discharged.

The object of the invention is to provide a device which comprises an internally and externally threaded element which is relatively easy to manufacture.

This objective is accomplished with the device according to the invention in that said second element is tubular, whereby the internal and external screw threads have been provided simultaneously by hydromechanical deformation of a metal tube.

By hydromechanical deformation of a tube a metal tubular element with internal and external screw thread can be obtained in a relatively simple and inexpensive way.

When a hydromechanical deformation operation is carried out, a metal tube is placed into a mould, whereby said mould is provided with a profile which corresponds with the desired external shape of the second element.

The ends of the tube are closed by movable, cylindrical pistons. Then a fluid, such as oil or water, is introduced into the tube under pressure, whereby the tube is deformed by the fluid pressure. During said deformation the cylindrical pistons are moved in a direction towards each other, whereby the pistons exert an axial pressure force on the tube.

The axial pressure force ensures a proper sealing action between the tube and the pistons, whilst the tube may also be upset, if desired. The axial pressure force and the fluid pressure press the tube against the walls of the mould, and the tube takes a shape which corresponds with the profile. It is also possible to use a deformable, non-compressible material such as rubber or sand, whereby said material is pressed into the tube by means of the pistons, as a result of which the tube is deformed.

External screw thread is thereby formed on the outer side of the tube thereby, whilst simultaneously internal screw thread is formed on the inner side of the tube.

It is noted that from Japanese patent application JP-A- 61-066625 a method for manufacturing an externally threaded element of plastic material is known, wherein a heated tube of plastic material is transformed by means of air pressure. One drawback of such a method is that it cannot be used for transforming metal tubes.

One embodiment of the device according to the invention is characterized in that balls are present between the internal screw thread of one element and the external screw thread of the third element being in engagement therewith.

The balls provide a proper bearing of the elements with respect to each other.

The invention will be explained in more detail with reference to the drawings, in which: Figure 1 shows a diagrammatic longitudinal section of a device according to the invention; Figure 2 shows a detail of the embodiment shown in Figure 1; Figure 3 shows a longitudinal section of a second embodiment of the device according to the invention; Figure 4 shows a cross-section of the device shown in Figure 3; Figure 5 shows a device for manufacturing a tubular element of the devices shown in Figures 2 and 3; Figure 6 shows a diagrammatic longitudinal section of another embodiment of the device according to the invention; Figure 7 shows a vertically adjustable treating table, which is provided with a device according to the invention.

Like parts are numbered alike in the figures.

Figure 1 shows a device 1 according to the invention, which comprises five tubular, concentrically positioned elements 2, 3, 4, 5, 6. The device furthermore comprises a supporting platform 7 positioned under said tubular elements, and a motor 9 disposed near the central axis 8 of said tubular elements. The first element 2 located near central axis 8 comprises a coupling 10 located near motor 9, which is in engagement with an outgoing shaft (not shown) of motor 9. First element 2 furthermore comprises external screw thread, which is in engagement with internal screw thread on a second element 3. Second element 3 is furthermore provided with external screw thread, which is in engagement with the internal screw thread of third element 4. Third element 4 is in engagement, via its external screw thread, with the internal screw thread of fourth element 5, which is in turn in engagement with the internal screw thread of fifth element 6 via its external screw thread. One end 11 of fifth element 6 remote from supporting platform 7 is connected to a vertically adjustable part 12 of an apparatus (not shown). Plate 12 is locked against rotation about central axis 8 by means which are known per se. Said means may for example comprise a lineair guide. It is also possible for the apparatus to comprise a number of vertically adjustable devices 1, which are each connected to plate 12. In this way plate 12 is automatically prevented from rotating.

In order to move plate 12 axially in a direction indicated by arrow P1, motor 9 drives the outgoing shaft of the motor, as a result of which first tubular element 2 is rotated in a direction indicated by arrow P2 via coupling 10. As a result of the rotation of first element 2, second element 3, which is in engagement with the external screw thread of first element 2 via its internal screw thread, will either rotate together with first element 2 in the direction indicated by arrow P2, or be translated in the direction indicated by arrow P1 together with third element 4, fourth element 5 and fifth element 6, which is locked against rotation. If second element 3 rotates in the direction indicated by arrow P2 together with first element 2, the third element may either rotate in the direction indicated by arrow P2 together with the second element, or be translated in the direction indicated by arrow P1 together with fourth element 5 and fifth element 6, which is locked against rotation. If first, second and third elements 2, 3, 4 are rotated in the direction indicated

by arrow P2, fourth element 5 may either be rotated in the direction indicated by arrow P2 as well, or be translated in the direction indicated by arrow P1. If the fourth element is rotated in the direction indicated by arrow P2 together with first, second and third elements 2, 3, 4, the fifth element 6, which is locked against rotation, will be translated in the direction indicated by arrow P1. Elements 2, 3, 4, 5, 6 can be translated in the direction indicated by arrow P1 until the bottom end of a tubular element is positioned near the upper end of the element present therein. Device 1 is provided with locking means, which prevent the tubular elements from becoming detached from each other. Said locking means may for example be comprised of a cap attached to one end of a tubular element, or of a termination of the screw thread on the tubular element.

The maximum mounting height that can be realised by means of device 1 is substantially five times the height of a single element. The minimum mounting height of device 1 substantially equals once the length of a tubular element. This makes it possible to give the device 1 according to the invention a relatively large adjusting range with a relatively small mounting height.

Figure 2 shows a detail of the device shown in Figure 1, in which three elements 2, 3, 4 are illustrated. Each element comprises a tube 13, whose cylindrical wall is deformed such that each tube 13 comprises internal screw thread 14 and external screw thread 15. The diameters of tubes 13 of elements 2, 3, 4 are such that the external screw thread of first element 2 is in engagement with the internal screw thread of element 3, and the external screw thread of element 3 is in engagement with the internal screw thread of third element 4.

Figures 3 and 4 show a longitudinal section and across- section respectively of another embodiment of the device shown in Figure 1. Tubular elements 2, 3, 4, 5 are each provided with internal screw thread 14 and external screw thread 15, whereby the internal and external screw threads have been provided simultaneously in the tubular element through deformation of the tubes. Balls 16 are present between the external screw thread of one element and the internal screw thread of a surrounding element. Said balls are kept a desired distance apart by means of a bearing cage 17. In the device rotary motion of one element is turned into translatory motion in axial direction P1 of another element via rolling

contacts with balls 16. Balls 16 provide a proper guidance among the various elements 2, 3, 4, 5. In addition to that balls 16 compensate for dimensional tolerances between the internal screw thread of one element and the external screw thread of an element co-operating therewith. By providing at least three balls distributed in regularly spaced-apart relationship over one turn and likewise providing three balls distributed over another turn, a device is obtained which is capable of taking up both axial loads and bending loads.

Figure 5 shows a mould 20 for manufacturing an internally and externally threaded element 2, 3, 4, 5, 6. The mould comprises two mould halves 21, which is provided with a profile 22 which corresponds with a profile to be formed on the outer side of a tube 13.

Mould 20 furthermore comprises two axially movable mandrels 23 positio ned on either side of profile 22. Each mandrel 23 is provided with a fluid intake 24, which is in communication with a fluid supplying device (not shown) via a flexible line (not shown). Mandrels 23 can be moved in a direction towards each other by means of a shifting device (not shown).

When a tubular element is to be formed an elongated metal tube 13 is placed into a mould half 21, after which the mandrels are placed in abutment with the ends of tube 13, and the other mould half 21 is placed over tube 13.

A fluid, for example oil or water, is supplied under pressure to the space 25 bounded by tube 13 and mandrels 23 via fluid intake 24. Simultaneously therewith mandrels 23 are moved in a direction towards each other by force, whereby an axial force is exerted on the ends of tube 13. The omnilateral pressure exerted by the fluid and the axial forces cause tube 13 to deform, whereby the wall of tube 13 comes to abut against profile 22. The side of tube 13 abutting against profile 22 makes up the external screw thread, whilst the side of tube 13 facing towards space 25 makes up the internal screw thread. The two screw threads are simultaneously formed in tube 13 as a result of the above-described hydromechanical deformation.

Figure 6 shows another embodiment of a device 30 according to the invention, which is provided with a first, externally threaded tube 31, two co-axial tubes 32, 33, which concentrically surround tube 31, and two co-axial tubes 34, 35, which concentrically surround tubes 32, 33. Tube 31 is provided with left-handed screw thread near an upper end and with right-handed screw thread near a lower end. The tubular elements 32, 34 surrounding the lower end of tubular element 31 are

likewise provided with right-handed screw thread, whilst the tubular elements 33, 35 surrounding the upper end of tubular element 31 are provided with left-handed screw thread. Tubular elements 32, 33 are threaded both internally and externally.

Rotation of the inner tubular element 31 causes tubular elements 33, 35 to move in the direction indicated by arrow P3, whilst tubular elements 32, 34 are moved in a direction P4 opposed to the direction indicated by arrow P3. Each tubular element is provided near its ends with a locking arrangement, which prevents said tubular elements from becoming detached from each other.

Figure 7 shows a treating table 40 according to the invention, which comprises a number of vertically adjustable legs 41, and a treating table 42 supported by legs 41. Treating table 42 is provided near its ends with a pivoted head support 43 and a pivoted foot support 44. Legs 41 are each provided with a device 1 according to the invention, by means of which treating table 42 can be adjusted for height.

The tubular elements are preferably made of a metal, such as stainless steel or aluminium.

It is also possible to use the device according to the invention as a coupling shaft, whereby two apparatuses are coupled by means of a device according to the invention, which apparatuses are first moved a selected distance apart until the maximum length is reached. Rotation of one apparatus will then result in rotation of the second apparatus as well.

The device according to the invention may be used for adjusting tables, chairs, apparatuses, etc. for height. It is also possible to use the device in place of a cylinder known per se comprising a piston which can be moved therein by means of oil.

The second element may also be used in other devices which require the use of an internally and externally threaded tube.