WO1998024613A1

WO1998024613A1 - Welding device for plastic pipes

Info

Publication number: WO1998024613A1
Application number: PCT/CH1996/000428
Authority: WO
Inventors: Rindo Marcassoli
Original assignee: Rindo Marcassoli
Priority date: 1996-12-04
Filing date: 1996-12-04
Publication date: 1998-06-11

Abstract

The proposed welding device is designed for situations in which one of the two plastic pipe sections to be joined is partly grouted in and thus already secured. The device therefore has a pipe clamp (22). The welding device is secured by means of a base plate (1) to the surface from which the grouted plastic pipe section (5) projects. The base plate (1) has a pivotable rail (2) on which a slide (10) is movably secured. A guide rod (18) is secured to the slide (10) via a rotating joint and a tilting joint (16). A pipe clamp (22) is secured to said guide rod (18) so as to be longitudinally movable. The guide rod (18) is also fitted with a yoke (21), the centre of which is connected to the guide rod (18) so as to move and rotate, and bears a heating plate (26) on one arm (25) and a motor-driven grinding disc (30) on the other.

Description

Welding device for plastic tubes

The invention relates to a welding device for connecting plastic pipes according to the preamble of patent claim 1.

In buildings, especially in industrial buildings, plastic tubes, which can serve different purposes, are built in ceilings or walls. Recesses are formed in the concrete to accommodate the pipes. The pipes used are usually made of thermoplastic.

The installation of pipes that run over the edge of the room is particularly difficult. The problems that arise are explained in more detail below with the aid of a pipeline which extends vertically in a wall to the edge of the room between the wall and the ceiling and from there runs horizontally through the ceiling.

When building a room, the walls are first bricked up. A vertical recess in the wall, which will later receive a pipe, is achieved by shaping the formwork accordingly. When the walls are finished, the ceiling is built. In a first step, you create the formwork. At the top of the recess in the wall, this casing consists of two horizontally guided beams, which are attached to the side walls of the recess. These beams are supported outside the recess by steel girders. The two beams carry a rectangular piece of formwork board within the wall recess. This piece of board is nailed to the beam. The size and shape of this piece of board correspond essentially to the cross section of the wall recess. The lower reinforcement is then produced.

The plumber then cuts a circular hole out of the formwork board piece. In this hole he sticks a leg of a plastic pipe elbow. The second leg of this pipe elbow extends above the lower ceiling reinforcement at right angles to the wall. The upper ceiling reinforcement is then produced. It will Pipe contra-angles are often tilted by the construction workers. Finally, the ceiling is concreted. After the concrete has dried out, the perforated boarding piece must be removed from the wall recess again. The mason usually uses a crowbar for this. He presses the piece of board down with this crowbar. At the beginning you need a lot of effort, because the nails with which the board piece is attached have to be loosened first. When the board piece is loosened, it is pulled down along the protruding pipe stub. The pipe stump is often damaged. It gets scratches and cracks or is compressed so that its cross-sectional shape changes.

To continue the pipeline, a plastic tube is welded onto the vertically protruding pipe stub. However, welding is not possible if the pipe stub is deformed and has an oval cross section instead of a round one. The section of the pipe stub immediately adjacent to the ceiling is normally not deformed because it is sufficiently stabilized by the concrete. In order to make the connection possible, the deformed, lower section of the tube end is sawn off. According to the prior art, a hand saw is used for this. However, using a hand saw poses problems because there is not enough space in the wall cut-out for unobstructed sawing. You always hit the hand saw on the back of the wall recess. The sawing work is therefore tedious and time-consuming.

As is known, manual welding devices or automatic welding devices are used for connecting plastic pipe pieces by welding.

The manual welding devices essentially consist of a round aluminum heating plate with a handle. The heating plate is heated electrically. A thermostat keeps its temperature constant at 220 ° C. So that the plastic of the pipes does not stick to the heating plate, it is coated with Teflon.

The welding comprises the following work steps: First, the end faces of the two plastic tubes to be welded are ground so that they are free of oxides. After that they are made by hand pressed against the two surfaces of the heating plate until a sufficiently thick layer of plastic has melted at the pipe ends. The heating plate is then removed and the heated ends of the two tubes are pressed together. In order for a stable connection to be created, a certain minimum pressure must be used. This pressure can no longer be achieved by hand in pipes with a diameter of over 63 mm.

For welding pipes with larger diameters, therefore, automatic welding devices must be used, which can generate the necessary force.

The automatic welding devices known from the prior art for connecting plastic pipes normally comprise two clamping devices for fastening the two plastic pipes to be connected and a heating device for heating the pipe ends. The clamping devices usually consist of two jaws, between which the plastic tube is clamped.

Patent specification CH 684 525 (publication date October 4, 1994) describes, for example, an automatic welding device in which one of two pipe clamping devices is attached to a sliding carriage. The other clamping device is stationary. The plate-shaped heating device can be pushed in between the two clamping devices.

Patent specification WO 92/09419 (publication date June 11, 1992) proposes an automatic welding device in which two clamping devices are fastened so as to be displaceable relative to one another on a rail. The clamping devices are each driven by a hydraulic cylinder. The heating device is composed of two plates. Pressure sensors are arranged between the two plates, which measure the pressure with which the pipe ends are pressed against the heating device. The measured pressure values are used to control the hydraulic cylinders. In addition to welding devices with plate-shaped heating devices, welding devices with cuffed or ring-shaped heating devices are also known. Patent specification WO 90/00112 (publication date January 11, 1990) proposes an automatic welding device whose heating device consists of electrically heatable jaws. The two jaws are pivotally connected to one another. When closed, the two jaws encompass the end sections of the plastic tube to be connected.

The heater is attached to a frame. On both sides of the heating device, a pipe clamping device is axially adjustable. The object of patent specification WO 90/11884 (published on October 18, 1990) is an automatic welding device with two clamping devices, which fix the two plastic tubes to be connected abutting one another on the end face. The heating device encompasses the end sections of the two plastic tubes and heats them from the outside.

At the end of the heating phase and at the beginning of the cooling phase, the end sections of the plastic tube are held with a vacuum on the inner circumference of the heating device. This can prevent a bulge from forming on the inside of the weld seam.

As is known, the formation of beads can also be prevented with the aid of support bodies which are inserted into the interior of the tube and the surface of which lies against the inside of the tube wall.

To prevent bulges on the inside of the weld seam, patent specification WO 88/06966 (publication date September 22, 1988) also proposes to provide undercuts on the end faces of the pipes to be connected. This creates space within the thickness of the tube wall for the softened plastic that is moved when the two plastic tubes are pressed together. The softened plastic is absorbed by the undercuts and therefore does not come out on the inside of the pipe wall.

Special welding devices and processes have been developed for welding composite pipes. Such welding devices and methods can be found in the two patents EP 0 531 750 (publication date March 17, 1993) and CH 685 814 (Date of publication 13.10.1995). They are unsuitable for single-layer plastic tubes.

The known automatic welding devices are unsuitable for welding plastic eyes in recesses in a wall, in the ceiling or in the floor of a building, because of the limited space available. So far, therefore, in these situations, even for pipes with a larger diameter than 63 mm, manual welding devices were used and the plastic tube was compressed by hand after heating, although in this way the contact pressure prescribed by the manufacturer cannot be achieved. The consequence of this was poor weld seams, which broke even at low loads. This sometimes caused considerable structural damage.

The object of the invention is therefore to create an automatic welding device which makes it possible to connect plastic tubes which are arranged in recesses in building walls, ceilings and floors with a sufficiently high pressure by welding.

The task is solved with the aid of the training features of patent claim 1. Advantageous developments of the invention are the subject of the dependent claims.

The proposed welding device is intended for situations in which one of the two pieces of plastic pipe to be connected is partially walled in and is thereby already fixed. No clamping device is consequently necessary for this plastic tube piece. The proposed welding device therefore has only one pipe clamping device.

The welding device is fastened with a base plate to the surface from which the walled-in piece of plastic pipe protrudes. The base plate carries a pivotable rail on which a slide is slidably attached. A guide rod is connected to the slide via a swivel joint and a tilt joint. On this guide rod, a tube tensioning device is fastened so that it can move in the longitudinal direction of the guide rod. The guide rod also carries a yoke. The yoke is in the middle the guide rod slidably and rotatably connected. It has a heating plate on one arm and a motor-driven grinding wheel on the other arm.

To grind the stub of the pipe, the yoke arm, which carries the grinding wheel, is pivoted into the recess in the wall, ceiling or floor and the yoke is moved in the direction of the base plate. To heat the pipe ends, first swivel the yoke arm with the heating plate into the recess. Then the yoke together with the pipe clamping device is pressed in the direction of the base plate until both the end face of the pipe stub and the end face of the free pipe piece lie against the heating plate. As soon as the plastic has melted sufficiently, the free piece of pipe is lifted from the heating plate and this from the pipe stub. The arm with the heating plate is then swung out of the recess. Finally, the pipe clamping device is moved again in the direction of the base plate and the heated end face of the free pipe section is pressed against the heated end face of the pipe stub.

The invention is described below using exemplary embodiments, inter alia in the drawings. Show it:

Figure 1 is a side view of a welding device according to the invention, which is attached to a building ceiling.

Fig. 2 is a plan view of a base plate of a welding machine ge ass Fig. 1;

3a shows a top view of the carriage of a welding device according to FIG. 1;

3b shows a side view of a slide according to FIG. 3a;

3c shows a cross section through a carriage according to FIG. 3a;

Fig. 4a is a plan view of an arm of a pipe clamping device of the welding device according to Fig. 1 with a clamped thin pipe and

ERSATZBWTUREG & 26) Fig. 4b is a plan view of an arm of a pipe clamping device ge ass Fig. 3a with a clamped thick pipe.

The embodiment shown in FIG. 1 of the proposed welding device is attached to the ceiling 52 of a room. A base plate 1 is used for fastening. The shape of the base plate 1 essentially corresponds to an acute-angled circular sector (cf. FIG. 2). The edge areas of the base plate 1 are raised. The base plate 1 is fastened to the ceiling 52 with a single screw 3. A dowel is used for fastening. To allow the screw 3 to pass through, the base plate is provided with a continuous round hole 4 near its pointed corner 6. A rail 2 is arranged on that side of the base plate 1 which faces away from the ceiling 52. The rail 2 is pivotally attached to the base plate 1. The pivot point is located near the sharp corner 6 of the base plate 1 on its bisector. The raised edge area 7 on the curved edge of the base plate 1 has a continuous groove. That end of the rail 2 which faces away from the pivot point projects into this groove and is guided through it. In addition, the rail 2 has a threaded bore on this end face, into which a locking screw 8 is screwed. With this locking screw 8, the rail 2 can be locked in any angular position. The rail 2 has an essentially U-shaped cross section (cf. FIG. 3c). The two legs of the rail 2 each have an inwardly projecting rib 9 at their free end.

A slide 10 is fastened axially displaceably to the rail 2. The carriage 10 has an essentially guader-shaped base body 11 (cf. FIGS. 3a to 3c). A web 12 projects from each of the two longitudinal narrow sides of the base body 11. The two webs 12 have an L-shaped cross section and extend over the entire length of the carriage 10. Furthermore, each narrow side has a longitudinal groove 13 directly next to the web 12. These longitudinal grooves 13 each receive a rib 9 on the rail 2. At the same time, the two webs 12 each have a free end of one leg of the rail 2, so that the carriage 10 is in Moved longitudinally of the rail 2, but can not be lifted off the rail 2.

The carriage 10 can be locked in any longitudinal position on the rail 2 with two locking screws 53. To accommodate the locking screws 53, the base body 11 is provided in the vicinity of its two end faces with a continuous threaded bore (see FIG. 3a).

In the center of that side of the slide 10 which faces away from the rail 2, a connecting piece 14 is rotatably attached. The connecting piece 14 can be locked in any rotational position with the aid of a set screw 15 (cf. FIGS. 3a and 3b). That section of the connecting piece 14 which faces away from the slide 10 is designed as a fork 17 of a joint 16. This fork joint 16 connects a guide rod 18 to the connecting piece 14. One end of the guide rod has a plate-shaped projection 19. The outer end face of this projection 19 is rounded. The projection 19 is inserted between the two arms of the joint fork 17. The axis of rotation of the fork joint 16 is formed by a cylindrical bolt 20 which penetrates the two arms of the joint fork 17 and the projection 19 of the guide rod 18. For this purpose, the fork arms and the projection 19 are provided with suitable, mutually aligned bores. The fork joint 16 can also be equipped with a set screw (not shown), which makes it possible to lock the joint 16 in any angular position.

A yoke 21, a tube tensioning device 22 and a pushing device 23 are fastened to the guide rod 18 (cf. FIG. 1).

The yoke 21 has a central guide part 24 and two arms 25 projecting laterally from this guide part 24. So that the guide part 24 can be pushed onto the guide rod 18, it is provided with a continuous, cylindrical bore, the diameter of which corresponds to the diameter of the guide rod 18. The yoke 21 is displaceable along the guide rod 18 and rotatable about the guide rod 18. The two arms 25 of the yoke 21 preferably lie in one plane. They are angled at an obtuse angle in the direction of the base plate 1. Each arm 25 preferably has a lockable one Tilt joint 54, so that the outer section of each arm 25 can be folded down.

A preferably round heating plate 26 is attached to the outer end of one arm 25. It runs parallel to the base plate 1. The heating plate 26 is heated by an electrical resistance heater. It can be made of aluminum and coated with Teflon. The power supply to the electrical resistance heating is carried out with the aid of a cable 27, which runs inside the arm 25 to the guide part 24 and exits the yoke 21 there.

A grinding device 28 is attached to the outer end of the other arm 25. It consists of a carrier plate 29 on which a grinding wheel 30 is rotatably attached. Both the carrier plate 29 and the grinding wheel 30 run parallel to the base plate 1. The grinding wheel 30 is arranged on that side of the carrier plate 29 which faces the base plate 1. It is attached to the carrier plate 29 with the aid of a shaft 31, which projects at a right angle from the center of the grinding wheel 30. This shaft 31 runs through a bore in the carrier plate 29. The projecting section of the shaft 31 can be clamped in a drill chuck of an electric drill 41. The drilling machine 41 drives the grinding wheel 30. It is also possible to drive the grinding wheel indirectly with the aid of a V-belt (not shown). In this case, the shaft 31 carries a belt pulley on which the V-belt runs.

The tube tensioning device 22 consists of a guide tube 32 and two holding arms 33. The inside diameter of the guide tube 32 corresponds to the outside diameter of the guide rod 18. The guide tube 32 is pushed onto the guide rod 18. It is further away from the base plate 1 than the guide part 24 of the yoke 21. A compression spring 34 is arranged between the guide part 24 of the yoke 21 and the guide tube 32 of the tube clamping device 22. It is also pushed onto the guide rod 18. The two holding arms 33 protrude laterally from the guide tube 32. Each holding arm 33 is divided into a rod-shaped inner section 35 and a fork-shaped outer section 36 (cf. FIGS. 4a and 4b). The rod-shaped sections 35 of the two Holding arms 33 run at right angles to the longitudinal axis of the guide tube 32 and lie in one plane. They are adjustable in length. This can be achieved in that the rod-shaped section 35 of each holding arm 33 is composed of a tubular part 37 and a rod-shaped part 38. The tubular part 37 is adjacent to the guide tube 32. Its inside diameter corresponds to the outside diameter of the rod-shaped part 38. The rod-shaped part 38 is inserted into the tubular part 37 and can be displaced in its longitudinal direction. It can be locked in any longitudinal position with the aid of a locking screw or clamp (not shown). In order to ensure that the extension length of the two holding arms 33 is always the same, the rod-shaped parts 38 of the two holding arms 33 can be connected to one another via a cross strut 39. The fork-shaped section 36 adjoins the outer end of the rod-shaped parts 38. The plastic pipe piece 42 to be fastened is positioned between the fork arms 40 and clamped with a steel band 43 per holding arm 33. So that plastic pipe pieces 42 with different diameters can be fixed, the angle between the fork arms 40 is smaller in an inner section than in an outer section. In addition, each fork arm 40 has two fastening devices for the steel band 43. A fastening device 44 lies at the outer end of the inner arm section. It is used when a small diameter plastic pipe piece 42 is to be fixed. The other fastening device 44 lies at the outer end of the outer arm section. It is used when a plastic pipe piece 42 with a larger diameter is to be fixed. The fastening devices 44 of a fork arm 40 of each holding arm 33 are designed as cable ties which fix the steel band 43 in a tensioned state.

The pushing device 23 consists of a push tube 45 and a drive. The inside diameter of this push tube 45 corresponds to the diameter of the guide rod 18. The push tube 45 is also pushed onto the guide rod 18. Eε is further away from the base plate 1 than the guide tube 32 of the tube clamping device 22 and the guide part 24 of the yoke 21. A compression spring 46 is also arranged between the guide tube 32 and the push tube 45. It is pushed onto the guide rod 18. On its outside the push tube 45 has a toothed rack 47 which runs parallel to its longitudinal axis. A gear 48 of the drive engages in the toothed rack 47. An angle piece 49 is attached to the lower end face of the guide rod 18, to which the toothed wheel 48 is rotatably attached. Daε gear 48 can be driven with a hand crank or an electric motor (not shown). When eε is rotated, the push tube 45 is displaced along the guide rod 18. Furthermore, a measuring rod 50 is attached to the outer surface of the push tube 45. This measuring rod 50 runs parallel to the longitudinal axis of the push tube 45 and protrudes beyond that end face of the push tube 45 which faces the base plate 1. A guide plate 51 protrudes laterally from the guide tube 32 of the tube tensioning device 22. This guide plate 51 has a through hole through which the measuring rod 50 projects. A measuring scale is attached to the measuring rod 50. If the thrust tube 45 is pushed in the direction of the base plate 1 by turning the toothed wheel 48, then it exerts a force on the compression spring 46 lying between the thrust tube 45 and the guide tube 32. The greater this force, the shorter this compression spring 46 and the smaller the distance between the push tube 45 and the guide tube 32. The reduction in distance is measured with the measuring rod 50. The measuring scale is so calibrated that the force corresponding to the respective distance can be read directly from it. The guide plate 51 serves as a pointer.

The proposed welding device is used in the following manner:

In a first step, the welding device is attached to the wall 52 from the recess in the wall in such a way that the pointed corner 6 of the fastening plate 1 faces the pipe stub 5 and the imaginary extension of the bisector of the fastening plate 1 running through the pointed corner 6 is as precise as possible runs through the middle of the Rohrstummelε 5. If there are deviations here, these can be adjusted the angular position of the rail 2 can be compensated. Furthermore, the distance between the mounting plate 1 and the pipe stub 5 must be less than about 30% of the length of the mounting plate 1.

In a further step, the end face of the pipe stub 5 is ground off. For this, the yoke arm 25, which carries the grinding wheel 30, is pivoted into the recess. The grinding wheel 30 is centered with respect to the pipe stub 5 by adjusting the angular length of the rail 2 and by moving the slide 10 on this rail 2. The electric drill 41 is then put into operation. The rotating grinding wheel 30 is pressed against the end face of the tube stub 5 by pressing the push tube 45 in the direction of the base plate 1 by rotating the gear 48. The pressure is transmitted to the yoke 21 via the two compression springs 34, 46 and the guide tube 32 of the tube clamping device 22.

When the tube stub 5 is ground to the desired extent, the thrust tube 45 is moved away from the base plate 1 again by turning the gear 48 and then the yoke 21 is pulled down, whereby the grinding wheel 30 is lifted from the end face of the tube stub 5. Subsequently, the drilling machine 41 is switched off and the yoke arm 25 with the grinding wheel 30 is pivoted out of the recess.

In the next step, the actual welding of the plastic pipe section 42 to the pipe stub 5 takes place. For this purpose, the yoke arm 25 that carries the heating plate 26 is pivoted into the recess and centered with respect to the pipe stub 5. The continuing plastic tube piece 42 is then fixed in the tube tensioning device 22. The plastic pipe section 42 is also centered with respect to the pipe stub 5 by changing the length of the holding arms 33 of the pipe clamping device 22 accordingly. When the heating plate 26 has reached its operating temperature, push the push tube 45 again by turning the toothed wheel 47 in the direction of the base plate 1. The pressure is applied to the guide tube 32 of the tube tensioning device 22 and the guide part 24 of the yoke via the two compression springs 34, 46 21 transferred. This has the effect that the heating plate 26 against the end of the tube stub 5 and the upper end of the continuing plastic tube 42 against the Heating plate 26 is pressed. The pressure is maintained until a sufficiently thick plastic layer is melted on the end faces of both tubes 5, 42. Then the push tube 45 is again moved away from the base plate 1 so that the yoke arm 25 with the heating plate 26 can be pivoted out of the recess again. Immediately afterwards, the heated end face of the plastic tube piece 42 is pressed against the heated end face of the tube stub 5 by pressing the push tube 45 again in the direction of the base plate 1. The measuring rod 50 can be used to determine whether the pressure exerted is sufficiently high.

After the weld seam has cooled sufficiently, the pipe tensioning device 22 can be released and the base plate 1 can be removed from the ceiling 52.

The proposed welding device enables for the first time the production of good quality weld seams in the narrow spatial conditions of a recess in a wall, in the ceiling or in the floor of a room. In addition, the working time required for welding two plastic pipes is considerably shorter when using the proposed welding device than when using manual welding devices.

Claims

1. A welding device for connecting plastic tubes (5, 42) to a tube tensioning device (22) for holding a plastic tube (5) to be welded to a stationary tube (5), with a heating device (26) that interposes the tubes to be connected (5, 42) can be pushed, and with movement devices (23, 2, 10) for moving the pipe clamping device and / or the heating device to and away from the fixed plastic tube, characterized in that the welding device has a fastening device (1) for fastening the Welding device near the stationary, fixed plastic tube (5) on which the movement devices (23, 2, 10) are attached.

2. Welding device according to claim 1, characterized in that the movement devices comprise a pushing device (23) for moving the plastic pipe to be welded on (42) and a pushing and rotating device (2, 10) for moving the heating device (26), the The pushing device and the pushing and rotating device are attached to a guide rod (18), on which the heating device is also attached via a supporting device (21) and the tube tensioning device (22).

3. Welding device according to claim 2, characterized in that the sliding and rotating device (2, 10) consists of a rail (2) pivotally attached to the base plate (1) and a slide (2) axially displaceably fastened to the rail (2) 10) is formed.

4. Welding device according to claim 3, characterized in that the guide rod (18) is fastened to the slide (10) via a rotary connection (14, 15, 16, 17, 19, 20) which can be locked in any angular position.

5. Welding device according to one of claims 2 to 4, characterized in that the supporting device is a yoke (21) with a central guide part (24) and laterally projecting arms (25) from this central guide part, the yoke around the guide rod (18th ) is rotatable and displaceable in the longitudinal direction thereof, and wherein the heating device (26) and a further provided grinding device (28) for grinding the surface of the locally fixed plastic ear (5) are arranged on respective arms of the yoke.

6. Welding device according to claim 5, characterized in that the heating device is a heating plate (26) and the grinding device (28) consists of a rotatable grinding wheel (30) arranged on a carrier plate (29), the grinding wheel for example by means of an electric drill ( 31, 41) is rotated.

7. Welding device according to one of claims 2 to 6, characterized in that the tube tensioning device (22) has one or more holding arms (33) consisting of a rod-shaped section (35) and a fork-shaped outer section

(36) for receiving the plastic pipe to be welded on

(42), with the plastic tube accommodated in the tube tensioning device by means of a steel band

(43) is held in the fork-shaped section of the holding arm in such a way that a holding arm is in each case designed as a rod-shaped part (38) which extends over a tubular part

(37) is attached to a guide tube (32) encompassing the guide rod (18), and that in the case of several holding arms, these holding arms are each connected via a transverse strut (39).

8. Welding device according to one of claims 5 to 7, characterized in that a compression spring (34) pushed onto the guide rod (18) is provided between the guide part (24) of the yoke (21) and the guide tube (32) of the tube tensioning device (22) is.

9. Welding device according to one of claims 2 to 8, characterized in that the pushing device (23) consists of a push tube (45) pushed onto the guide rod (18) and a drive, between the guide tube (32) and the push tube (45 ) a compression spring (46) pushed onto the guide rod (18) is arranged.

10. Welding device according to claim 9, characterized in that the thrust tube (45) has on its outside a toothed rack (47) which runs parallel to its longitudinal axis and in which a gearwheel (48) of the drive engages, on one of it the lower end of the guide rod (18) attached angle piece (49) is rotatably attached.

11. Welding device according to one of claims 1 to 10, characterized in that a measuring device (50) for measuring the force exerted when the plastic pipe (42) to be welded is moved in the direction of the fixed plastic tube (5) is also provided.

12. Welding device according to claim 11, if it refers back to one of claims 9 or 10, characterized in that the measuring device is formed by means of a measuring rod (50) attached to the outer surface of the push tube (45) and provided with a measuring scale, which is parallel runs to the longitudinal axis of the thrust tube and extends beyond its end face through a hole in a guide plate (51) serving as a measuring pointer and laterally projecting from the guide tube (32) of the tube tensioning device (22) in the direction of the fastening device (1).