The invention relates to a machine for wrapping loads with stretchable film.

The vertical and top surface of loads can also be wrapped using the machine.

There is a great variety of load stretch wrapping machines known in the practice.

Using the machine described in the German publication of registration number DE 3633680 the load is put on a turntable and a shrinkable cap made of a shrinkable film is put on the load.

The load wrapped with the shrinkable cap is rotated by the turntable and a burner nozzle fastened on a vertically moving carriage shrinks the upper and lower parts of the shrinkable cap circularly and the lateral part along a screw line performing a programmed movement.

The application of the shrinkable cap provides appropriate protection to the load owing to the wrapping the upper part of the load.

The disadvantage of the machine is that the shrink-hood must be made previously and put onto the load. The manufacturing of the cap requires manual labor, in addition the specific demand of film is great, as well as the energy requirement of the shrinking is also great enough.

In the last decade the significantly cost effective wrapping with stretchable (ductile) film is applied in more and more increasing degree because of the above mentioned disadvantage of the wrapping with shrinkable film.

The specific demand of film of the wrapping with stretchable film weight approximately one fifth part of that of the shrink hood and the indispensable high energy requirement of the shrinking does not arise. The common distinctive features both of the simplest hand- operated machines and the fully automated ones applied for the stretch wrapping with stretchable film are that they have turntables and a carriage applicable for the vertical movement of the film.

The above mentioned structural units of the machines make possible that the pre- stretched film wraps the load along a screw line as result of the rotating movement of the load and the vertical movement of the film. The stretch of the film, the overlap and the number of film windings can be changed depending on the properties of the load and the requirements of fastening. The patent description of registration number of HUP 210302 discloses a very advantageously applicable machine enabling the stretch wrapping of loads with stretchable film also on the top surface of the load with the film applied for the wrapping of lateral surface.

The solution ensures the perfect protection of the goods through continuity of the film, rendering unnecessary applying a shrink hood in this way.

The essence of the machine according to the solution is that the film applied for stretch wrapping the lateral surface of the load by winding up along a screw line is led above the load and tilted from vertical position to horizontal one during the rotation of the load. The film is then tilted back to the vertical position during a descending movement and the wrapping of the upper part is made in this way.

The machine according to the patent has a base frame, a turntable mounted in it and a mast fastened on the base frame. A carriage vertically moving the film is mounted on the mentioned mast and bushing is supporting the carriage.

A shaft is inserted into the bushing and a reel of film is connected to the one end and the tilting actuator is connected to the other end of the shaft.

A sensor for the vertical (basic) position and a sensor for the horizontal position are mounted on the base frame, triggering flags mounted on the turntable associated to the corners of the load, as operating elements controlling the tilting.

The disadvantage of the machine is that the stretch tension value can attain only the nearest digital value according to the step values, so it cannot be followed proportionally, consequently the tension of the film cannot be set precisely.

As the tension continuously changes, the film can occasionally be torn by the sharp corners of a load. There is another problem that the film can break to inside the corners of the most loaded lower layers of the loads packed in cardboard and corrugated paper boxes.

It is a widespread solution to sense the tension of the film by leading the film around a swing axle loaded with a spring. Using this solution, when the tension of the film between the load and the pre-stretching unit increases, it forces to deviate the swingarm loaded with spring and there are digital sensors (usually inductive ones) arranged along the circular arc described by the shaft, and the sensors transmit signals of discrete values about the tension of the film to the control system, which responds to them by changing the braking force of the film.

According to another solution the swingarm deviating along a circular arc under influence of the tension of the film is pulled or pushed by one or more springs with quasi constant force - between the two end positions of the deviation - to the direction corresponding to the smaller tension of the film. The problem with this solution is that the pulling or pushing force of the springs is not linear along the length of pulling or pushing, consequently a pre-stretching unit with springs has also not linear but progressive behavior.

A possible consequence of this operation is that a machine combined with digital control is amenable to oscillation and sets tension of the film in wide range. Consequently an excessive tension can arise, which can result in tearing or loosening. It can even cause the winding up of the film onto the rubber covered cylinders, and force to stop the packing process. The pre-stretching of film with loading by spring has also another problem, that it cannot be adjusted, repeated and measured.

We set ourselves an aim to develop an improved machine eliminating the above mentioned disadvantages.

We reached this aim in such a way that we completed the machine with a tiltable film tension setting unit, which has pre-stretching rollers provided with a pre-stretching motor connected to the frame of a film carrying and pre-stretching unit, a leading roller containing an arm provided with a roller, a tension setting pneumatic cylinder, which has a piston provided with magnet, with an analogous transducer of displacement, a pressure regulator and a frequency changer.

The invention relates to an machine for stretch wrapping vertical and top surface of loads with stretchable film having a base frame, a turntable mounted in it and a mast fastened on the base frame.

A carriage vertically moving the film is mounted on the mentioned mast and bushing is supporting the carriage.

A shaft is inserted into the bushing and a reel of film is connected to the one end and the tilting actuator is connected to the other end of the shaft.

A sensor for the vertical (basic) position and a sensor for the horizontal position are mounted on the base frame, triggering flags mounted on the turntable associated to the corners of the load, as operating elements controlling the tilting. The characteristics of the machine are that it has a swinging film tension setting unit connected to the other end of the pin, which has pre-stretching rollers provided with a pre-stretching motor and fastened to the frame of the film carrying and pre-stretching unit, a leading roller containing an arm provided with a roller, a tension setting pneumatic cylinder having a piston with magnet and an analogous displacement transducer, furthermore a pressure regulator and a frequency changer, where the film winded off from the film reel is led between pre-stretching rollers connected one to another with a transmission and one of them is driven by a pre-stretching motor, then the film is led around a leading roller having an arm moving around a centre of rotation and provided with a roller, this arm is connected to the piston provided with magnet and arranged in the film tension setting pneumatic working cylinder, the latter is provided with an analogous displacement transducer, the piston provided with magnet is connected to the pressure regulator and the frequency changer and the frequency changer is connected also to the pre-stretching motor.

The turntable of the machine is driven with a turntable driving motor.

The piston provided with magnet of the machine is moved between two end positions in relation to the set reference value through the pressure regulator.

The machine according to the invention is shown in the Figures 1 and 2.

The Figure 1 shows the schematic representation of the machine, while the Figure 2 shows the regulating elements of the machine and their connections.

The Figure 1 shows an advantageous embodiment of the machine. The solution covers among others also those machines having more than two pre-stretching rollers and their drive can be either identical to the one shown or different from it.

The base frame 1, the turntable 2 mounted in it and the load 13 is positioned on the turntable 2 are shown in the Figure 1.

The carriage 4 vertically moving the film reel 7 is fastened to the mast 3 mounted on the base frame 1 and bushing 5 are formed on the carriage 4.

A shaft 6 is inserted into bushing 5, a tilting actuator 8 is connected to the one end and a film tension setting unit 22 is connected to the other end of it.

The home position sensor 9 and the horizontal position sensor 10 are arranged on the base frame 1, and (tilting controlling) triggering flags 12 A, 12B, 12C, 12D are controlling the tilting associated to the corners of the load 14, as well as a home position sensor 9 are mounted on the turntable 2.

The swinging film tension setting unit 22 fastened to the frame of the film holding and pre-stretching unit 23 has pre-stretching rollers 7A provided with a pre-stretching motor 20, a leading roller 7B containing an arm provided with a roller 7C, a tension setting pneumatic cylinder 16 having an analogous displacement transducer 17 and supplied with a piston provided with magnet 16 A, a pressure regulator 18 and a frequency changer The film winded off from the film reel 7 is led between the pre-stretching rollers 7A connected one to another with a transmission 19 and one of them is driven with the pre- stretching motor 20, then the film is led around a leading roller 7B having an arm moving around a centre of rotation and the arm is provided with a roller 7C.

The arm provided with the roller 7C is connected to a piston provided with magnet 16A and the piston provided with magnet 16A is arranged in a tension setting pneumatic cylinder 16 provided with an analogous displacement transducer 17.

The piston provided with magnet 16A is connected to a pressure regulator 18 and a frequency changer 15. The frequency changer 15 is connected also to the pre-stretching motor 20 of the pre-stretching rollers 7A.

The turntable 2 is driven with a turntable driving motor 21.

The piston provided with magnet 16A is moved by a pressure regulator 18 between two end positions in relation to the set reference value.

We applied advantageously the product SM6 of the company Bosch Rexroth or the one named SMAT of the company Festo as analogous displacement transducer 17.

The transmission 19 can be of any usual type, advantageously one with chain, V-belt, gears, timing belt etc.

The Figure 2 shows the tension setting pneumatic cylinder 16 having the piston provided with magnet 16A and the analogous displacement transducer 17 connected to the pressure regulator 18.

The machine contains a tension setting pneumatic cylinder 16 equipped with a piston provided with magnet 16 A, charged on one side and a pressure regulator 18 connected to it, applicable for setting the tension of the film in relation to an targeted reference value. The tension setting pneumatic cylinder 16 is provided with an analogous displacement transducer 17 measuring the position of the piston provided with magnet 16A. The transducer transforms the position to an electric signal characterizing it and regulates with that the film pre-stretching motor 20 through a frequency changer 15. In the case where the tensile force characterizing the tension of the film is less than the original reference value set by the tension setting pneumatic cylinder 16 and the pressure regulator 18 belonging to it, the piston provided with magnet 16A moves towards the one end position.

The rotary speed of the pre-stretching motor 20 is then diminished; it wraps the load 13 with less film during a unit of time until that film quantity during the unit of time becomes equal to or less than the quantity sufficient to wrap the load 13. The load 13 "pulls" the film because of that, it becomes stretched and the force characterizing its tension moves the arm provided with roller 7C towards the position characterizing the tersioned film. The piston provided with magnet 16A of the tension setting pneumatic cylinder 16 will be moved towards the other end position at the same time. Consequently, the analogous displacement transducer 17 sends accelerating signal to the frequency changer 15, which sends accelerating signal to the pre-stretching motor 20.

The current position of the piston provided with magnet 16A is regulated by the resultant of the pressure set and the pulling force arising on the film. The pulling or pushing force of the tension setting pneumatic cylinder 16 can be set in the whole length of path, the pressure can be measured, so the stretching force can be repeated.

The horizontal axis of the film tension setting unit 22 encloses an angle of twentyfive degrees with the radial direction of the main rotating movement, so the upper edge of the film turned to horizontal on the side of the real or imaginary centre of the main rotating movement is not elongated in such a great extent that it could lose the ability of shrinking back.

The advantages of the machine according to the invention are as follows:

It follows the angular displacement directly proportional to the film tension between the load 13 and the pre-stretching rollers 7A with an analogous signal of a contactless displacement transducer 17, which can accept not only a small number of digital values but even several hundreds of individual values. The measurement of the tension is more accurate, the regulating action is quicker and the difference of the tensions to be compensated becomes smaller.

The pre-stretching motor responds quicker, more accurately and with smaller redundancy under the influence of the increased tension

The required value of the film tension can be better approached and kept by means of the regulating action of increased accuracy during the whole duration of the wrapping.

The swinging film tension setting unit 22, which can be operated with motor-driven pre- stretching and at several pre-sets of pressure, adjustable in analogous way can be a separate complement of the machine wrapping with stretchable film, which can be built on all types of the machines wrapping with stretchable film, e. g. to the constructions with rotating arm or rotating ring, and the wrapping with film can be solved on a machine wrapping with stretchable film of any type with a relatively small cost using the stretchable film used for wrapping the lateral surfaces of the load 13. List of symbols

1 base frame

2 turntable

3 mast

4 carriage

5 bushing

6 shaft

7 film reel

7A pre-stretching rollers

7B leading roller

7C arm provided with roller

8 tilting actuator

9 home position sensor

10 horizontal position sensor

11 home position actuating element

12A, B, C, D (tilting controlling) triggering flags

13 load

14 corners of the load

15 frequency changer

16 tension setting pneumatic cylinder

16A piston provided with magnet

17 analogous displacement transducer

18 pressure regulator

19 transmission

20 pre-stretching motor

21 turntable driving motor

22 film tension setting unit

23 frame of the film holding and pre-stretching unit