PRE-STRETCHING WRAPPING FILM

Abstract: Apparatus for pre-stretching wrapping film (33) includes an assembly of rotating arms (35) mounted on a fixed shaft (22). The arms (35) are driven by a wheel (39) which is rotated about an axis (49) off-set from the axis of the fixed shaft (22). Rollers (46) connect the wheel (39) to an adjustable guide ring (34) and rollers (47) run in a channel (48) in the lower face of the lower brace (29) of the arm (35). As the wheel (39) is rotated about its axis, the arms (25) rotate about fixed shaft (22) so that the included angle between adjacent arms (25) increases and decreases cyclically so that the linear speed of the film (33) will be increased so as to stretch the film as it passes around the assembly of rotating arms.

Description: PRE-STRETCHING WRAPPING FILM FIELD OF INVENTION This invention relates to the pre-stretching of polyethylene and other stretchable wrapping film. BACKGROUND ART Tensioning of low density polyethylene film by pre- stretching to a pre-determined degree is well known in the art of stretch wrapping. Such pre-stretching is commonly achieved by passing the film successively over two rollers rotating at different speeds. United States patent specifications 4,302,920; 4,387,548; 4,387,552; 4,336,679; 4,429,514 and 4,712,686 are indicative of the two roller approach of the prior art. DISCLOSURE OF INVENTION It is an object of this invention to provide an improved arrangement for pre-stretching wrapping film which utilises a single rotating member. According to the invention there is provided an apparatus for pre-stretching wrapping film comprising a plurality of spaced apart arms rotatably mounted on a fixed shaft, drive means adapted to be rotated about an axis off¬ set from the axis of the fixed shaft and means connecting the drive means to the arms whereby rotation of the drive means about the off-set axis causes the included angle between adjacent arms to increase and decrease cyclically as the drive means is rotated. The drive means may be rotated by action of the film being wrapped around a load at a driven wrapping station. In other forms of the invention, the drive means may be rotated by a constant speed motor or by a constant torque device. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings in which:- Fig. 1 is a perspective view of an apparatus for pre-stretching wrapping film according to one embodiment of the invention, Fig. 2 is an enlarged, partly broken away perspective view of the apparatus shown in Fig. 1, Fig. 3 is an enlarged, partly broken away perspective view of the rotating arm assembly of the apparatus shown in Fig. 1, and, Fig. 4 is a plan view of the rotating arm assembly shown in Fig. 3. BEST MODE FOR CARRYING OUT THE INVENTION The pre-stretching apparatus 10 shown in the drawings is mounted on a vertical mast 11 on a base 12 and can be moved up and down by the endless chain 13 which is driven by motor 14. Although the pre-stretching apparatus 10 is shown in a vertical disposition, it is to be appreciated that it could assume whatever disposition is appropriate for the wrapping task to be done. The pre-stretching apparatus 10 includes a top plate 20 and a lower plate 21 which support a fixed shaft 22 (see Fig. 2) held in position by collar 23 above the top plate 20 and a collar (not shown) beneath the lower plate 21. A plurality of arms or frame members 25 are rotatably mounted on the fixed shaft 22 by upper and lower bearing arms 26. Although six arms 25 are shown in the drawings, the number of arms may be varied to suit any particular task. In this instance, the arms 25 are vertically disposed and comprise an inner upright member 27 and an outer upright member 28 interconnected by lower, mid and upper braces 29, 30 and 31 respectively. On the outer face of the outer upright member 28 there is a high friction pad 32 (such as polyurethane) adapted to engage the wrapping film 33. An adjustable guide ring 34 is mounted on the lower plate 21 by pivot means 35 and a bolt 36 in adjustment slot 37. The guide ring 34 has a groove 38 in its inner circumferential face. Above the guide ring 34 there is a driven wheel 39 which, in this instance, is engaged by chain 40 driven by a sprochet 41 on shaft 42. Drive motor 43 is coupled to a slipping clutch 44 mounted on the lower end of shaft 42. The driven wheel 39 (which constitutes the drive means for the assembly of rotating arms 25) may be driven in a number of ways. The slipping clutch arrangement described above provides a constant torque which is less than the torque required to provide the predetermined pre-stretching of the film 33 appropriate to the film material and to the wrapping task. The film 33 is wrapped around a load mounted on a driven wrapping station. Rotation of the wrapping station wraps the film 33 around the load and pulls the film through the pre-stretching apparatus 10. The slipping clutch 44 may be replaced by a constant torque motor to provide the torque assistance necessary to complement that provided by the action of the rotating station on the film. In some circumstances it is appropriate to replace the constant torque drive with a constant speed motor so that the wrapping action does not apply additional force to pull the film through the pre-stretching apparatus. The speed of the constant speed motor may, of course, be varied to suit a particular wrapping task. In other circumstances, it may be appropriate to dispense with any drive to the wheel 39 other than the force applied to the film 33 by the action of the rotating station on the film. That is to say, the rotation of the arms 25 is achieved solely by the pulling action of the film as it is wrapped around the load. Spaced around the driven wheel 39 are six roller assemblies 45 (one for each frame member or arm 25). Each roller assembly 45 has a lower roller 46 which runs in the groove 38 of the guide ring 34 and an upper roller 47 which runs in a channel 48 in the lower face of the lower brace 29 of frame members 25. In this instance, the upper and lower rollers are mounted on a common shaft 24 but this need not be so. Although there need be an upper roller 47 for each arm 25, there need not be the same number of lower rollers 46. As can be seen in Fig 4, the axis 49 of driven wheel 39 is displaced from the centreline 50 of the fixed shaft 22. As the driven wheel 39 is rotated about its axis 49, the frame members 25 rotate about fixed shaft 22 so that the included angle between adjacent frame members 25 increases and decreases cyclically and the angular velocity of a particular arm 25 will vary during its rotation around fixed shaft 22. The wrapping film 33 on roll 51 is fed around unpowered guide roller 52 and engages the pads 32 of the arms 25 where the angular spacing between adjacent frame members 25 is a minimum and leaves the rotating arm assembly where the angular spacing is a maximum. After the film 33 leaves the rotating assembly it passes around unpowered guide rollers 53, 54 and 55 and to the wrapping station (not shown). The location of the unpowered guide rollers may be varied as required by a particular wrapping task. The degree of pre-stretching can be varied by changing the spacing between the axis 49 of driven wheel 39 and the centreline 50 of the fixed shaft 22. This is achieved by pivoting the guide ring 34 about its pivot means 35. The degree of pre-stretching to be effected by the apparatus will depend upon the nature of the film and the nature of the wrapping task. For example, fragile loads cannot be used to provide the necessary force to pull the film around the arm assembly. In cases where the arms are driven by an external means such as a constant speed or constant torque device, the arms 25 will provide a primary stretching of the film and a secondary stretching will be effected when the peripheral speed of the film created by the action of the wrapping station is greater than the peripheral speed of the film 33 as it leaves the assembly of rotating arms 25. Various modifications may be made in details of design and construction without departing from the scope and ambit of the invention.

Claims: CLAIMS 1. Apparatus for pre-stretching wrapping film comprising plurality of spaced apart arms rotatably mounted on a fixed shaft, drive means adapted to be rotated about an axis off¬ set from the axis of the fixed shaft and means connecting the drive means to the arms whereby rotation of the drive means about the off-set axis causes the included angle between adjacent arms to increase and decrease cyclically as the drive means is rotated. 2. Apparatus according to claim 1 wherein the drive means includes a driven wheel coupled to a powered drive. 3. Apparatus according to claim 2 wherein the connecting means includes roller means slidably engaged with each arm and adapted to move radially with respect to the arm as the drive means is rotated. 4. Apparatus according to claim 3 wherein the driven wheel is mounted above a guide wheel and the roller means includes a lower roller adapted to engage a groove in the guide wheel and an upper roller engaged with the arm. 5. Apparatus according to any one of the preceding claims and including a feed roller adapted to direct the film to the arms where the spacing between adjacent arms is a minimum and a discharge roller around which the film passes as it leaves the adjacent arms at maximum angular spacing. 6. Apparatus according to any one of the preceding claims and further including means for adjusting the distance between the axis of the drive means and the axis of the fixed shaft. 7. Apparatus according to any one of the preceding claims wherein the film is wrapped around a load at a rotatable wrapping station after it leave the pre-stretching apparatus and wherein rotation of the drive means is effected, at least in part, by the action of the film as it is wrapped around the load. 8. Apparatus according to any one of the preceding claims and further including means for rotating the drive means. 9. Apparatus according to claim 8 wherein the means for rotating the drive means is a constant torque device. 10. Apparatus according to claim 8 wherein the means for rotating the drive means is a constant speed device.

Inventors: DOWN IAN FRANCIS, DOWN, IAN, FRANCIS
Applicants: GD ENG PTY LTD, G.D. ENGINEERING PTY. LIMITED
Application year: 1989
Main classification: B29C55
IPC codes: B29C55