GB2107668A

GB2107668A - Stretch wrapping apparatus

Info

Publication number: GB2107668A
Application number: GB08130915A
Authority: GB
Inventors: Peter George Dedman
Original assignee: INPAC AUTOMATION Ltd
Current assignee: INPAC AUTOMATION Ltd
Priority date: 1981-10-13
Filing date: 1981-10-13
Publication date: 1983-05-05
Also published as: GB2107668B

Abstract

Apparatus for wrapping a load, comprising an article or articles, with a stretch film includes a turntable adapted to receive the load, drive means for rotating the turntable, and a dispenser reel that holds a rolled-up film of the stretchable material. A pair of pre-stretching rollers (1, 2) is provided between the dispenser reel and the turntable through which the film takes a sinuous path to the load, the surface of the rollers (1, 2) being such that there is minimal slip between the film and the rollers. A pair of mating gears (7, 8) is associated with the rollers and has a ratio greater than unity. The gear (7) is permanently coupled to its respective roller (2) so that they rotate as one and the gear (8) is coupled to its respective roller (1) through slipping friction clutch (11, 15). The apparatus is further provided with control means (17) operable to engage the clutch (11, 15) with an intended limited torque capacity to maintain a velocity ratio of the rollers (1, 2) appropriate to a desired percentage film stretch. <IMAGE>

Description

SPECIFICATION Stretch wrapping apparatus The present invention relates to apparatus for wrapping a load with a stretch film.

Stretch wrapping apparatus is known of the kind in which stretchable film from a dispenser reel is wound onto a load, e.g. a palletised load, on a turntable. It is known to stretch the web of film between the dispenser reel and the load by applying torsional resistance over the outer surface of the dispenser reel by braking the dispenser reel shaft (U.K. Patent Specification Nos. 1,471,054 and 1,546,523). It is also known to pass the film web around successive rollers geared or independently driven to rotate at different rates, the surface of the rollers and the wrap of the film being such that there is minimal slip between the film and the rollers.The theoretical degree of stretch of the film is given by the velocity ratio of the rollers (assuming them to be of equal size) but the actual elongation is somewhat less than this due to slippage which is a function of the roller material and amount of wrap. Typically the percentage stretch is between 10% and 110%.

Deformation of the film takes place between the rollers because of their different rates of rotation but because of the short distance between them this deformation tends to be two-dimensional since the friction between the film and the rollers minimise reduction in film width or necking.

But roller stretch methods may employ a variety of different fiims at different times including low density polyethylene in cast or blown forms, ethyl vinyl acetate and polyvinyl chloride. All these films have different stretch characteristics and so require different velocity ratios between the rollers. In known systems, however, change in the velocity ratio requires manual change of the gears or introduction of a variable drive arrangement.

It has now been appreciated that for a given film and a given rate of deformation a constant tension will be present in the web of film between the two rollers. The torque experienced by the gears is proportional to this tension and if the torque that the gears can sustain is limited the velocity ratio of the rollers will automatically adjust itself to maintain the required deformation. Accordingly a friction clutch is interposed between one of the gears and its associated roller and means is provided for adjusting the force acting on the friction surfaces in order to vary the slipping torque.

The invention provides apparatus for wrapping a load with a stretch film comprising in combination: a turntable adapted to receive a load comprising an article or articles to be wrapped; drive means for rotating the turntable; a dispenser reel that holds a rolled-up film of the stretchable material; a pair of rollers between the dispenser reel and the turntable through which thefilm takes a sinuous path to the load, the surfaces of the rollers being such that there is minimal slip between the film and the rollers; a pair of mating gears associated with the input and output roller respectively and having a ratio greater than unity, one of the gears being permanently coupled to its respective roller through slipping friction clutch means; and control means operable to engage the clutch with an intended limited torque capacity to maintain a velocity ratio of the rollers appropriate to a desired percentage film stretch.

The adjustment of the force acting on the friction surfaces of the clutch (which is normally disengaged in the sense that positive steps have to be taken to engage the friction surfaces) in order to maintain an intended torque capacity may be by mechanical displacement of the springs of a mechanical clutch, by varying the pressure of a pneumatic or hydraulic clutch or by varying the field current of an electric clutch. The present arrangement has the advantage that it is simple and permits easy operator adjustment of the degree of stretch. Furthermore, in the case of fluid operated and eiectrical clutches the stretching load may be applied progressively to the film using control arrangements described in our patent specification No. 1,546,523.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure lisa diagrammatic partially sectioned view of a roller stretch unit to be interposed between the film dispenser and the turntable of a spiral wrapping machineforpalletised loads; and Figure 2 is a diagrammatic view of the rollers and associated gears showing also the film path.

Apparatus of the present kind is for the wrapping of articles which will normally be stacked on a pallet of size up to 1500mm by 1500mm and it hasa turntable adapted to receive the load and drive means by which the turntable may be rotated. A dispenser reel holds a rolled-up film of stretchable material whose leading edge is wrapped around the load on the turntable. Then as the turntable rotates film is drawn off the roll and encloses the load, the film being supplied under restraint so that the web of film being wound onto the load is under tension.

More particularly the leading edge of the film takes a path from the dispenser reel to the load sinuously through a pair of closely spaced rollers that in use are arranged to rotate so that the film is stretched between them and the rotatory load on the turntable only has to be sufficient to maintain the tension on the film web but does not produce stretching in the film.

Commonly the width of the film dispenser reel is about one third to one quarter of the height of the intended load on the pallet and the reel is reciprocated vertically by suitable means so that the load becomes spiral wrapped.

In Figures 1 and 2 film 19 from a dispenser is wrapped sinuously around an entry roller 1 and an exit roller 2 that are fixed to their respective shafts 4 and 3 and are supported by means of bearings 5 and retainer caps 6 in carriage plates 18. The two rollers are geared together by two permanently meshing gears 8 and 7 having a ratio of about 3, gear 7 being pinned to shaft 3 by means of pin 10, whereas gear 8 is free to rotate on shaft 4 a plain bearing 9 being interposed. Gear 8 and its attached parts are also free to move endwise within the constraints of bearing cap 6 on which it rests and a driving disc 11 of a friction clutch which fits on splines cut in shaft 4.

The disc 11 is maintained against a shoulder on shaft 4 by a circlip 13 that locates in a groove in the shaft.

Friction facings 12 are bonded to disc 11.

A flexible diaphragm 14 to which a pressure disc 15 is rivetted is sandwiched between gear 8 and air chamber 16. A rotary fluid coupling 17 is screened into the top of the air chamber 16 and is connected to a compressed air supply having means of pressure reguiation only or pressure and flow regulation as described in Patent Specification No. 1,546,523. In the latter case a compressed air supply is connected through a solenoid-operated valve, a flow reguiator and a reservoir to the air chamber 16, a quick exhaust valve also being provided downstream of the reservoir.Compressed air entering the chamber 16 deflects the diaphragm 14 downwards causing the pressure disc 15 to press against the driving disc facing 12 and thence causes the air chamber and gear assembly 8 and 16 to rise until the inner face of gear 8 presses against the lower friction facing 12 of the driving disc 11. The rollers 1 and 2 are now torsionally coupled and this coupling will sustain a torque proportional to the pressure in the air chamber 16. The force needed to produce a given stretch in the film passing between rollers 1 and 2 is constant for a given film.So if the torque applied to roller 1 by the clutch is set so that the required force is applied, the necessary velocity ratio between rollers 1 and 2 will be maintained since the clutch will not slip when the force applied to the film is below that needed to stretch the film and correspondingly it will slip when the force is above that needed.

Replacing the rotating fluid coupling with a screwed shaft having a spring loaded end cap would, of course, achieve the same result with the torque sustained being proportional to the compression of the spring. But the pneumatic method described above is preferred since it lends itself to automatic control and remote setting. As weli as giving the facility of variable tension it also affords automatic detensioning at the end of a wrapping cycle and controlled rate of tension build up at the start of a wrapping cycle by the method described in our Patent No. 1,546,523. But the frictional torque of the clutch could also be adjusted by hydraulic or electromagnetic method of frictional force variaion.

Claims

1. Apparatus for wrapping a load with a stretch film comprising combination: a turntable adapted to receive a load comprising an article or articles to be wrapped; drive means for rotating the turntable; a dispenser reel that holds a rolled-up film of the stretchable material; a pair of rollers between the dispenser reel and the turntable through which the film takes a sinuous path to the load, the surfaces of the rollers being such that there is minimal slip between the film and the rollers; a pair of mating gears associated with the input and output roller respectively and having a ratio greater than unity, one of the gears being permanently coupled to its respective roller so that they rotate as one and the other gear being coupled to its respective roller through slipping clutch means; and control means operable to engage the clutch with an intended limited torque capacity to maintain a velocity ratio of the rollers appropriate to a desired percentage film stretch.

2. Apparatus according to claim 1 wherein the turntable is adapted to receive a load stacked on a pallet typically of size upto 1500mm by 1500mm.

3. Apparatus according to Claim 2 wherein the width of the film on the reel is less than the height of the intended load stacked on the pallet and means reciprocates the reel vertically relative to the pallet so that the load becomes spiral wrapped.

4. Apparatus according to Claim 1,2 or 3 wherein the ratio of the mating gears is about 3.

4. Apparatus according to any preceding claim wherein a plain bearing is interposed between said other gear and a shaft of said respective roller, a clutch disc is coupled to said shaft for rotation as one therewith and means operable to press the clutch disc against said other gear so that they are torsionally coupled.

6. Apparatus according to Claim 5 wherein said means operates to increase the pressure between said clutch disc and said other gear in a progressive manner.

7. Apparatus according to Claim 5 or 6 wherein pneumatically operated means couples said clutch disc to said other gear.

8. Apparaus according to Claim 7 wherein said pneumatically operated means includes a variable flow regulatorthat controls the build up of pressure in an air chamber of a clutch actuator.

9. Apparatus according to claim 5 or 6 wherein said clutch disc is spring loaded into engagement with said other gear.

10. Apparatus according to any preceding claim wherein the clutch means is associated with the input roller.

11. Apparatus for wrapping a load with a stretch film including film stretch means substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.