The present invention relates to the maintenance of a consistent tension in material drawn off a roll of material. One field of interest is the adjustment of tension to create a uniform stretch of plastics material being drawn off a roll to be used for wrapping a farm bale.

Farm round bales, i.e. straw, or green fodder for silage making, are often wrapped in plastics material to conserve the product by keeping excess air out. "Round" bales are wrapped by drawing material from a roll of plastics material on a spindle by turning the bale about two axes at right angles to one another, as is described for example in Patent Specification NO.EPO 0 110. One side of the plastics material is usually coated with an adhesive layer so that overlapping plastic layers will adhere to one another.

A problem has been experienced in this method in that it has not been possible to regulate the tension in the plastics material between the roll and the bale. If the tension is not constant it is not possible to obtain a uniform stretch on the material resulting in unsatisfactorily wrapped bales. The problem to be solved is rendered more arduous because of the wide_j?ange of countries in which this method is used, with the associated wide range of temperatures.

U.S. Patent Specification No 4,004,750 seeks to solve this problem by mounting the spindle on a housing which is oveable on a carriage along guide rails towards and away from the direction of withdrawal under a resilient bias.

In other applications the material has been passed through a set of rollers parallel to the roll to try to create a constant tension. In this case that is not appropriate as the rollers would remove some of the adhesive coating.

It is the object of the present invention to provide an improved apparatus for meeting the problem;

According to the present invention there is provided apparatus for enabling material to be drawn off a roll in a direction of withdrawal at an optimum tension, the apparatus comprising a spindle for carrying the roll, the spindle being mounted for rotation in a housing, and a speed controller to apply a control to the rotation of the spindle, characterised in that the spindle housing is mounted to accommodate an angular displacement about an axis transverse to the direction of withdrawal of the material in response to a tension change in the material, means being provided to operate the speed controller in response to the size of the said displacement to adjust the speed of rotation of the spindle to adjust the tension in the material.

The angular displacement made by the spindle can be a pivotal one about an axis transverse to the axis of the spindle; a rotational one about an axis substantially parallel to the spindle and preferably coaxial; or can be a very small displacement caused by a deformation of the material forming or supporting the spindle housing.

The means provided to detect the angular displacement may be mechanical or electrical/electronic and any suitable control mechanism can be used to control the amount of change of speed provided by the speed controller. The speed controller may be either a motion retarder which brakes the rotation of the spindle or a motor which drives the spindle.

The operation of the speed controller may be to retard the spindle in the case of too little tension in the material by applying a brake or slowing a motor or to increase the speed of rotation of the spindle in the case of too much tension by releasing a brake or increasing the speed of a motor. It is not necessary that every displacement of the spindle housing causes operation of the speed controller. For example displacement in one direction might operate the controller while displacement in the other does not. The speed controller might also only operate on displacements above a predetermined size.

The spindle housing is preferably stationary and sustains the said displacement from a fixed position. The invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a sketch of apparatus according to a first embodiment of the invention, Figure 2 is a sketch of apparatus according to a second embodiment of the invention,

Figure 3 is a sketch of an apparatus according to a third embodiment of the invention, Figure 4 is a circuit diagram of the controller used in the apparatus of Figure 3.

The drawings of all embodiments illustrate a plastics material sheet 11 which is being drawn off a roll 12 automatically in a direction D (direction of withdrawal) , for example by the mechanical turning of a round bale (not shown) . The roll 12 is firmly held on a spindle 14 between a spiked base plate 15 which pierces the cardboard core of the roll and a screwed-on retaining ring 16. The tension in the material between the roll 12 and the bale to be wrapped causes the material to stretch, which is desirable, but the stretch needs to be consistent for good results.

The spindle 14 is supported at one end only in two spaced sets of bearings 18 and 20 carried in a cylindrical bearing housing 22. The spindle idles in the bearings, the turning motion being promoted by the drawing-off of material during use. The bearing housing 22 is firmly mounted to a mounting base bar 23 at a rigid hinge block 24 having an axis 26 which is substantially at right angles both to the spindle axis and to the direction of withdrawal D of the plastics material. It is important that this hinge has no appreciable side play so as to prevent unwanted oscillation. The bearing housing 22 is pivotable towards the direction D against a resilient bias for example a compression spring 28 such as a pneumatic spring, a coil spring or a piece of resilient material such as rubber. In the embodiment of Figure 1 the air spring 28 is pivotally mounted between the base bar 23 and the housing 22 via respective pivot blocks or brackets 27,29 about respective horizontal transverse axes 31,33 parallel to the pivotal axis 26 of the spindle.The force of the spring 28 is set according to the tension required on the material sheet 11. This force will vary according to the stretch of material required and the prevailing temperature. At its most open limit the spring 28 allows the housing 22 to pivot away from the direction D.

The device of Figure 1 is provided with a spindle rotary motion retarder comprising an actuator 30 and a braking mechanism 38. The actuator 30 is in the form of a hydraulic or air brake 31 comprising a piston 32 fixed at one end to the mounting base bar 23 and slidable at the other end into and out of a hydraulic or pneumatic chamber 34 having a feed tube 36 connected to actuate the braking mechanism 38. In this case the braking mechanism is a disc brake, fluid from the actuator 30 causing clamps 40 to move towards a disc 42 rigidly

mounted on the spindle 14 to retard the rotation. The chamber 34 is rigidly mounted to a flange 35 on the housing 22.

It will be appreciated that the actual braking mechanism can be chosen as appropriate, for example a cone brake. The same principle can be used to control an electronic retardation device or a hysteresis brake for example by using a load cell.

The operation of the device illustrated in Figure 1 will now be described:

As material is drawn off the roll a tension is set up in the material. If that tension is substantially equal to the one that is desired the spindle will remain substantially vertical. If the tension increases beyond that level the spindle is drawn to pivot forwardly which releases tension. The resilient bias in the spring 28 produces a counter force so that the pivotal movement of the spindle stops when the tension is the required value. If the circumstances change and the tension falls the spring 28 pushes the spindle upwardly until the tension returns to the desired value. If the spindle has been returned to the vertical and still the tension is too slack the spindle is urged away from the direction of withdrawal D. Pivotal movement in this direction causes the hydraulic chamber to be moved downwardly. The piston 32 is thus urged into the chamber 34 which in turn causes hydraulic fluid to operate the disc brake. The more the spindle is pivoted the more the piston is pushed into the chamber and the greater the braking effect on the disc 42.

As the material tension increases again so the spindle is drawn again to the vertical. This is an ongoing automatic operation which will maintain a substantially consistent tension in the material. The required tension can be adjusted by adjusting the strength of the bias 28.

An alternative braking system is illustrated in Figure 2. In this embodiment the rotary motion retarder comprises a disc 52, mounted to rotate and pivot with the spindle, and a brake block 53 having an arcuate portion (not shown) directed towards the disc 52 the surface of which is lined with friction material to produce a reduction in speed of rotation of the disc when in contact therewith. The arcuate portion of the block has a radius substantially equal to that of the disc so that on contact a reasonable circumference of the disc bears against the brake.The brake block 53 is supported firmly in position by a mounting block 54 secured to the base 23. The firm connection of the brake block to the mounting block may be via a universal joint to ensure that the friction material on the brake block is parallel to the disk when the two are in contact to maintain the maximum braking effect. hen the spindle is moved away from the direction of withdrawal under the resilient bias of the spring 28 to a position the far side of the vertical (to the right as illustrated in the drawing) the disc 52 engages the friction lining of the brake block 53. Further pivotal movement of the spindle will create a greater pressure between the disc and the brake block which will increase the retardation. It will be appreciated that the brake block may be shaped differently or indeed may be located so that contact is made with the top or bottom rather than the side of the disc. One possibility is to make the disc hollow to form a drum and to arrange the brake block inside the drum so that the trailing edge of the inside of the drum engages the brake rather than the leading edge of the outside of the drum.

The amount of pressure that is exerted between the disc 52 and the brake block 53 will depend upon the distance between the hinge 26 and the disc 52 and also

the angle of the spring 28. Both these variables can be adjusted in order to achieve the desired pressure.

It will be noted that the hinge block 24 has been replaced by two hinge brackets 71 firmly fitted to the base and hinged directly at 26 one to each side of the bearing housing 22. The position of the hinge is further away from the base 23 to increase the retardation force applied to the disc.

The position of the disc on the spindle can be adjusted. The spindle is extended to pass through the bearing housing to the space therebelow between the pivot brackets and the disc is fixed integrally with the spindle under the bearing housing. The disc will obviously have to be of a sufficiently small diameter to turn between the brackets. In this arrangement the friction brake will be arranged either at a lower position, opposite the disc, or alternatively as a friction pad mounted on the base plate. In the latter case on pivotal movement of the spindle, behind the vertical the disc will bear at its lower edge on the friction pad. This would further simplify the device.As a still further modification to any of the illustrated embodiments the disc may be fitted to the spindle between the bearings 18 and 20. The Embodiment of Figure 2 also incorporates a mechanism for compensating for changes in temperature. When the temperature increases the plastics material 11 expands and so the retarding force needs to be adjusted accordingly.The spring 28 is replaced by a vertically operating air spring 61 firmly attached to the base 23. The piston 63 of the spring includes a horizontal portion such as a plate. The action of the piston 63 is transferred to an arm 65 by means of a roller bearing 66 in engagement with the horizontal portion of the piston. The arm 65 is in two parts and tapers from a wider end of a first part 65a which is integrally mounted with the

bearing casing 22 to a narrower end of a second part 65b which carries the roller bearing. The upper face of the arm is angled to the horizontal to direct the pushing force on the bearing housing above the pivot 26 and the two parts are separated by a horizontal hinge 67. Spaced from the hinge 67 the two parts of the arm are connected by a horizontally arranged strip 69 of high expansion material such as a high expansion plastics material.

When the temperature increases the strip 69 expands so slightly raising the part of the arm 65 which is attached to the piston 63. This in turn reduces the air spring pressure.

The device of Figure 2 operates in the same way as that of Figure 1 in principle with the exception of the operation of the braking mechanism and the resilient bias. It will be appreciated that the second embodiment is a simpler one requiring fewer components and is therefore cheaper to produce and easier to maintain.

In the embodiment of Figures 3 and.4 the roll of material is fitted to a spindle 14 and the spindle mounted in the housing 22 in the same way as in the previous embodiments. However in this embodiment the hinge block supporting the spindle housing is replaced by a flexible and resilient I-shaped pillar 75 the axis of which is coaxial with the spindle axis. The material and thickness of the pillar must be such that it can accommodate a degree of deflection and be sufficiently resilient to return to its original state or position. EN19 steel and HT30 Aluminium would be suitable. Tension applied to the spindle via the amount of pull on the material being drawn off the roll results in a leaning or bending of the spindle about an axis in the neck of the pillar caused by a temporary deformation of the material of the pillar under the load. The displacement of the spindle is only a few thousandths of an inch but a load cell 76 in the form of a bridge of strain gauges

(not shown) attached to the pillar can pick up the change and output a load signal 76A proportional to the tension in the material.

The spindle is braked by a speed controller in the form of an electronic rotary retarder 77 located adjacent the spindle in or near the spindle housing 22. An electromagnetic powder clutch brake has been found to be effective but other suitable brakes could be used for example an electromechanical friction brake or a hysteresis brake. The rotary retarder, or brake as it will hereafter be known, is activated or driven by current passing through a coil 78 the size of which current is controlled from an electronic control circuit 77. When the apparatus is set up the current supplied to the brake is at its maximum which results in the brake being full on. As the current decreases so the braking force decreases. A gear box 81 is incorporated in the drive to the motion retarder to increase the speed of rotation of the clutch brake to increase. the efficiency of the brake. For example the rotation of a friction clutch at 350 rpm can be increased to more than 1500 rpm in this way.

The load cell will output a load signal 76A which is amplified by amplifier 82 the output of which is a signal Ei . The output of the amplifier 82 is electrically connected to the negative input of a direct-current differential amplifier or operational amplifier 83 powered with a voltage ±A which may typically be ±12V or ±24V. The positive input of amplifier 83 is electrically connected to the slider of a potentiometer 84 to receive a positive set-point voltage p representing a critically high tension at which the material will snap. This set point is adjustable as required for different operating conditions. A temperature sensitive resistance 85 is included in the potentiometer, thereby providing a

degree of temperature compensation by displacing the set point.

The signal output E2 from amplifier 83 E2 = Gain(p_ -Ei) with Gain = R/r where R is the resistance of a feed-back resistor and r is the resistance of a series-connected resistor between the output of the amplifier 82 and the input of the amplifier 83. This output signal E2 is passed on to the positive input of a direct-current differential or operational amplifier 88 via a low-pass RC filter. The power supply to amplifier 88 is -A to the negative terminal and + (A-Z) to the positive terminal. The negative input of the amplifier 88 is connected to the slider of a potentiometer 87 in a variable feedback circuit to receive a signal f_. The current output from amplifier 88 will be a function of E2 -f. i.e. F(E2 - f) . The amplifier 88 tracks the amplifier 83 and makes adjustments accordingly. The lower voltage at the positive terminal which is delivered via a zener diode (not shown) enables the implementation of the control to balance. When there is little or no tension Ei =0, so p>Eι , so a large positive output E2 appears from amplifier 83. Amplifier 88 gives a positive output signal F(E2 -f) which causes current to flow through its output resistor 89.

At the output side of the control circuit a Darlington transistor pair 91 feeds current from a positive battery terminal +B which is usually of the same value as the voltage A, in this case 12V or 24V. This transistor pair 91 is permanently switched on, the current being adjusted by the signal F(E2-f) . When the output signal from the amplifier 88 is positive, current flows through the Darlington pair 91 and through the coil 78 which activates the brake. The amount of current flowing through the brake coil will increase with an increase in the signal from the amplifier 88. With no

load on the film material being drawn off the roll, the brake will be hard on and a large voltage will appear at the live end of the resistor R2 in series with the brake coil 78. This switches on a bypass transistor 95 electrically connected between the live end of the coil 78 and the base of the first transistor 91a of the Darlington pair with the result that the Darlington pair is deprived of current. This continues until equilibrium is reached. The feed back setting determines how much current flows through the brake coil at equilibrium. The by-pass transistor 95 also acts as short circuit protection.

When a load is applied to the material is such that Ei>p_, then E2 becomes negative. Assuming that f_ does not immediately become negative, then the F(E2-f) will be negative. That will deprive the Darlington pair 91 of drive current, thereby preventing current passing to the coil. The brake will be released almost instantly, the plastics material sheet will be free to be drawn out, the load cell signal will signify no load and the brake will be reapplied.

In normal circumstances Ei < p_ therefore E2 will be positive and proportional to the safety margin between the present tension and the set value p. As the tension Ei increases so the value E2 decreases, so (E2-f) decreases. Less current is therefore available for driving the brake, so f decreases, thereby increasing (E2-f_) . This increases the current through the coil, the brake is applied harder, f. increases etc to maintain an equilibrium.

Capacitors and resistors are added where necessary to give the preferred values and damp the circuit.

The feed back loop to the amplifier 88 and the Darlington pair has its equilibrium point set by the potentiometer providing the feedback signal f_. This loop

acts to keep the current flowing through the brake tracking the signal E2.

The controls-circuit described above has other applications in situations where it is necessary to κ reduce one variable as a result in the increase of a second variable, - for example in the control of central heating systems. It may be elected where relevant to gang the potentiometers.

The control system may be replaced by a suitable analog or digital control system.

A further modification can be made by replacing the brake or motion retarder in any of the described embodiments with a motor which drives the spindle. The adjustment in speed of rotation of the spindle would then take place by altering the speed of the motor. With an electrical or electronic control system this would be accomplished by controlling the current applied to the motor. Thus for example the current in the coil 91 could be used to drive a motor.