The invention relates to stretch wrapping a load with packaging material and more particularly to controlling the tension of the packaging material during the wrapping process.
Stretch wrapping devices have been used to wrap a web of tensioned packaging material around the outside of a load by providing relative rotation between the load and a packaging material dispenser. In some instances, the dispenser includes prestretch rollers which rotate at different speeds to stretch the web before it is wrapped on the load, and a dancer roller which is used to help control the dispensing of the film and sense a film web break to stop the dispensing of the film.
The stretch wrapping process begins by loading a roll of stretch wrap packaging material, such as a stretch wrap film web, in the dispenser. After loading the roll, the leading end of the film web is threaded through the prestretch rollers and around the dancer roller and then secured to the load or to a clamp.
The tension in the film web between the web dispenser and the load can be provided in many different ways. For example, it can be provided by a brake which restrains film roll as the web is dispensed. It also can be provided by a roller between the film roll and the load which is connected to a brake. It further can be provided by a prestretch mechanism which, in addition to prestretching the film, restrains the film between the dispenser and the load to tension the web.
The tension imported to the film web by the prestretch mechanism can be provided solely by a mechanical interconnection between the upstream and downstream prestretch rollers. The tension also can be provided, regulated or reduced by a speed or torque control which uses an external power source to control the speed or torque of the prestretch rollers. Examples of such arrangements are shown in U.S. Pat. Nos. 4,302,920, 4,418,510, 4,387,548, and 4,866,909 which are incorporated herein by reference.
In a number of instances, it is desirable to disengage the mechanical linkage between the prestretch rollers or disengage the prestretch rollers from the external power source. For example, it is useful when the web is threaded through the prestretch rollers.
In some instances, such as with unstable loads, it may be desirable to dispense the first circumferential wrap of the web around the load at substantially less tension in the web than subsequent circumferential wraps around the load. This enables the load to be secured in position before wrapping it with a highly tensioned web which is required to effectively contain the load. Such highly tensioned web would otherwise undesirably permit the load to be displaced if it had not previously been wrapped with the web while at a low tension.
In stretch wrapping operations where the film web dispenser is stationary, it is easy to provide mechanisms to electronically control the components of the film web dispenser because the electronic circuitry between power sources, control mechanisms and the film web dispenser does not cross between two moving units. However, when the film web dispenser is rotated around a load, it is necessary to use commutators to provide electrical communication between components in the film web dispenser and control mechanisms or power sources which do not rotate with the film web dispenser.
Previous attempts to disengage the prestretch rollers or the external power source, or control the tension of the film web in a moving web dispenser have not been found to be completely satisfactory. In some wrapping machines, the mechanical drive between the two prestretch rollers, and the mechanical drive between the prestretch rollers and an external power source previously had been coupled by respective wrap spring clutches. These clutches were engaged and disengaged by a mechanical linkage which included moveable pawls that cooperated with ratchet collars on the clutches.
In one instance, the clutches were engaged and disengaged by a mechanical linkage actuated by a spring loaded dancer roller. So long as the film web pressed on the dancer roller, the clutches would stay engaged. This kept the prestretch rollers engaged and interconnected to prestretch the film web. It also kept the prestretch rollers engaged to an external power source. The power source regulated the tension of the film web between the web dispenser and the load, generally by reducing that tension during normal wrapping operations.
If the film web broke and ceased pressing on the dancer roller, the dancer roller would move because it was spring loaded. Such movement would be transmitted through a linkage to disengage the clutches. This caused the prestretch rollers to mechanically disengage from each other and from the external power source to prevent undesirable consequences such as winding the broken film web around the prestretch rollers. The use of a spring loaded dancer roller to engage and disengage the tensioning system also had the disadvantage of requiring an undesirably high minimal level of tension on the film to restrain the dancer roller against its spring loading during operation.
Another arrangement which mechanically engaged and disengaged clutch mechanisms on the web dispenser used an arm which mechanically reached in to toggle a control mechanism on the orbiting web dispenser at a predetermined point in its orbit. The control mechanism could then be toggled back to reengage the clutch mechanism when the web dispenser again reached that point in its orbit.
Yet another arrangement to engage and disengage clutches included a spring loaded follower roller which extended from the orbiting web dispenser. The follower roller was engageable with the inner radial surface of a curved plate mounted on the support frame of the stretch wrapping apparatus at one segment of the dispenser's orbit. When the plate was radially pivoted into a ramped position, it engaged the spring loaded follower roller and disengaged the clutch mechanisms on the web dispenser. The clutch mechanisms would reengage after the follower roller on the web dispenser passed the plate or the plate was pivoted out of position.
These earlier arrangements had the disadvantage of being unable to use a mechanical device to immediately and flexibly control the tension on the film web.