In the papermaking industry, it is generally known that paper to be converted into a consumer product such as paper towels, bath tissue, facial tissue, and the like is initially manufactured and wound into large rolls. By way of example only, these rolls, commonly known as parent rolls, may be on the order of 10 feet in diameter and 100 inches across and generally comprise a suitable paper wound on a core. In the usual case, a paper converting facility will have on hand a sufficient inventory of parent rolls to be able to meet the expected demand for the paper conversion as the paper product(s) are being manufactured.
Because of the soft nature of the paper used to manufacture paper towels, bath tissue, facial tissue, and the like, it is common for parent rolls to become out-of-round. Not only the soft nature of the paper, but also the physical size of the parent rolls, the length of time during which the parent rolls are stored, and the fact that roll grabbers used to transport parent rolls grab them about their circumference can contribute to this problem. As a result, by the time many parent rolls are placed on an unwind stand they have changed from the desired cylindrical shape to an out-of-round shape.
In extreme cases, the parent rolls can become oblong or generally egg-shaped. But, even when the parent roll is are only slightly out-of-round, there are considerable problems. In an ideal case with a perfectly round parent roll, the feed rate of a web material coming off of a rotating parent roll can be equal to the driving speed of a surface driven parent roll. However, with an out-of-round parent roll the feed rate can likely vary from the driving speed of a surface drive parent roll depending upon the radius at the web takeoff point at any moment in time.
With regard to the foregoing, it will be appreciated that the described condition assumes that the rotational speed of the parent roll remains substantially constant throughout any particular rotational cycle of the parent roll.
If the rotational speed remains substantially constant, the feed rate of a web material coming off of an out-of-round parent roll will necessarily vary during any particular rotational cycle depending upon the degree to which the parent roll is out-of-round. In practice, however, parent rolls are surface driven which means that if the radius at the drive point changes, the rotational speed can also change generally causing variations in the feed rate. Since the paper converting equipment downstream of the unwind stand is generally designed to operate based upon the assumption that the feed rate of a web material coming off of a rotating parent roll will always be equal to the driving speed of the parent roll, there are problems created by web tension spikes and slackening.
While a tension control system is typically associated with the equipment used in a paper converting facility, the rotational speed and the takeoff point radius can be constantly changing in nearly every case. At least to some extent, this change is unaccounted for by typical tension control systems. It can be dependent upon the degree to which the parent roll is out-of-round and can result in web feed rate variations and corresponding tension spikes and slackening.
With an out-of-round parent roll, the instantaneous feed rate of the web material can be dependent upon the relationship at any point in time of the radius at the drive point and the radius at the web takeoff point. Generally and theoretically, where the out-of-round parent roll is generally oblong or egg-shaped, there will be two generally diametrically opposed points where the radius of the roll is greatest. These two points will be spaced approximately 90° from the corresponding generally diametrically opposed points where the radius of a roll is smallest. However, it is known that out-of-round parent rolls may not be perfectly oblong or elliptical but, rather, they may assume a somewhat flattened condition resembling a flat tire, or an oblong or egg-shape, or any other out-of-round shape depending upon many different factors.
Regardless of the exact shape of the parent roll, at least one point in the rotation of the parent roll exists where the relationship between the web take off point radius and the parent roll drive point radius that results in the minimum feed rate of paper to the line. At this point, the web tension can spike since the feed rate of the web material is at a minimum and less than what is expected by the paper converting equipment downstream of the unwind stand. Similarly, there can exist at least one point in the rotation of the parent roll where the relationship between the web take off point radius and the parent roll drive point radius results in the maximum feed rate of paper to the line. At this point, the web tension can slacken since the feed rate of the web material can be at a maximum and more than what is expected by the paper converting equipment downstream of the unwind stand. Since neither condition is conducive to efficiently operating paper converting equipment for manufacturing paper products such as paper towels, bath tissue and the like, and a spike in the web tension can even result in a break in the web material requiring a paper converting line to be shut down, there clearly is a need to overcome this problem.
In particular, the fact that out-of-round parent rolls create variable web feed rates and corresponding web tension spikes and web tension slackening has required that the unwind stand and associated paper converting equipment operating downstream thereof be run at a slower speed in many instances thereby creating an adverse impact on manufacturing efficiency.
While various efforts have been made in the past to overcome one or more of the foregoing problems with out-of-round parent rolls, there has remained a need to successfully address the problems presented by web feed rate variations and corresponding web tension spikes and web tension slackening.