The present invention involves an improved chopper for chopping continuous or very long loose items such as fiber, fiber strands, yarn, wire, string, ribbon, tape and the like by pulling the item(s) into the chopper while the loose items are held tightly against the surface of a rotating backup roll with a rotating idler roll biased against the backup roll and carrying the item(s) on into a nip between a rotating blade roll and the rotating backup roll where they are separated into short pieces. More specifically the present invention involves a chopper having an improved idler roll which allows the idler roll to reliably self align with the surface of a back up roll to more reliably and more effectively hold down and pull item(s) being pulled and chopped such as loose wet or dry strands of glass fibers with, or without, a chemical sizing thereon.
It has long been known to chop continuous fibers or fiber strands into short lengths of about 3 inches or shorter. Billions of pounds of such product including chopped glass fibers and fiber strands are produced each year in process and chopping apparatus such as disclosed in U.S. Pat. Nos. 5,970,837, 4,398,934, 3,508,461, and 3,869,268, the disclosures of which are incorporated herein by reference. The choppers disclosed in these patents comprise a blade roll containing a plurality of spaced apart blades for separating the fibers into short lengths, a backup roll, usually driven, which the blades work against to effect the separation and which pulls the fibers or fiber strands and an idler roll to hold the fibers or fiber strands down onto the surface of the backup roll. In the chopped fiber processes disclosed in these patents, the chopper is usually the most productivity limiting equipment in the processes that are round the clock, 24 hours per day, type processes. Therefore, improvements in the chopper, which allow the chopper to pull and chop faster, for longer times between maintenance shutdowns, and/or to pull and chop more fibers or fiber strands at one time, have an extremely positive impact on productivity and production costs.
In the published prior art the idler roll has been mounted and held against the surface of the back up roll in a generally rigid manner and with a variable force such as a force applied by an air or hydraulic cylinder. The working surface of the idler roll has been knurled and chrome plated in the past to better grip the fibers or fiber strands to prevent the fibers or fiber strands from sticking to the roll and wrapping and to avoid having to shut the chopper down to remove the wrap. Normally several strands, up to 14 or more, are fed into the chopper, each strand containing 2000 or more fibers. As more fiber strands and fibers are fed into the chopper it becomes more difficult to pull all of the strands and fibers at the same speed, so more pressure is applied to the cylinder pushing the idler roll against the backup roll with more force. If all of the strands or fibers are not pulled at the same speed, the slower strands and fibers will have a greater fiber diameter which is unacceptable and the bushings of the slower strands frequently will not operate at the proper temperature causing more frequent breakouts and/or additional fiber diameter variations, both of which are unacceptable. Also, fiber slippage can cause some of the fibers to be cut to shorter lengths than desired resulting in an unacceptable product.
As the pulling speed is increased, and/or as the number of strands and fibers are increased, above about 3000-4000 ft./min. (FPM), depending on the product, the present state of the art choppers begin to vibrate and the idler roll begins to allow one or more of the strands to slip some thus reducing the pulling speed of one or more of the strands. Also, if all of the strands are not pressed between the idler roll and the elastomer layer of the backup roll, a strand can slip partially out of the nip leaving some of the fibers unchopped, producing double cuts and stringers in the chopped product and causing the product to be scrapped. This condition worsens as the backup roll and blades become worn with use, typically about 6-24 hours following a chopper rebuild in which the blade roll is replaced with a roll having new blades and the backup roll is replaced with either a refurbished or new backup roll. The backup roll has a working layer of polyurethane at least about a half inch thick on the surface of a metal wheel. As the chopper life increases the surface of the backup roll becomes chopped up due to the blades pressing and cutting into the polyurethane.
Periodically the backup roll must be removed and either the polyurethane layer turned down on a lathe or sander or must be replaced with a new layer of polyurethane. It would be very desirable to be able to run the chopper at higher speed without vibration and strand slippage to make good chopped fiber than is possible with the current state of the art choppers because the remainder of the process is capable of producing fiber that can be pulled much faster in terms of speed and pounds of fiber per hour than is currently possible. Also, in the preferred state of the art chopper the idler roll mount is exposed to broken fiber and sizing being thrown from the various rolls on the chopper and must be frequently cleaned to avoid serious malfunction at current speeds.
An improved idler roll mount is disclosed in U.S. Pat. No. 6,723,670, and this mount works very well but requires the entire yoke to pivot to keep the idler roll in the desired alignment and this added inertia is a disadvantage, especially at high pulling speeds above about 3000 FPM. It is known that the reason for certain strands slipping on the idler/backup roll of the current choppers is the manner in which the idler roll is mounted, the manner in which the force is applied to hold the idler roll onto the surface of the backup roll and the fact that the peripheral surface of a new and used backup rolls often have a slight taper from side-to-side or misalignment with the idler roll, either initially or after the new backup roll has been in use for a few hours. A mechanism that would achieve at high pulling speeds with the same desired alignment now achieved at lower pulling speeds with this device would be highly desirable and valuable.