Apr 23, 2019
Rope systems are designed to carry a "tail" of paper through sections of a paper machine to thread it prior to widening the sheet to full width. There are one or more points in the process where it is desirable to drop the tail and widen the sheet prior to threading up the next section. Such points (or stations) are referred to as "tail drops" or drop stations. These stations also provide a position in the process where the sheet can be broken down when problems occur in following sections.
On many machines, the only location where a sheet drop is provided (relative to rope systems) is at the last dryer in a section.
Since paper machine dryers are broken up into separate groups (or sections) for drive purposes, each section must have its own complete and independent rope system. When threading a machine, it is desirable to have the paper tail go through all dryer sections without having to be handled. Therefore, a means of interfacing adjacent rope systems has to be devised so that the tail is automatically passed from each rope section to the next. This interaction is known as "Automatic Transfer".
There are several ways that such a transfer can be designed. One of these involves tearing some of the tail off at each transfer. A second way is to pass the tail intact from one section to the next. Some machines were built when the state of the art was to tear the tail at each transfer. With this system, some rope sheaves at the transfer have large flanges on one side for the sole purpose of severing the tail. There are some poor features in this "tear-off" concept:
To compound the problem, a machine may have "up transfers" between dryer sections. That is, the tail is transferred from one section to the next on up-runs of ropes (due to dryer drive gearing on the back side). The tail shreds created by the tear-off transfer are released into the upper hood where they become difficult to purge.
At this mill, there were frequent and worsening issues with transfers from one rope run to the next. Some rope runs were old and originally designed for tear-off transfers. In addition, on PM2 there were up transfers between dryer sections. Pieces of tail were going up into the hood and falling down on the felt and conveyer on down to the wet end then down into the sheet run causing sheet breaks.
Transfers on both machines were improved with some simple modifications that permit transfer of tails intact from one section to the next, eliminating - or at least minimizing - creation of shreds.
Figures 1 throu 8 show the rope systems as they previously existed as well as after modifications were made to improve transfers. Figures 1 to 4 are for #1 machine. Figures 5 to 8 are transfers and areas of concern for #2 machine. (Note: With the machines in operation, it was impossible to access some locations, so final location of some of the new sheaves was determined at installation.)
We addressed the problem of providing clean tail drops at last dryers. This involved the use of kicker sheaves on the opposite-the-tail side of each rope to knock, or kick, residual tail off the respective ropes to alleviate fouling of return runs. These kickers were provided as an essential ingredient of a clean transfer as well. Tail-drop rope sheave placement was determined at installation.
On #2 machine, some special problem areas were addressed. Ropes previously ran over paper tension rolls making them less effective in sensing paper tension. By changes in the rope runs, the ropes are now removed from the paper tension rolls eliminating this influence (Figures 7 & 8) In the old breaker stack area, we changed the transfer from 3rd to 4th section to be made at the first tension roll (Figure 7). All ropes are removed from the second tension roll so that it could accurately control paper tension in this draw.
In connection with the above, all rope tension control air cylinders were being run in bottomed out condition. Such operation completely nullified rope tension control. Rope tension was set by wind-up on the boat winch. All control was lost and there was no forgiveness in the system. When a paper wad would go through a sheave nip, a wreck of the system could occur. Limit switches were existing for the bottom of each cylinder, and would trigger visual indication on the operator benchboard when a switch was engaged. We installed second limit switches, connected in parallel with the existing switch on each cylinder to indicate when the cylinders are stroked out.
We strongly recommended that the tension devices be operated as designed - somewhere near mid-stroke on the cylinder - so air cylinder pressure can control rope tension.
One of the advantages of air cylinder control of rope tensions is that the device lends itself to two-mode control of rope tensions. By addition of a few simple controls the mill can now have "RUN" tension (about 15 pounds) and "THREAD" tension (about 30 pounds). Such control of rope tension extends rope life considerably.
In general, while unorun dryer sections have improved runnability considerably, they have also created some special problems for threading. To name a few:
Rope geometry at transfers is very important. Off-going ropes should pass in close proximity of the dryer where on-going ropes nip. Whenever possible the ropes should separate in two planes. In the machine-direction, the first off rope should lift off the tail sandwiched between the ropes. If the wrong sequence is used, the tail can end up wrapping the other rope. In the cross-direction, the gap must be enough to release the tail. (Figure 10) On up-transfers, the gap geometry becomes very critical, if too little, the tail gets carried up into the hood, if too loose, the tail will fallout before a transfer can be made.
The rope systems on both machines were made much more reliable by making a minimum of changes. Basically, Valmet changed the transfer philosophy from tear-off to tail-release. This resulted in less fouling of return runs, and on #2, less debris on the upper hood. If tail drops at last dryers can be worked out, more viable systems will result.
For assistance with modernizing your rope runs, or converting your machine to ropeless threading, contact your Valmet representative.