In the day-to-day operation of paper machines, little thought is typically applied to carrier rope runs until something changes that affects downtime related to sheet transfers and/or rope replacement.
Premature rope failures should never have to be a cause for downtime. When ropes fail prematurely, effort should be applied to determine the assignable cause for the shorter life. It is important to retain a sample of the rope that failed, along with the splice area. Your rope supplier can assist you in this, but this paper contains the basics for crew training and shows common problem areas to examine.
When discussing rope related issues, it is important to understand and use the correct terminology. Ropes are made by building various layers of the final structure. Figure 1 shows the proper terminology used to describe the components of a rope.
Individual yarns are made from fibers, several of which yarns are twisted into strands. These strands can then be twisted or braided into ropes of various types and sizes. Those familiar with the art, know that these ropes can be built with or without a hollow core.
Difference Among Types
Figure 1 demonstrates the differing types of ropes commonly used in the mill. The top is a 3-strand twisted rope, then a hollow-core braided rope, number 3 is a center-core braided rope, and at the bottom is a center-core braid in which is a braided core.
Typically, the various types are available in 2 or 3 common diameters and are available in an array of colors. There will be more about colors later in this paper. Here would be a common product line for a carrier rope supplier:
|Rope Type||Typical Sizes|
|Braided, hollow- and center-core||9.5 mm, 11 mm, 12.7 mm|
|Double braided, hollow- and center-core||9.5 mm, 12.7 mm|
|Twisted, 3-strand||9.5 mm, 11 mm, 12.7 mm|
Ropes are made of either nylon or polyester, or in rare cases, a combination of the two. Nylon, since its inception, has been known for its excellent abrasion resistance. Since nylon is hydrophilic, however, it will elongate in wet environments. Nylon fibers are not subject to hydrolysis.
Polyester is a more hydrophobic material, so it has very little reaction to moisture. By nature, it stretches little under normal running tensions, and would be the desired material on sections with limited stretcher adjustment. Because of the molecular structure of polyester, it can hydrolyze when exposed to high temperature, in the presence of high humidity.
Specifically, there is no requirement for high rope strength, so strength is frequently sacrificed for improved abrasion resistance. More important than strength is a high modulus, meaning that the rope elongates a minimal amount during operation, since elongation reduces running tension over time unless tension is compensated for. That being said, these ropes are still very strong and can cause significant damage if they jump off a sheave or break under running tension.
|Rope Diameter||Tensile Strength, Hollow Core/Center Core|
|9.5 mm||2408/2608 kg|
|11 mm||2898/3370 kg|
|12.5 mm||3387/4135 kg|
Note: Splices reduce strength by 10%, and knots reduce strength by 20-30%.
As was noted above, splicing a rope, as necessary as it is, reduces the tensile strength significantly, rendering it the weakest point in the rope. Additionally, rope splices are always thicker than the body of the rope, causing an increase in abrasion as that thicker portion of the rope goes around sheaves or as it makes contact with other ropes. There are 3 critical factors in rope splicing:
- The sheave groove must accommodate the thickness of the rope AND the splice, as shown in Figure 2
- The splice must be as thin as possible. This is a major factor in converting from twisted to braided ropes. Braided ropes are spliced with the "Chinese Handcuff" technique, resulting in a much thinner splice than with a 3-strand twisted rope.
- Spices must be tapered to the greatest extent practical, to extend rope life. The single largest cause for rope failure is a poorly tapered splice, as is shown in Figure 3.
Proper Groove Width is Critical
Figure 2 illustrates sheave groove matched properly to the splice diameter.
Twisted Rope Splice Taper Variations
Figure 3 is a demonstration of insufficient splice taper (top) vs. proper taper (middle) compared to rope diameter.
Rope Run Best Practices
1. Good Sheet Transfer
Certainly, one of the main purposes of the rope run is to transfer the tail from one dryer section to the next. Figure 4 shows the proper geometry. "Release Ropes" A and B run together from the last dryer can in one section, until they are approximately 18" (45.7 cm) from the next dryer, at which point they begin to open in order to release the tail to the "Catching Ropes". These catching ropes, D and E, form a nip which will then secure the tail for transporting the tail through that next section of dryer cans.
Correct Geometryfor Proper Tail Transfer
Figure 4 demonstrates the correct geometry to ensure good transfer.
In order for the ropes to do their job without dropping the tail, the inner rope must be UNDER the tail and the outer rope, conversely, is OVER the tail as shown in Figure 5.
The Position of the "Catching" Ropes for Good Transport of the Tail
Figure 5 shows that the inner rope is under the tail
2. Dryer Can and Sheave Grooves
Transferring the tail through a dryer section can be a challenge. To prevent "dropping" the tail the rope speed and the dyer can speed must match as closely as possible. Note that the deeper the groove on the edge of the dryer can, the slower is the surface speed driving the ropes. A general rule is that shallower is better, up to the point that the ropes will not track properly. This phenomenon is shown in Figure 6.
Minimizing the Speed Differential Between the Dyer Rope Groove and Can Surface
Figure 6 illustrates the surface speed difference phenomenon
As mentioned earlier, sheave grooves must be matched to the rope that is being run. This should always be a consideration when changing rope diameters or purchasing new sheaves. Remember that the sheave groove needs to be matched not to the rope diameter, but rather to the splice diameter. Figure 7 shows a splice running through 3 differing sheave grooves. Only the center sheave is sized correctly.
Match the Sheave Groove to the Diameter of the Splice
Figure 7. Only the center sheave groove is sized correctly.
3. Sizing Sheaves Correctly
There is never a problem oversizing a sheave, except the added cost. Problems most frequently seen are sheaves that are undersized for the angle of wrap. Figure 8 shows the minimum sizes of sheaves for the various angles of wrap.
Best Practice Recommendations for Minimum Sheave Diameters
Figure 8 shows minimum sheave diameters based upon angle of wrap.
Note that larger diameter sheaves are always a good idea on machines running faster that 1000 mpm. Excessive sheave bearing failures on high speed machines dictate larger sheaves to reduce the number of rotations per unit of time.
4. Rope Entry and Exit Angles
No papermaker wants to have a carrier rope to "jump" the sheave. When this happens damage to machine components can occur, as can injury to associates who happen to be in that part of the basement. When a rope comes off a sheave without having broken, the most common culprit is improper entry and or exit angles. Figure 9 demonstrates correct and incorrect geometry. Always enter the sheave straight to prevent ropes from jumping off the sheave. In the illustration, the rope and sheave geometry on the left is correct, while the rope and sheave geometry on the right will likely cause trouble.
Different Entry Angles Create Different Results
Figure 9 shows correct incoming geometry (left) and improper geometry (right)
The rope should never make contact with any part of the sheave other than the groove. The ropes are driven by the dryer cans and maintain that speed. The flange of the sheave has a much higher surface speed than the groove, and, as such, will create accelerated wear on the rope and premature failure. For that reason and the high risk of ropes jumping the sheave, it is recommended that 30 degrees is the maximum exit angle from the sheave. This is shown in a visual representation in Figure 10.
Maximum Exit Angle
Figure 10 shows the maximum exit angle to prevent the rope from making contact with the flange.
5. Rope Stretcher Systems
Ropes can be tensioned by a series of suspended weights (gravity system), or by pneumatic loading. Slower machines can get away with gravity systems, but the precision of pneumatic tensioning is always recommended. Guidelines for stretcher type and rope tension guidelines are shown in Figures 11 and 12.
Figure 11 are "rules of thumb" for stretcher type. Good rope run geometry allows higher speeds for gravity systems.
Rope Tension Recommendations
Figure 12 shows generally accepted guidelines for rope tensions, as measured by a tensiometer.
6. Selecting the Proper Sheave
Sheaves are made in various shapes and configurations for specific applications. When a sheave requires replacement, the maintenance and operational associates should be trained on the differences. In general, tail carrying sheaves have shallow flanges to avoid stretching or breaking the tail. Rope return sheaves have taller flanges to avoid rope jumping out of the sheave. As a general rule, tail carrying, or "Transfer" sheaves have wrap angles of less than 15 degrees. Figure 13 shows typical sheave profiles as they appear in supplier catalogs.
Sheave Geometries as They Appear in Supplier Catalogs
Figure 13. Rope return sheaves and tail carrying (transfer) sheaves
Rules of Thumb for Maintenance and Operational Personnel
- Rope Runs are generally required for machines running over 100 mpm. Some modern machines are equipped with ropeless transfer systems.
- Rope runs should follow the sheet run as closely as possible to keep the tail tight.
- Rope tension can be kept low when not threading the sheet, to extend rope life. Never allow the ropes to become so slack that they vibrate or "bounce" between sheaves.
- Target a maximum of 2 m between a rope carrying sheave and a roll.
- Return rope sheaves should not be more that 4 m apart.
- Ropes should enter the sheave straight (right angle to the rotational axis).
- Ropes can leave the sheave at an angle, as long as the rope does not make contact with the flange.
- Sheaves should not be angled more than 30 degrees from the vertical.
- Inside and outside ropes should not cross. For this reason, run different color ropes inside vs. outside.
- Wrap on sheaves should be as low as possible to minimize wear and splice fatigue. If excessive wear and/or splice failure occurs on high wrap positions, consider adding additional sheaves.
- Sheave inside diameters should be as large as possible (5:1 bend ratio up to 90 degrees, 8:1 bend ratio for larger than 90-degree wrap.
- Ropes must not run against stationary objects or other ropes.