III. BASIC WINDING

Today center winding is the most prevalent type of winding, and the basic principles remain the same whether the operation takes place in a vacuum or in ordinary atmosphere or in a pressurized chamber.

In center winding (Figure III-1) the winding force is derived solely from the rewind shafts and is transmitted to the winding web through the core and layers of material that have already been rewound. The winding force is generated by a electric motor, a fluid motor, or a slip clutch.

Normally after slitting there are two rewind positions, when center winding, generally located one above the other, to ensure positive separation of the rolls. Adjacent cuts are wound alternately on the upper and lower rewind mandrels.

Therefore, in center winding it is easiest to wind at constant torque. When it is necessary to wind to larger diameters on a given core size, programmed torque is used. It is seldom advisable to attempt constant tension winding in a center wind operation.

The bottom line in Figure III-3 shows the basic constant torque decreasing tension curve. The upper line is the constant torque curve. This is one form of programmed torque. The ideal arrangement for building a good roll is somewhere in between as is shown in the programmed torque curve. The exact ideal curve varies with the material and winding conditions and is usually best found by experimenting with varying programs until ideal performance is arrived at.

The above conditions hold true in all center winding operations.

The duplex center winder is an application of center winding utilizing differential rewinding. Typical of this type of machine are the Dusenbery Series 465, 635, 735, 835 and their many derivatives. Differential rewinding is utilized in these machines. Engineered for versatility, differential rewinding is an arrangement whereby each individual rewinding roll is completely isolated from all others. This is essential to satisfactorily rewind materials of non-uniform gauge.

Without differential action, the heavy gauge areas of the material would wind tightly while lighter gauge areas would wind loosely. The former condition may result in stretching of the material and/or collapsing of the core. The latter condition may cause the roll to telescope and/or fall apart in handling. High quality rewinding, therefore, depends directly on differential winding. High quality rewinding, therefore, depends directly on differential winding (Figure III-4).

Differential rewinding is accomplished on duplex center winding machines through a system of alternating cores and spacers on each mandrel. Figure III-4 shows this. The spacers are keyed so that they are free to slide axially on the mandrel, while being restrained from rotating. An adjustable air pressure system is used to exert a force axially along the mandrel. Each core is squeezed between the spacers adjacent to it. The rewind mandrel is driven faster than the slit strips are fed onto the core. Thus, the cores are held back by the slit strips and are forced to slip with respect to the mandrel and the keyed spacers. The adjoining faces of the cores and spacers act as slip faces of slip clutches, providing a driving torque on each core proportional to the axial pressure exerted. Each core therefore has the equivalent of its own individual slip clutch drive.

The tension of all slit strips is the same since all cores are acted on by the same axial force acting on identical slipping surfaces. Each core is its own slip clutch and, therefore, each slit strip is free to wind at its own speed and tension regardless of the size or condition of the rolls being rewound on either side of it.

For maximum versatility, four sets of rewind tension controls, one at each end of each mandrel, are installed as standard equipment on this type of machine. Thus, by utilizing each of the four different tension control zones it is possible to slit and rewind up to four different slit widths concurrently from a single web and maintain the correct tension for each individual slit width.

Any burrs or turned over edges on the inside diameter of the core will cause binding between the core and the mandrel. Burrs catch in the mandrel keyway or act as a point contact with the rewind spacer. These conditions cause an erratic slipping action resulting in erratic tensions on the rewound web strip.

Since the cores are stacked together with the rewind spacers, any cumulative size variation in the individual elements will cause successive increases in misalignment between the cores and the slit webs. Techniques here, therefore, necessitate holding close tolerance on the length or width of the core used.

Where commercially cut narrow-width cores are used and erratic operation exists due to poor core edges and/or excessive friction between the core I.D. and the mandrel body, core adapters have been developed. Core adapters are used to eliminate contact between the core body, the rewind mandrel, and the rewind spacer (Figure III-5).

All core adapters serve to control the friction between the mandrel, rewind spacer and the core. By placing the core on the core adapter and utilizing a low friction insert in the core adapter as the slipping surface on the mandrel and against the rewind spacer, very fine tensions can be maintained on all operations, especially when thin, stretchy films are being rewound.

Design of the flush-sided adapter eliminates the weakness of the recessed type but requires the operator to subtract the width of the flange from the width of the cut to determine the length of rewind spacer required.

Adapters are employed where cores cannot be obtained to the tolerance required. Use of adapters, as shown in Figure III-8 will eliminate the error introduced by the core length and will permit the use of varying core lengths on the same adapter by merely changing the spacers between them.

Spacer length for this type of core adapter is computed as follows: Spacer width equals (2 x slit width) minus core adapter width.

High torque core drivers (Figure III-9) are used to obtain rewind tensions higher than those that can be generated between the side of a paper core and a standard rewind spacer. This is in the range of 100 lb. or more. These core drivers act as individual slip clutches that drive the cores while serving as thermal insulators.

High torque adapters (Figure III-10) are used to obtain the higher rewind tensions while eliminating the problem of the core collapsing and seizing. Here the core is supported by the pilot of the adapters and not by the mandrel.

All types of core adapters can be used for rewinding on large size cores where mounting and handling make it impractical or expensive to use a mandrel of matching diameter. The type of core adapter selected depends on the intended use.

Individual top riding rolls are used to obtain straight, smooth sided rewound rolls, at high and low speed, when winding off-gauge, high-slip light tension "problem" materials. Of all the progress which has been made with regard to the slitting and rewinding of these materials, the use of the individual top riding roll for each individual rewound roll is probably the most significant. Efficient subsequent operations, to a large extent, depend on the smooth edged, even density rolls of packaging materials which can be produced on the slitting-rewinding equipment. Four major factors must be compensated for in order to produce the desired slit and rewound roll. These factors are: gauge variation, gauge bands, slack zones and entrained surface air, which acts as a lubricant when an excessive amount is trapped and wound within the rewound roll. (Refer to Material Conditions.)

1 - Air becomes a lubricant, and acts as a frictionless separator between the layers of material allowing shifting of second, third and lower layers within the convolutions of the roll.

2 - Air is trapped between the layers of material and is wound into the roll. Once inside the rewound roll, it forms tires or balloons or bubbles, which can have various detrimental effects depending upon the characteristics required of the finished roll.

The top riding rolls may be 3" I.D. paper cores mounted on two flanged ball bearings, in turn mounted on aluminum tubular center shafts. These are held in position by easily removed arms and are quickly changed from one width to another by simply changing the paper core or the whole assembly.

For maximum utilization of these individual top riding rolls, it is necessary to have the incoming web enter into contact with the rewinding roll at the point of contact with the individual top riding roll or better yet, to wrap around the top riding roll. If the incoming web contacts the rewind roll some distance ahead of the individual top riding roll, the tracking and guiding effect of the top riding roll is lost. Once this has occurred, it is extremely difficult, if not impossible, to force the air out. Therefore, Dusenbery has developed their machines so that the incoming web wraps around the individual top riding roll as shown in Figure III-11. This action positively eliminates any possibility of web contact with the rewinding roll other than at the immediate point of contact between the individual top riding roll and the rewinding roll. Hence, the best possible condition for the positive elimination of air exists. Further, the web tension assists in maintaining contact between the individual top riding roll and the rewinding roll.

Top riding rolls are available in four varieties: Paper Core, Aluminum Tube, Rubber Covered Aluminum Shell and Filament Wound Tubing.

The paper core top riding roll has advantages in the following areas:

a. Fast, easy set-up.

b. Economical.

c. Works particularly well on materials which have a rough surface or materials with a high coefficient of friction.

d. Works well with printed plastic materials.

Disadvantages of paper core top riding roll are primarily:

a. Limits the running speed of materials with hi-slip characteristics to below 500 ft. per minute.

b. Paper cores have a length limitation of 24", maximum as over 24" in length, paper cores normally are not straight.

The rubber covered aluminum shell top riding roll offers advantages in the following areas:

a. Will permit machine speed to the maximum for the material being processed.

b. Equally efficient at any length.

c. Available as either a soft or hard rubber (durometer) consequently several different types can be kept on hand to provide for winding a complete range of materials.

d. Particularly effective for materials in the "hi-slip" light tension classifications.

e. Rubber top riding rolls tend to tract difficult material more easily.

Disadvantages of rubber covered top riding rolls are primarily:

a. Not as readily available as paper.

b. Relatively more expensive.

Filament wound fiberglass tubing has all the advantages of the aluminum tubing and although it is more expensive, it is less susceptible to damage.

Numerous tests, both in Production and Laboratory Research, have resulted in the following table of recommendations for winding various types of materials at various speeds.

Key:

     A - Low web sped (Under 300 FPM)

     B - High web speed (to 1500 FPM) - Rough faced material (Hi coefficient friction)

     C - High web speed (to 1500 FPM) - High Slip (Low coefficient friction)

     D - Aluminum Foil

     NR- Not required but can be used

     X - Will not work - definitely not recommended

     * - Denotes that hold-down springs may be required

For example, a roll with a gauge band build-up being wound without top riding rolls will develop tension concentration in the gauge band area resulting in uneven distribution of web tension across the full slit width. Hence, for a 10" wide roll winding at one pound per inch of web width (10 lb. total) all 10 lb. will be concentrated at the gauge band. Thus, if the gauge band is 2" wide we will have a concentration of 5 lb. per inch instead of the desired 1 lb. This excess tension will cause distortion of most plastic films causing further build-up of the gauge band. This in turn causes wrinkles to form in the web coming into the roll. Refer to Figure III-12.

Because top riding rolls are maintained in constant positive contact with the rewound rolls, the air which would be trapped between the layers of film is ironed out. Top riding rolls are sometimes referred to as "lay-on rolls" or "ironing rolls". They also serve to distribute rewind tension evenly across the slit width. Because parallelism between the top riding roll and the rewind mandrel is maintained, uneven web tension due to build-up on the rewound roll is eliminated. This action produces a wrinkle free rewound roll; and the rolls are not affected by off-gauge material conditions in the adjacent rewinding roll.

In slitter-rewinders, the most common type of center winding is the duplex type winder.

This type of machine is designated 280, 800 and 802 in the Dusenbery range. These machines have independent rewind arms for each slit strip.

The winding configuration used on these machines is that in which the web after slitting is partially wrapped around a driven roll known as a winding drum. The slit webs are then secured alternately to cores located on two banks of rewind arm assemblies positioned at front and rear of the drum.

In this winding method, each slit strip has its own rewind mandrel (Figure III-14). In the 820 series, only one rewind mandrel is provided, because this machine is primarily concerned with rewinding. Slitting, other than edge trimming, is secondary.

The rewind mandrels are mounted at the upper ends of a pair of arms, which are pivoted and can move in an arc as the material is wound upon the cores located on the mandrels.

To achieve the desired roll density, air must be eliminated from between the layers as they are wound. The contact pressure "N", shown in Figure III-14, does this. The thickness of the air layer and the air pressure created by this air film at the nip point (Figure III-11) increases as the surface speed of the web increases, therefore, contact pressure must be increased with web speed to prevent air from winding into the roll.

To control the tension in the web, it is unwound under controlled tension from the mill roll. Every roll in the machine prior to winding is driven at a controlled surface speed. The greater the accuracy of this control the less chance of imposing undo tension or sag in the web prior to reaching the winding drum.

As the material rewinds, it is under the influence of the two primary forces: contact force"N" and force "F", resulting from center torque produced by a motor driving each rewind shaft. Force "F" has, as its main purpose, the job of overcoming the internal bearing and rolling friction of the rewind core and its mounting.

In practice, if the material has a high coefficient of friction when rubbed upon itself (i.e. greater than 0.4), then no center wind torque should be necessary. As the coefficient of friction falls, the amount of center torque required increases.

Experience has shown that center wind torque exceeding the force derived from the contact pressure is seldom necessary. Consequently, its effect upon the tensions induced in the material during winding is very small. Therefore, the decrease in "F" with increasing roll diameter, and a constant center torque, is insignificant.

Any material with a coefficient of friction equal to 0.1 is considered special material and should be given special attention and tests.

Theoretically Perfect Rolling Conditions vs. Slip

When the two rolling surfaces have a high coefficient of friction, they roll together perfectly and there is no relative difference in their surface speeds. If one is driven (i.e. the winding drum), the other will follow perfectly. If layers of material are placed between them, then slip will occur proportional to the degree of slip between the material surfaces and to the amount of drag imposed upon the non-driven roll (i.e. the rewinding roll) by its bearings and weight.

The rewind roll must then be driven to the extent necessary to overcome this retardation while matching the driven roll surface speed. If the rewind is driven with a force greater than the friction forces between the contacting surfaces, it will attempt to go faster than the winding drum and the material between may be stretched.

Factors affecting roll density and tension in the rewound materials are:

1. Relationship between winding drum surface hardness and contact pressure.

2. Relationship between rewinding roll hardness and contact pressure.

3. Air wound into the rewinding roll.

4. Unwind tension of the mill roll.

5. Amount of overdrive between the unwind and rewind.

a. Between driven rolls in the machine.

b. Between winding drum and the remainder of the driven rolls in the machine (this is adjustable).

Theoretically, if the materials were perfect in thickness across the web width, there would be no need for individual rewind arms. Since this is seldom so, the reason for the use of individual rewind arms is the same as discussed in relation to center winding.

The effect of variation in web thickness raises a further problem in surface winding. If one part of the web width is thinner than an adjacent piece or strip, one roll will be smaller in diameter than the roll adjacent to it. If both rolls are mounted on a common shaft the smaller diameter roll will lose contact with the winding drum and change its tension pattern and density.

The individual arms overcome this problem by allowing the rolls to contact the drum irrespective of the size of the adjacent roll. The arms are also adjustable to various widths of cut.

If the winding drum and the rewinding roll are of a hardness that prevents their deformation by the contact pressure, then a perfect rolling condition exists and the material rolls from the winding drum on the rewinding roll without distortion or stress. This condition is shown in Figure III-17.

Should the winding drum be soft enough to be deformed, the path the material must travel before being wound into the rewinding roll, is lengthened by the amount of this deformation. This condition increases web tension and if the tension is high enough the material will be elongated at this point.

This condition is shown in Figure III-16. Conversely if the roll being wound is too soft, the same conditions exist. Therefore, it is obvious that these conditions must be matched to obtain ideal roll configuration. Once this relationship is understood these factors can be used to control roll deformation, unwind tension, and winding drum speed to help produce the type of rewound roll desired.

Unwind tension is a direct factor in the tension and density of the rewinding roll for there is no section in a surface-center winder (except at the winding drum) where material tension can be varied.

A high unwind tension contributes to a high rewind tension and a dense rewound roll. A light tension produces a softer roll.

Winding Drum Speed

Close control of the relative speed of the winding drum is obtained through the variable pitch pulley driving the winding drum. By increasing the winding drum overdrive, tension is increased in the material as it is pulled from the knife shaft to the winding drum. The converse is also true where lower tensions are desired.

If there is any variation in gauge the winding drum will be in contact with the largest diameter of the roll or wherever the gauge band exists. That section of the web that does not rest on the gauge band will follow along without undue stress.

In surface winding the tension in the web is created by the driven rolls in the system. Once the web gets on the winding drum it rolls onto the rewind without further tension being applied. Hence all stress is uniformly distributed throughout the length of the web going through the machine and no further stress is created at the windup.

If the winding roll is in contact at its largest diameter it will run at the surface speed of the largest diameter allowing the rest of the web to wind without tension on it.

An added advantage of surface winding is that one can build up to any roll diameter desired because there is no pull on the web emanating from the core, hence no stress, as is found in center winding.

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