Sheraton Roll Cover Configuration 16K Photo

The papermaker has long recognized that a 4-5 Hz shake applied throughout the forming section of slow speed papermachines (i.e., less than 200 fpm or 60 m/min) is perhaps the most favorable environment available for producing uniform, quality papers. This situation exists because such a shake creates an almost continuous oscillating CD shear or fine-scale turbulence throughout the forming zone.

However, few paper machines run this slow any longer. When a comparable shake is applied to a high speed machine, like one running at 2000 fpm, it introduces a fresh CD pulse into the stock only once every 3-4 feet of travel down the machine, thereby losing most of its effectiveness.

So few machines today running over 1000 fpm employ a shake. Instead, they depend on a number of other devices, primarily free- draining and vacuum-augmented foils to maintain stock dispersed throughout the forming zone. These devices have never been as effective as a 5 Hz shake on a 200 fpm machine.

The Sheraton Roll offers an alternative method for applying a continuous, vertical, fine-scale, high speed pulsing action or vibration to the stock throughout an extended length of the forming section. As can be seen in the top picture of the cover, the Sheraton Roll itself is shaped like a gear. When it is mounted in contact with a forming fabric like a table roll, it causes the section of fabric directly above it to oscillate vertically. The amplitude of this oscillation, H, is given by the equation

where G is the free distance between its teeth (see sketch #1), and D is the outer diameter of the Roll.

When a Sheraton Roll is driven by the forming fabric so that the speed of its surface equals that of the fabric, the length of fabric which oscillates vertically is extremely short. On the other hand, when the Roll is driven by a variable-speed motor, the entire section of free fabric and the stock it supports between the Roll and the next point-of-contact of the fabric with a stationery element (such as a foil blade) can be oscillated _ or close to its resonant frequency as shown in the lower photo of the cover. This type of vibration creates a very effective form of fiber dispersing turbulence in a relatively long length of the fabric. When this action is applied to the stock at the beginning of the forming section so that there is considerable dewatering and sheet formation in its presence, the uniformity or formation quality of the sheet is improved considerably; see sketch #2.

The vibration of a section of a forming fabric at resonance differs little from that of any other flexible element such as a violin string, for example. What it means is that the entire element between its two supports or contact points oscillates vertically as a unit; see sketch #3. The time, T, required for one such oscillation to occur depends on (1) the mass or weight, W,, of the element; (2) its length, L; and (3) the tension of force, T, applied to it. Dividing the time T, into one (or 1/T) gives the resonant frequency, R , of the element.

The resonant frequency of an element is unique. That is, a change in any of the three primary variables, W, L, or T, will cause a corresponding change in R_f according to equation (1).

When applied to a section of forming fabric being oscillated by a Sheraton Roll and supporting a partially-formed wet web and the residual stock above it, there is also a unique or resonant frequency at which it oscillates vertically as a unit. Such an oscillation or vibration can introduce an enormous amount of fine-scale energy into the stock, so much so that in practice Sheraton Rolls usually do not oscillate the system right at R_f but rather at some nearby frequency. For more details and the equations describing the resonant frequency of forming fabrics, see references (1) and (2).

A Sheraton Roll causes a section of fabric and the stock it supports to vibrate at resonance when:

1. The time, T_r, which it takes a tooth of the Sheraton Roll to raise the point of Roll/Fabric contact from zero to maximum (see sketch #4) equals:

2. The time it takes for the vertical motion of the fabric to travel from the tooth to the next downstream contacting element, or T/2.

On the other hand, when T_r above is longer or shorter than T/2, the point of Roll/Fabric contact begins to descend after or before the vertical motion of the fabric reaches the following contact point. As a result, an interfering vertical motion is introduced down the fabric so that its length L no longer rises and falls as a unit. These situations describe all non-resonant vibrations, and they introduce far less energy into the stock that the unique resonant frequency.

In practice, the free length of fabric oscillated should be 2-3 feet long. The resonant frequency of such a length of fabric at typical fabric tensions (say 30#/in) in the forming of medium- to-heavy weight papers (50# - 100#/3000 ft²) is in the 30-100 Hz or pulses-per-second range. Knowing the number of teeth of a Sheraton Roll, this frequency is readily translated

into a Roll speed. For example, a Roll with 15 teeth which is to produce a frequency of 60 Hz must make 4 (60 pulses/15 pulses per revolution) rotations per second, or turn at 240 RPM.

As mentioned above and illustrated by equation (1), an increase (or decrease) in the mass W being vibrated causes a corresponding decrease (or increase) in the resonant frequency. First, however, a word about the nature of the mass W being vibrated. It is not all of the stock above the fabric. Rather, it appears to be equal to the mass of the fabric plus that of the partially-formed wet web, but not the residual free stock above it. For example, if one quarter of a 100# (per 3000 ft²) sheet has been formed on a section of fabric with a weight of 40#/3000 ft², than

where the factor 20 accounts for the fact that the wet web being oscillated is at an average consistency of 5%, or has a water content 19 times its dry mass.

The fine-scale turbulence introduced into the residual stock above the partially-formed wet sheet appears to be comparable in nature to that introduced by other devices such as the shake, foil blades pulses, etc. As a result, it suffers from the same shortcomings, primarily in that it decays extremely rapidly once the stock passes over the foil blade which ends the vertical oscillation of the fabric. Irrespective of the nature of this residual stock, it reflocs in a matter of milliseconds or a few inches of travel down the fabric. Thus, to gain any "formation benefit from the vibrations induced into the stock by a Sheraton Roll, a considerable amount of sheet formation must occur in its presence or immediately thereafter.

In practice, these concepts translate into the following operating principles:

1. The Sheraton Roll should be located as close as possible to the forming board so that it acts on as much of the underwatered headbox discharge as possible. On machines where there is considerable dewatering, and hence sheet formation over the forming board, (1/4 - 1/3 of the total is not uncommon), these activities should be limited as much as possible. This can be done by either employing only the lead blade of the forming board (and removing its other blades), or by closing it off with one wide or several contiguous blades; see sketches #5a and 5b. In either case, the bulk of the initial dewatering will occur in the presence of the vertical vibrations introduced by the Sheraton Roll.

All of this is not to say that Sheraton Rolls cannot be employed further down the table. Their action will be the same, but the benefits derived will be less because less sheet formation can occur in the presence of Roll-induced vibrations.

It should be pointed out that the vibrations in fabrics induced by Sheraton Rolls will cause some increase in dewatering. This increase occurs primarily because the vertical vibration of the fabric tends to loosen up the formed or forming wet web, thereby decreasing its drainage resistance and allowing more dewatering.

While Sheraton Rolls can be employed on machines operating at any speed, in practice their use should be limited to those requiring more fiber dispersing energy, not less such as high speed news or LWC machines

(3000 fpm and up). As a general rule of thumb, the top operating speed of Sheraton Rolls on lightweight papers (say 50-60#) should be 1000-1200 fpm, whole on heavyweights such as boards, 1800-2000 fpm.

The motors which are employed to drive Sheraton Rolls are as varied as the papermachines on which they are mounted. Obviously, they must be waterproof. They should be geared down at least 2:1 to provide a positive drive. It should be noted that the speed of the fabric and the surface speed of the Sheraton Roll are two independent variables. That is, the fabric speed does not appear in equation (1). Thus on some machines, the surface speed of the Sheraton Roll is faster, and on others slower than the machine speed. These differentials are generally inconsequential to the operation of the paper machine.

The gear-like outer structure of the Sheraton Roll is cut into an epoxy mantel reinforced by a layered fiberglass web which can be applied to virtually any steel roll. Typically, table roll cores are employed. In fact, they can be applied to old table rolls (in good shape, obviously) after removal of their old rubber cover.

Typically, the epoxy/ fiberglass mantel is about 1" thick, and the teeth are cut 1/2 into it.

(1) Kallmes, O., Marinari, G., & Perez, M., TAPPI Journal 72, 4:71-5, April, 1989

(2) Kufferath, W., & Kallmes, O., Proceedings of EUCEPA Conference, Stockholm, Sweden, May, 1990.