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ished product roll and to keep air from being

trapped between the plies of material, which

will prevent telescoping. In addition, flying-

splice rewind units generally have secondary

(or follower) rider rolls. These secondary

rolls ride against the rolls throughout the

rewinding and splicing sequence. When the

rewind arms of the primary rider roll lose

contact with a roll that is in the final state of

being wound, the follower rider roll contin-

ues pressing against the full roll to prevent

air entrapment and telescoping. The secon-

dary rider rolls can be either spring, hydrau-

lic or pneumatically loaded.

Flying Splice. The flying-splice rewind uses

the same basic center-winding principle, but

has two core shafts mounted on a turret. The

drive and auxiliary equipment are more

sophisticated and aim to splice at full operat-

ing speeds. Normally, each shaft is driven

through a slipping clutch and/or separate,

direct drive motors to permit speed control

of each shaft. Center-shaft, flying-splice units

have become the most popular and widely

used winders today, especially in the pro-

cessing of extensible films and packaging

materials.

Unloading. Roll unloading equipment is

available in the form of hydraulic or pneu-

matic roll-lowering arms for the single-posi-

tion winder. Floor dollies or overhead hoists

are normally used for unloading the rolls

from either the center-shaft or flying-splice

rewind units.

Slitting. Roll slitting on center winders has

grown in use. Score and razor-blade slitting

attachments are available as auxiliary equip-

ment. Shear slitting is also used in some

instances where materials call for it. A

spreader bar, driven expander roll or slatted

expander roll is used after slitting to prevent

web interleaving.

Web Guide. Edge guiding on rewinds gener-

ally employs one of two approaches. One is

to independently guide the web to the wind-

ing roll; the other is to side shift the rewind-

ing unit to “chase” the web edge. If the

winder is quite large, the provisions for

shifting it may be relatively expensive com-

pared with an independent guide ahead of a

stationary rewind. Recently, on presses with

web-scanning inspection devices ahead of

the rewind unit, an independent web guide

has been installed beneath the scanner

viewing platform to maintain web alignment

through the viewing section.

Splicing. Semi-automatic or automatic splic-

ing attachments can be fitted to turret-type

rewind units. The operator starts the splic-

ing sequence by rotating the turret to a pre-

set splicing position. When the core shaft is

running at or close to the printed web speed,

the operator manually makes a splice by cut-

ting the web while simultaneously pushing it

against the new, pre-glued or taped core.

This same sequence has been fully automat-

ed, thanks to hydraulic or pneumatic cylin-

der-actuated cut-off mechanisms and elec-

tric sequence controls.

Large-roll diameter flying-splice units are

required for additional footage when pro-

cessing laminates. These units allow the han-

dling of larger rolls and further reduce down-

time. Along with the larger diameter rolls,

core sizes are also increasing in order to keep

the roll buildup ratio more in line with the

capabilities of present rewind drives.

REWIND TENSION SYSTEMS

There are two basic tension-control sys-

tems: constant tension and taper tension. In

a constant-tension rewind system, the ten-

sion in the sheet being wound is the same on

the first wrap at the core as at the last wrap

on the roll. In a taper system, the tension in

the last wind of the roll is less than the ten-

sion in the web at the core. For instance, if a

web experiences two pounds of tension per

linear inch at the core and finishes winding

the roll at one pound per linear inch, the ten-

sion experienced is referred to as a 2:1 taper.

PRESSES AND PRESS EQUIPMENT 53

54 FLEXOGRAPHY: PRINCIPLES & PRACTICES

A center rewind unit can offer either con-

stant or taper tension. A drum rewind unit

offers only constant tension. Since most

flexographic presses use center-shaft rewind

units, more attention will be given to these

tension systems.

A good rewind tension system should be

capable of winding rolls with straight edges

and uniform density, while preserving the

accuracy of register and repeat length in the

print stations. To accomplish these require-

ments, the unit must be able to rapidly and

accurately follow the acceleration and decel-

eration of the printing press, compensate for

change in the diameter of the winding roll,

and be able to make rapid speed transfers

when used with flying-splice rewind units.

The drive must be able to handle wide or nar-

row webs; flexible, stiff or stretchable mate-

rials; and, in some cases, perforated webs.

POWER REQUIREMENT

Specifying a rewind system and sizing the

rewind motors is a job for the press design-

er. However, it is useful to know the para-

meters and how to apply them when adapt-

ing a press to suit a new substrate applica-

tion. The following are some of the factors

used to determine the rewind horsepower

needed: line speed expressed in feet per

minute; core tension expressed in pounds

per web inch; web width expressed in inch-

es; the mathematical horsepower constant;

and the ratio of the full-roll diameter over

the core diameter.

Two other factors should also be consid-

ered. The first is taper, which is selected

core tension divided by full roll tension, if

taper winding is required. The other is an

experience factor, and this has to do with the

windage and friction losses that are present

in the various mechanical components of

any particular rewind stand.

In setting up the various formulae to size a

particular winder drive, the following para-

meters must be known:

• line speed (feet/minute);

• line acceleration or speed change over

the time taken (feet/minute

);

• core tension (pounds/inch);

• web width (inches);

• taper tension when used (core tension/

full roll tension);

• core diameter (inches);

• full roll diameter (inches);

• full roll weight; and

• experience factor for rewind.

The general equation for determining the

approximate rewind drive horsepower is:

HORSE



2π TORQUE  REVOLUTIONS/MINUTE

POWER

33,000

WINDER



TENSION



WIDTH



ROLL

TORQUE DIAMETER

CORE



Line Speed

RPM

π  ROLL DIAMETER

Winding horsepower for a constant line

speed therefore is directly proportional to

web tension, web width and line speed and

is independent of roll diameter. However,

whenever line speed is changed, the inertia

of the wound roll must be overcome either

by a braking system to slow the roll or addi-

tional horsepower to accelerate.

ROLL



ROLL



ROLL



CORE

INERTIA MASS DIAMETER

DIAMETER

ROLL



2  LINE SPEED CHANGE

ACCELERATION

ROLL DIAMETER

INERTIA



ROLL



ROLL

TORQUE INERTIA ACCELERATION

For any given roll diameter and line speed

acceleration, winding torque is proportional

to the roll diameter, and roll acceleration is

inversely proportional to the roll diameter.

Therefore, the horsepower required to over-

come the roll inertia torque on press start-up

[]

or speed change depends largely on the time

it takes to run the press up to speed and is

proportional to the roll diameter.

Many materials wind best by taper ten-

sion; some wind best by constant tension. A

representative list of materials and their sug-

gested winding tensions and recommended

winding method is shown in Table 3. A web

wound by either method must be controlled

by a sensing device.

Constant Tension

The most commonly used controller for a

constant-tension winding system is a dancer

roll. The dancer may be loaded by weights,

air or a torque motor. A typical weight-loaded

dancer system was previously described in

the section on dancer rollers (Figure

Any movement of the dancer roll signals the

rewind drive to speed up or slow down,

maintaining a given dancer-roll position.

Another method to control the rewind drive

is a load cell transducer.

The rewind motor can either be a DC or a

constant-speed AC motor fitted with an

eddy-current clutch. Direct current motors

are more expensive, but because of their

superior performance, are now virtually

standard on new equipment.

Taper Tension

The controller for a taper tension wind can

be set up in several ways. One method is by

an electronic controller using solid-state cir-

PRESSES AND PRESS EQUIPMENT 55

Table 3

MATERIAL TENSION PREFERRED WINDING METHOD

(lb per mil per inch of width)

Aluminum Foils 0.5 to 0.15 Taper 1.5 to 1

Cellophanes 0.5 Taper 1.25 to 1

Cellulose Acetate 0.25 to 0.5 Taper 1.25 to 1

Ethyl Cellulose 0.5 Taper 1.25 to 1

Glassine 1.0 to 2.0 Taper 1.1 to 1

Methyl Cellulose 0.5 Taper 1.25 to 1

Polyester 0.5 to 1.0 Constant or Taper 1.25 to 1

Polyethylene 0.2 Constant or Taper 1.25 to 1

Polypropylene 0.187 to 0.25 Constant or Taper 1.25 to 1

Polystyrene 1.0 Taper

Rubber Hydrochloride 0.06 to 0.25 Constant

Vinyl Chloride Copolymers 0.06 to 0.187 Constant

Vinylindene Chloride Copolymers 0.06 to 0.187 Constant

Paper & Laminates ( per inch of width)

20# 0.5 to 1.0 Taper 1.5 to 1

40# 1.0 to 2.0 Taper 1.5 to 1

50# 1.25 to 2.5 Taper 1.5 to 1

60# 1.5 to 3.0 Taper 1.5 to 1

80# 2.0 to 4.0 Taper 2 to 1

85# 2.0 to 4.3 Taper 2 to 1

REWIND TENSION RANGES

56 FLEXOGRAPHY: PRINCIPLES & PRACTICES

cuitry. The controller receives two electrical

signals from the main drive system. One sig-

nal must be proportional to line speed and

the second proportional to acceleration.

The signal proportional to line speed is

usually obtained from a tachometer genera-

tor mounted on the main drive. A signal pro-

portional to the main drive acceleration is

obtained from the main-drive control sys-

tem. The controller regulates the winder

drive shaft output horsepower, whether it

be a DC motor or an eddy-current clutch.

Adjustments are provided in the controller

to allow tension to be decreased as the roll

size increases, providing a taper tension.

With this type of equipment a wide range of

tension patterns may be obtained with a sin-

gle-winder drive and controller.

The operator can vary the tension level to

accommodate different material thickness

and width combinations. The operator first

sets a tension value; the regulator will

decrease this value according to the taper

pattern programmed into its system. It is

possible to program several different tapers

into the system and have an operator selec-

tor switch to choose the taper pattern

desired during any given run. Since this type

of rewind drive only develops torque, it

does not know which portion goes into fric-

tion and windage and which portion goes to

the web, hence the experience level of the

press operator is important.

As mentioned in the rewind horsepower

formula, the drive manufacturer must take

operator experience into account in adjust-

ing for windage and friction losses when siz-

ing the rewind motor. In the previously dis-

cussed alternative dancer-roll system, the

tension in the web is only a function of the

loading of the dancer roll and, therefore,

friction, windage and inertia do not affect

the drive operation.

Another type of controller system is the

force transducer ( Figure

). It is similar to

the dancer-position drive, except that the

dancer roll is replaced with a fixed roll cou-

pled to a load cell arrangement, sometimes

called a strain-gauge amplifier. This system

has very precise and linear deflection-ver-

sus-load characteristics. Consequently, the

deflection of this load cell is directly pro-

portional to the web tension exerted upon

it. A small deflection (0.001") can cover the

entire tension range desired. This sensing

system is therefore very responsive and

accurate. It has the ability to readily adjust

taper with an electrical potentiometer

(operator’s tension setter) and it provides a

readout of the tension on a calibrated meter.

Table 4

SINGLE DUAL

MOTOR MOTOR CONTROL TIE-IN W/ TENSION ADJUSTMENT

DRIVE TYPE COST COST COMPLEXITY MAIN DRIVE RANGE TAPER

■ TORQUE REGULATED (TR) 130% 210% Medium Yes Medium

Yes

(Taper Tension)

■ DANCER POSITION (DP) 100% 180% Low No Medium

(Constant Tension)

■ CELLULOSE ACETATE 170% 240% High Yes Wide Yes (in TR mode)

■ FORCE TRANSDUCER 200% 330% High No Wide

Yes

(Load Cell)

Better at high end.

Better at low end.

Direct reading.

REWIND DRIVE CHARACTERISTICS

Since this system has no material storage,

there is nothing to absorb the tension

shocks resulting from out-of-round rolls,

splicing, or other irregularities. Compen-

sating systems are provided in its design to

handle these conditions.

Table 4 sums up the characteristics of the

drives that have been discussed. The

dancer-position drive is used as a base point

or the 100% reference line. For different dri-

ves, the relative cost, control complexity,

tie-in with the main drive, tension range and

adjustable taper have been compared.

The dancer-position system has low con-

trol complexity, medium tension range and

does a good job of winding materials requir-

ing low tensions. The taper-drive system has

a medium tension range and does a better

job of winding materials requiring higher

tensions. It requires a tie-in with the main

drive, has medium control complexity and

costs more than the dancer-position system.

A combination system of both taper and

dancer position is commonly used, allowing

for a wider range of tension values. In this

case, when winding heavier materials, the

dancer roll is bypassed and the drive winds

in the taper mode. When winding lighter

materials, the dancer roll may be used and

the control is then electrically transferred to

the dancer-position mode.

The force transducer system has a wide

tension range and features a direct tension

readout. This system does not require a tie-in

with the main drive and is readily adjustable

to taper controls. The system rates a “medi-

um” in complexity and expense.

Rewind drives, in the case of flying-splice

systems, can be equipped with either single

or dual motors. Dual motor drives have sev-

eral benefits, and individual core-shaft elec-

trical braking is available. No auxiliary core-

shaft clutches are required, and it’s easy to

get a very efficient, high-speed transfer from

the full roll to the empty core. On single-

motor systems tied into flying-splice rewind

units, it is necessary to use clutches to

engage the drive motor to each of the core

shafts. Although it is possible to speed match

the empty core to the line speed by “slipping”

the full-roll clutch, this method is not as reli-

able or as smooth as the two-motor system.

For these reasons, two-motor systems are

now standard on most modern equipment.

SURFACE-REWIND

TENSION SYSTEMS

The tension system used in most surface

rewind units is simpler than those just dis-

cussed. The double-drum or single-drum

surface rewind is normally geared to the

main drive so that the winding drums are

driving at a speed proportional to the line

speed. In order to establish a given tension,

a variable-speed drive is usually employed.

The operator adjusts the rewind to run

faster than the press. The material slippage

on the winding drums establishes the ten-

sion.

In certain applications, surface winders

are driven by a follower-type DC or eddy-

current, clutch-and-drive motor combina-

tion. Again, an operator sets the basic wind-

ing tension on one winding drum, while

another drum continually tries to tighten the

roll. The differential speed between winding

drums is usually obtained by gearing arrange-

ments. However, twin DC motors have been

used. The operator can then select a different

speed setting between the winding drums.

This approach allows a wide range of roll

hardness, depending on the difference in the

speed settings of the two drums.

With a single-drum system, there is no dif-

ferential or tightening effect. The winding

tension is set by the operator through the

mechanical or electrical speed controls.

PRESSES AND PRESS EQUIPMENT 57

58 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Pneumatic Shafts

and Chucks

henever there is a roll or

web, regardless of the

material, it has to be

unwound and usually

rewound. In most cases

the roll has a core of

some type, which could be paper/fiber,

metal, plastic or wood, that has to be held

securely to provide rewinding tension or

braking torque. There are a variety of man-

drels, shafts and chucks in use and most are

mechanically or pneumatically operated.

Mechanical shafts and chucks have been

widely used for conventional web handling

operations and are well-known in the field.

Pneumatically-operated shafts and chucks,

because of their simplicity and reliability, are

accepted as standard by engineers, produc-

tion supervisors and maintenance personnel

knowledgeable in manufacturing and con-

verting operations where web materials are

being processed. This next section will

explain the operation and application of

pneumatic shafts – more commonly called

air shafts – and chucks.

AIR SHAFTS

Air shafts are used for both conventional

and more demanding applications. They

offer many advantages over the older

mechanical shafts mainly because of their

light weight and simplicity of operation. In

addition, a high strength-to-weight ratio

results in minimum deflection, and the abil-

ity to maintain a full grip across the entire

length of the core or roll face allows air

shafts to handle very high torque. Narrower

rolls can be handled because of low deflec-

tion from loads concentrated near the cen-

ter. Low maintenance and easy in-plant

repair procedures contribute to the popular-

ity and extensive use of air shafts.

Although the air-shaft design is basically

simple, a variety of styles are available and

each shaft is custom-made to suit specific

requirements of an application. Each of

these types is designed around one basic

principle, i.e., a metal tubular bar acts as the

load-carrying member. The body of the shaft

has a number of drilled holes or slots into

which are fitted buttons or lugs backed with

steel pressure flanges. Upon introduction of

air into the shaft, an internal air bladder

expands and forces the buttons or lugs radi-

ally outward until the inside diameter of the

core is securely gripped along its full length.

The internal air bladder is made of a tear-

resistant neoprene or similar material and

has bonded ends or metal fittings to form an

airtight flexible chamber. Air pressure of

approximately 80 psi is necessary to ensure

that the outward thrust is sufficient to grip

the core. When the air is released, the shaft

deflates, causing the spring-loaded buttons

or lugs to retract below the outside surface

of the body and allow fast shaft removal.

There are five basic types of air shafts:

large-button, lug, small-button, leaf and fiber-

glass-sleeve (Figure

). The best applica-

tions are described in the following para-

graphs.

Large Button Type. For rugged, heavy duty

standard core winding or unwinding. Big

buttons 0.625" in diameter are spaced 1.75"

apart in multiple rows.

Lug Type. The lug type of air shaft is similar

to the large-button type, but with bars 0.25"

wide by 3" long instead of buttons.

Small Button Type. For stacked or multiple

cores when slitting and rewinding narrow

webs. Small buttons 0.375" in diameter are

spaced 0.75” apart in multiple rows. This

shaft has the capability of gripping individ-

ual narrow cores stacked on the shaft even if

the core tolerances vary from one core to

the next.

Leaf Type. For rugged heavy-duty coreless

winding or where thin walled cores are

used, this type provides a continuous grip-

ping action throughout the entire expand-

ing face, making it suitable for winding

multiple rolls or rolls without cores.

Fiberglass-Sleeve Type. A variation of the leaf

type, this air shaft is used when light-han-

dling weight is of prime importance. Two

fiberglass sleeves used in lieu of leaves and

in conjunction with a high tensile aluminum-

shaft body account for the weight saving and

make for a lightweight rugged unit.

SPECIAL AIR SHAFTS

There are many specially designed models

of air shafts for specific requirements. Four

unique and popular types are described

PRESSES AND PRESS EQUIPMENT 59

Four of the five basic

types of air shaffts. The

large button type is best

used with heavy duty

winding and unwinding,

while the small button

type is bet used with

multiple cores. The leaf

type is effective in core-

less windings, while the

fiberglass-sleeve type is

used an an altenative to

the leaf type in light-

weight applications.

Large Button Small Button

Leaf Fiberglass Sleeve

60 FLEXOGRAPHY: PRINCIPLES & PRACTICES

below. However, thousands more have been

built for just about every conceivable wind-

ing or unwind operation and it would be

impossible to describe each one. Torque fig-

ures and additional engineering data are

available from manufacturers. In general, air

shafts have a superior performance record

versus mechanical shafts.

Fixed Leaf Type. This shaft gives maximum

concentricity in close-tolerance cores. One

leaf is located and secured in the fixed posi-

tion, its radius matching the inside diameter

of the core (Figure

). The remaining

leaves expand and retract in the normal

manner, supplying the gripping effects and

forcing the core against the fixed radius of

the leaf. The core is therefore securely held

concentric with the shaft body in between

the leaves and turned to a desired outside

diameter (determined by the core) for a sim-

ilar effect.

Trapper Leaf Type. In coreless applications,

this leaf type grips the leading edge of the

web or webs (Figure

). One leaf is fixed in

the expanded position with a bar (or small

button) used to secure the web material

against the underside of the fixed leaf when

the shaft is inflated. The web is released

when the shaft deflates, allowing its with-

drawal from the finished wound coreless

roll. Adhesive tape or any other method used

to secure the web or webs for start-up is

eliminated.

Square Shaft. The unique design of this type

enables it to securely grip square bore cores.

The pneumatic feature eliminates the clatter

and vibration usually associated with this

type of application in square bore, metal or

wooden cores (Figure

AIR CHUCKS

Pneumatic chucks offer many advantages

over mechanical types. They insert easily

into the core end and allow the introduction

of air, resulting in a high-torque grip. When

air is released, the chuck slides out easily

without damage to the core ends. From an

operator standpoint, the simple procedure

A fixed-leaf type special

air shaft gives maximum

concentricity in close-

tolerance cores.

A trapper-leaf type spe-

cial air shaft is used in

coreless applications.

This leaf type grips the

leading edge of the web

or webs through use of

a bar or small button.

The unique design of

the square type air shaft

enables it to securely

grip square bore cores.

Inflated

Deflated

Roll Core

Inflated

Deflated

Inflated

Deflated

Roll Core

of inserting air pressure makes chucks easy

to use and as with air shafts, contributes to

their popularity and extensive use.

Maintenance, when required, is extremely

simple and easily carried out on site.

Air chucks are bored to fit over and clamp

onto a variety of core bars, round or square,

and in some cases are mounted on smaller

air shafts.

Slitter Knife Mounting. These special shafts

are designed and custom built to extremely

close tolerances for mounting and driving

slitter blades, individual hubs and other sim-

ilar slitting equipment.

PRESSES AND PRESS EQUIPMENT 61

62 FLEXOGRAPHY: PRINCIPLES & PRACTICES

he simplest approach to control-

ling a web is edge guiding. It is

generally applicable in cases

where it is desirable to maintain

the edge in constant reference

to the press and where the sen-

sor can be repositioned in accordance with

changing web widths. Control systems are

available with capabilities for guiding on

either edge, center guiding, or electric eye

line guiding.

Sensing of the web edge is generally accom-

plished using a pneumatic detector that pro-

vides a low-pressure signal proportional to

the web position. Photoelectric sensing is

generally used only in those cases where

guiding relative to a guideline registered with

the printing is required to accommodate criti-

cal slitting requirements on the press.

Center guiding is used in situations

where web widths vary during operation

and it is therefore desirable to keep the

material centered relative to the press,

rather than referenced to one edge. This

kind of guiding can be accomplished

through the use of fixed sensors located

on each side of the web, or through sys-

tems providing automatically adjusted

sensors for continually following the web

as its width varies. The choice between

fixed and moving sensors depends upon

the degree of web width variation.

The hardware for center-guide systems is

more complex and the setup and operation

are more complicated than the simple edge-

guide system. However, in those cases

where the benefits of center guiding are

required, it can be a highly successful way of

dealing with the printing requirement.

WEB GUIDES

The typical web guiding system consists of

several components integrated into a closed

control loop. Figure

illustrates a system

showing the sensor, controller, hydraulic

actuating cylinder and web. No particular

type of guiding apparatus is shown in this

illustration, since it depends on the specifics

of the installation.

In the typical closed-loop condition control

system, the set point, or command, is the pre-

positioning of the sensor on the press to

determine the guide point. The sensor then

produces error signals, which go to the con-

troller. At the controller, the servo valve

translates the low-level error signal into a

high-level hydraulic output to the guide cylin-

der. The guide mechanism, in turn, produces

an output. This output is a velocity differen-

tial across the web in response to the flow

from the controller. This velocity differential

is transmitted to the web through the guiding

device, and the web is repositioned at the

sensor, providing the necessary feedback.

Web Guiding

This air-pressure

hydraulic guiding

system includes the

sensor, controller,

hydraulic actuating

cylinder and web.

Note no particular type

of guiding apparatus is

shown, since it depends

on the specifics of the

installation.