Roll Forming Handbook / Edited by George T. Halmos

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The roll design, including the bolts holding the rollscan determine the narrowest product

that can be made on the mill.

Bottom and top hold-down devicesmay havetobemade easily removable or adjustable

to suit different widths.

Sometimes, it is awkwardtocheck the shaft shoulder alignment in cantilever mills, and

the alignment and parallelness in duplex mills. The alignment can be made easier and be

checked much quicker with astraight edge if the rolldesigner speciﬁes that the length of

all rolls should be kept within ^ 0.0005 in. (0.013 mm).

(Figure 5.51), can have25to60% less deﬂection (or in other words, 25 to 60% higher load

capacity) than the cantilever mill withthe same shaft diameter and roll length.

In these mills, the drive-side rolls are attached to the drive-side stands and the operator-

side rolls to the operator-side stands (Figure 5.52). The rolls are lockedonsleeveswhich rotate

with the shaft but allow movement along the axis of the shafts when the stands are moved in

or out.

Most items that havetobeconsidered when designing rolls for duplex mills also have to be

takeninto consideration for the duplex through shaft mills. The differenceisthat due to the

added sleeves, the roll diameters of the throughshaft mill will be larger and the horizontal

centers are usually greater.Adjustable center-supportrollsfor the product can be installed on

the shafts.

(d) Standard Mills (Shafts Supported at Both Ends).The majorityofroll forming mills havethe

shafts supported by bearings at both ends (Figure5.53). Most of the previously mentioned

factors and several others havetobeconsidered for standard mills.

FIGURE 5.50 Forming of asymmetrical sections requirebetter guides than asymmetrical one.

FIGURE 5.51 Through shaft duplex mill.

Roll Forming Handbook5 -32

5.5.3 Shaft Diameter

The shaft diameter is established by the mill

manufacturer,taking the maximum thickness,

maximum strength and the maximum width

of the material into consideration. It is the roll

designer’sresponsibilitytopointout before

roll design starts if in his/her opinion the shaft

diameter of the mill is not sufﬁcient to form the

product.

The “makeshift” arrangements to support

insufﬁcient diameter bottom shafts or top shafts

with rollsorapplying ayoketohold the top

and bottom shaft between stands together are

not recommended. This “emergency” measure

makes mill adjustment and rollchanges very

difﬁcult and it should be avoided.

5.5.4 Horizontal Distance

Horizontal distanceisthe dimension from the shaft center to shaft center in the horizontal direction

(see “HD” in Figure5.54). This pass-to-pass distanceiscritical when forming the following products:

FIGURE 5.53 In “standard” mills, the shafts are supported at both ends. (Courtesy of Ardcor.)

FIGURE 5.52 In through shaft mills, the roll supporting

sleeves are attached to the stands.

Roll Design 5 -33

(a) Wide Panels.The bend line not only movesupwards, but the edges and awideﬂat segment of the

coil moveinwards (Figure5.11a). If the space between the stands is not sufﬁciently long,then the

product will be wavy and distorted.

Example

Aset of rolls produces good-qualitywall panels in

amill with a24-in. (610-mm) horizontal distance.

The same set of rolls is placed on another mill

which has the same number of passes, same shaft

diameter,samerollspace andsametop-to-

bottom shaft rpm ratio,but the horizontal center

is shorter (e.g., 21 or 18 in., or 530 to 460 mm). It

is conceivablethat the same rolls will not form

acceptable qualitypanels, especially from lighter

gauges in the mills with shorter center distance.

(b) Deep Sections.Deep sections, particularly

those made fromthick or highstrength

material,require sufﬁcienthorizontal

center so they can be formed without

undue stresses. The deeper the section,

the larger diameter the forming rollswill

be. The larger the rolls are, the larger the

FIGURE 5.55 Roll forming deep section in amill with

shorthorizontal center will generate undue highstresses.

FIGURE 5.54 Mill dimensions must be considered during roll design.

Roll Forming Handbook5 -34

horizontal centersshould be (Figure

5.55). Too-short horizontal distancewill

create ﬂare, waviness, and other devi-

ations from straightness and ﬂatness.

Example

It is planned to roll form 6-in. (ca. 150-mm)

deep sections from0.375-in. (9.5-mm) thick

material in amillwith5-in. (125-mm)

diameter shafts. The companyhas three mills,

one with 24-in. (610-mm), one with 36-in.

(915-mm), and one with 48-in. (1220-mm)

horizontal centers.

The minimum roll diameter is assumed to

be 8in. (203 mm), and the maximum roll

diameter will be twice the section depth plus

lead-in length, say, 24.5 in. (622 mm). The

mill with the 24-in. horizontal center cannot

be considered because the rolls will not ﬁt on

that mill. The mill with 36-in. (915-mm)

horizontal center will provide amere 9-in.

(229-mm) spacebetween rolls while the 48 in.

(1220 mm) mill will have23-in. (585-mm)

spacebetween the rolls. The 36-in. (915-mm)

center distancewill allowonly limited form-

ing; therefore, morepasses are required. The

spacebetween the rolls is not sufﬁcient to

installand adjust suitable sizeside-rolls.

Therefore, the best mill on which to form

the section is that with 48-in. (1220-mm)

horizontal center distance.

Convenience.The horizontal distance

is notcritical when shallow,small

sections are formed (Figure5.56). In

this case, deformation is restricted to afew inches ahead of the contact points of the rolls.

Therefore, whether the horizontal distanceis8,12, 16, or 20 in. (200, 300, 400, or 500 mm) will

havenoinﬂuence on the roll design.

However,insome rollforming mills, especially those designed to form shortprecut sections,

the rolls are so close that it is difﬁcult, if not impossible, to install and adjust the side-rolls or other

units. Side-roll stands are occasionally installed at the “empty” roll stands which do not havemain

rolls or are mounted on the stands. Nevertheless, tight space can makesetup and checking of the

roll gaps withfeeler gauges extremelydifﬁcult.

(d) Short Precut Pieces.Toguide ashortpiecethrough the mill, the minimum length of the partshould

be alittle bit morethan twice the horizontal center distance(HD; Figure5.57). For example, if the

HD of amill is 8in. (200 mm), then the shortest precut pieceshould be at least 2 £ 8 þ 1 ¼ 17 in.

(425 mm), assuming that the partlength is at least double of the blank (strip) width (see also

Section 5.5.2b).

FIGURE 5.56 The horizontal center (HD) may not be

important when shallow sections are roll formed from thin

or low strength material.

Roll Design 5 -35

5.5.5 Vertical Distance

The vertical distance (VD) of Figure5.54 is

the maximum distancebetween the center of

thetop andbottomshafts. Thevertical

distanceisusually adjustable and if so should

be expressed with the minimum and maxi-

mum values. Aminimum vertical distanceis

required to accommodate thetop and

bottom rolls and the section in between

them. Figure 5.58 shows twosections. The

maximum height of the section to be considered as the limiting factor is not necessarily the ﬁnished

section, and it can be at one of the previous passes.

The maximum and minimum distances shown in the equipment sales literatureare occasionally not

accurate. When the vertical distancewill be close to the maximum or minimum, it is advisable to check

the actual maximum and minimum vertical distances of the equipment before roll design commences.

When the designer opts for the minimum vertical distance(the smallest possible roll diameter) for

economical reason, then some “extra”should always be added to the rolldiameters as a“safety” factor

and for futurerecuts.

It is not always necessarytohavethe same vertical centers at all passes of the mill. Increasing the

vertical centers as the section grows in height is quite common.

In the case of being in apredicament, the vertical distance maybeincreased by placing spacers

between the stands and the horizontal cross-bars as shown in Figure 5.59.The thickness of the special

spacers is limited by the size of the stand, engagement of the bearing blocks, and the method of driving

the top shaft. Link types of gear head maynot be suitable for this solution unless the top drive is

disconnected.

5.5.6 Distance between the Bottom Shaft and Mill Base

In most (althoughnot all) mills, the bottom shaft is in aﬁxedposition and is not adjustable. The distance

between the center of the shaft and the mill base (or the highest point anywhere underneath the shaft)

constrains the maximum bottom roll diameter.Inspite of such aclear-cut and simple fact, many

designers made the mistakeofspecifying rolls withlarger diameter than the space available for

installation (Figure5.60).

FIGURE 5.58 Maximum section height ( H

max

)must ﬁt between rolls.

FIGURE 5.57 The length of aprecut piece should be

morethan twice the horizontal distance between passes.

Roll Forming Handbook5 -36

5.5.7 Key Sizes and Keyways

The keysizes varyfrom mill to mill, and fromroll

set to roll set due to the lack of anystandard.

The keysize is afunction of the transmitted

torque while the shaft diameters are designed for

deﬂection. However,key sizes can be standardized

to suit shaft diameters.

Delta’s “Roll Former’sGuide

”software uses

Equation 5.2 to calculate keysizes. The calculated

size has to be rounded (usually up) to the next

standardkey size.

ð imperialÞ W

inch

¼ 0 : 11d

0 : 65

þ 0 : 002ð D

1 : 43

max

Þþt

0 : 51

ð metricÞ W

¼ 0 : 34d

0 : 65

þ 0 : 000506ð D

1 : 43

max

Þþ4 : 8 t

0 : 51

ð 5 : 2 Þ

where

W ¼ keywidth

d ¼ shaft diameter

max

¼ largest roll diameter

t ¼ metal thickness

k ¼ keyheight (frequentlyequal to W )

The roll designer should incorporate matching keyways into all rolls and spacers, except the shoulder

alignment spacers. The gap between the key and keyway in the rolls and spacers (“b ”dimension) should

be sufﬁciently large to assure easy installation and removal of the rolls.

Keyway width in rolls ¼ key width þ b

Keyway depth in rolls

¼ one half of key height þ b or 2 b

Forrecommended “ b ”dimensions in small and

medium size rollssee Table 5.2.

Forlarge diameter rolls, the key sizes must be

increased because the inertia of 10- to 20-in. (250

to 500-mm) diameter rolls will destroysmall

keyways in the shafts if the mill is frequently and

quickly stopped and started.

In these large diameter rolls, the keyway should

be only slightly wider than the key(b ¼ 0.005 to

0.010 in. ¼ 0.13 to 0.25 mm).

The “ b ”clearance also depends on the accuracy

of the keycutting equipment.

5.5.8 TopShaft Drive

The rolldesigner must knowifonly aselected

number of top shafts are driven. Driven top

shafts, with added friction, provide abetter strip

“pull” from the uncoiler and moreevenspeed.

FIGURE 5.60 Maximumbottom roll diameter is

restricted by the distance between the bottom shaft and

mill base and other protrusions.

FIGURE 5.59 Ve rtical height (HV)can be increased in

certain mills.

Roll Design 5 -37

Nondriventop shafts are less expensive, but the mill maynot havesufﬁcient friction to pull the material

when only the ﬁrst few passes are engaged. Mills with bottom-driven-only last passes may not generate

sufﬁcient “pull” (tension) to avoidbuckling between the passes and to push the material through the

straightener or curving unit and the cutoffdie.

Formills with partial spur-gear drives at selected locations, the roll designer must specify the passes

which requiretop shaft drive. Usually,atleast the ﬁrst two and the last twopasses are driven. Depending

on the circumstances, additional passes could also havetop drives. If aspur-gear on the bottom roll

directly drives aspur-gear installed on the top roll(or vice versa), then the gear pitch diameters will also

determine the pitch diameters of the rolls. The roll pitch diameter must match the gear pitch diameter

with allowancetoadjust for material thickness

variation and limited recuts of the rolls.

Rollforming mills with the facilityfor quick

disconnection of the top rolldrive are the most

versatile for selecting the appropriate top shafts

to be driven.

In deep sections, the surface speed differential

between different parts of the rollsand the

material can be large. To minimize the friction,

not only the top shaft is nondriven, but indivi-

dual segments of therolls areoccasionally

allowedtorun freeonbearings installed on the

shafts (Figure5.61). The reduced friction will

reduce tool wear,horsepowerrequirement and

surface scratches. It is the tool designer’sfunction

to determine which rolls should be free-running.

The roll designer also has to cooperate with the

machine designer to ensurethat the bearings of

the free-running rollscan be properly lubricated.

5.5.9 Bottom Shaft Drive

It is accepted by the roll forming industrythat, as arule, all bottom shafts are driven. However,

occasionally not driving some bottom shafts has advantages.

Example

Abottom-drivenmill, with afew top-drivenpasses, forms abuilding panel withseven ribs (Figure5.62).

The forming startsbymaking the center ribs in ﬁve passes. Forming continues with the adjacent two ribs,

made in the next ﬁve passes, then the next tworibs in ﬁve passes, and, ﬁnally,the two outside ribs in six

passes. As the rib heights are gradually formed to full height, the diameter of the driven bottom rolls

increases. If the section depth is 1in. (25.4 mm), then the circumferenceofthe larger rolls is about 6in.

(152 mm) larger than the rolls that starttoform the ribs. Because the shafts are running at the same rpm,

the last few rollsforming each rib are “pulling” the material faster then the ﬁrst tworolls. Actual

observation showed that passes 1and 2, 6and 7, 11 and 12, and 16 and 17 (circled in Figure5.62), which

starttoform the ribs, havenegativetorque, or in other words, are breaking,pulling back the formed

section. The ﬁrst twopasses cannot be disconnected because they are required to pull the front end of

FIGURE 5.61 “Free-running”rolls reduce friction

between rolls and formed sections.

TABLE 5.2 Keyway Width in Rolls

KeyWidth b

3/16 to 1/2 in. (5 to 12.5 mm) 1/16 in. (1.5 mm)

Over 0.5 in. (12.5 mm) 1/8 (3 mm)

Roll Forming Handbook5 -38

each coil into the mill, but disconnecting the other 6passes (6, 7, 11, 12, 16, and 17) would reduce the

energyrequired to form the panel, would reduce tool wear and surface marks. It would also provide less

ﬂuctuation in the panel speed, which would improvelength tolerance. Unfortunately,the construction of

most mills does not allowobserving the negativetorque (braking) without instrumentation. (Note that

the actual forming was different then described above; the forming sequence has been simpliﬁed for this

example.)

5.5.10 Rpm Ratio

To establish the roll diameter,the rolldesigner must knowthe rpm ratio between the top and bottom

shafts. Aratio is usually given by the manufacturer as 1:1, 1:1.33, 1:1.42 or other ﬁguresthat are based on

the gear ratio of the drive. The above-mentioned ﬁgures mean that the bottom shaft runs at the same

rpm, or 1.33 or 1.42 times faster then the top shaft. Therefore, the bottom roll diameter can be 1.33 or

1.42 times smaller than the top diameter.This arrangement is economical for “U” channels and for

similar sections formed with legs up,but it is not economical, and in most cases not suitable for sections

with legs down.

The rolldesigner must also check the means provided to quickly disconnect the top rolls and the

necessitytoapply drive to the top rolls or to use freerunning rolls.

5.5.11 Pass Line Height

Pass line is the distancebetween the mill bed and the bottom of the section at the last pass, at the point

wherethe surface speeds of the top and bottom rolls are equal (Figure5.63). It is prudent to havethe pass

line height for each mill established (standardized) by the plant management. If possible, all the sections

formed in mill should have the same pass line height. This will eliminate the time-consuming

adjustments of the entryguides and the straightener height, and costly cutoffpress height adjustments.

Oncethe pass line and the cutoffpress heights are established, the press position need never be adjusted.

The pass line height speciﬁed by the management should be checked by the roll designer to ensurethat

it does not interfere with good forming practice. Owing to the small increase in roll diameters from pass

to pass, the pass line is most frequently not parallel to the mill bed as explained in Section 5.6.4d.

FIGURE 5.62 A7-rib panel roll formed in 21 passes (schematic representation).

Roll Design 5 -39

5.5.12 Additional Items

The roll designer must know whether entryguide, straightener,lubrication, and other critical items are

available on the mill.

5.5.13 Direction of the Line

The direction of the line (left to right, or right to left) is importantasitdetermines the operator side of

the mill.

5.5.14 Motor and Roll Forming Speed

It should be discussed beforeroll design commences if, in the opinion of the roll designer,the speciﬁed

horsepowerappears to be insufﬁcient or if the line speed is not correct.

The calculated speed is always an estimate. The actual speed will be inﬂuenced by the speed differential

between the rollsand the strip,friction modulus (lubricant), setup (pressures), and other factors.

5.5.15 Available Number of Passes

The roll designer must check whether the estimated number of passes required to form asection is

available in the mill. If thereare not enoughpasses, then the design may havetobedifferent and more

side-roll passes havetobeincorporated, or additional passes havetobeadded to the mill. If these

improvements are not feasible, then the design assignment may havetobedeclined. One of the most

common mistakes made by roll designers is trying to form the product with fewer passes than required

for good forming simply to match the available number of pass on the mill. Sometimes, the designer does

not haveany other alternatives, but this approach frequently leads to inferior product quality,

productivityproblems, or completely discarded tooling.

5.6 Other Tool Design Considerations

5.6.1 Roll Change Requirements

Rolls havetoberemoved and reinstalled on the mill shafts frequently or occasionally,but at least once in

everysix months. The type of the mill and the frequency and method of roll changing haveaprofound

inﬂuence on roll design.

(a) Duplex Mills.Inmost duplex mills, the rolls are held in their position by abolt threaded into the

center of the shaft. The majorityofthe duplex mills are dedicated lines and do not requireroll

changes. Therefore, the interchangeabilityofrolls is seldom considered during roll design.

(b) Standard Mills.Roll sets are designed for speciﬁc proﬁles. When the proﬁle is changed, the

complete set of rolls is removed and replaced with the rolls for the new proﬁle. Multipurpose

rolls, usually employedfor the same proﬁle formed in different widths, are changed by partially

FIGURE 5.63 Pass line height above the mill bed is where the surface speeds of the top and bottom rolls are equal.

Roll Forming Handbook5 -40

removing and exchanging the sequence of rolls or spacers. Wherequick-change spacers are used

the rollscan be moved sideways on the shafts without removing them.

serious design consideration. However,occasional removal of the rolls (say,once in every

6months) is still recommended to prevent the “seizing” of the rolls on the shafts by rust, loose

particles, or aged lubricant.

5.6.2 Splitting Rolls

Rolls are split into smaller lengths for several reasons.

(a) Roll Weight.The price of arollset is related to both the number of rolls and the weight. The

weight of aset of rollscan seldom be reduced but designers often attempt to reducethe price by

making the longest thus the least number of rolls. However,roll lengths are also inﬂuenced by

other factors.

One consideration is the weight of asingle roll. It is difﬁcult to install and remove rolls

weighing over 50 to 70 lb (22 to 31 kg). At some companies, an even lowersafe lifting weight is

speciﬁed by safetystandards, regulatorybodies, or union agreement. However,tofully conform

to the weight limit, large diameter rollswould yield rolls so short(thin) that they would be

distorted during heat treatment. The weight of large diameter and dimensionally stable rolls can

be considerably abovethe safe weight limit. In this case, as well as speeding up roll change, it is

sensible to consider mechanized roll unloading/loading.

(b) Mechanical Installation/Removal.After the removal of the outboard stand, one method is to place

“dummy” shafts to the end of the mill shafts and slide the rolls from the mill shaft to the dummy

shaft. The dummy shaft has to be mechanically held in position and lifted. It is the roll designer’s

responsibilitytopoint out to the customer if the rollweights are over the safe lifting limit.

6in. (150 mm). The gap between the shaft and boreofthe rolls is so small that the smallest bend

in the shaft will make it difﬁcult or impossible to install or remove the rolls. To minimize this

problem, it is acommon practicetorelievethe center partofthe bore of rolls longer than 2.5 to

3.5 in. (65 to 90 mm; for further details, see Figure 5.64). If the boreissorelieved, then it is

important to check the load on the remaining contact surface with the shaft. To oshorta

roll/shaft contact area can damage the surface of the shaft.

Rolls that are too short(thin) result in other problems. First of all, it is difﬁcult to keep thin

rolls ﬂat during heat treatment. The second difﬁculty installing them on or removing them from

the shafts. Thin rolls can easily jam on the shafts. To avoid jamming,experienced operators use

the next longer roll or spacer to keep the thin rolls perpendicular to the shaft during

installation/removal (Figure5.65). Thin rolls are also more susceptible to cracking or chipping.

(d) Machinability.Turning technologyand the geometryofthe turning tools limit the shape that can

be turned. Figure5.66 shows atypical example when rolls havetobesplit to facilitate machining.

(e) Multiple Applications of Rolls.The location of roll splitting can be dictated by multiple product

widths. For example, if “U” channels are formed in 4-, 6-, and 8-in. (100-, 150-, and 200-mm)

FIGURE 5.64 The bore of long rolls is relieved to ease installation.

Roll Design 5 -41