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PRESSES AND PRESS EQUIPMENT 133

Helical Gears. The helical gear (Figure

)

has its teeth machined in the form of a helix,

at an angle to the gear body. The angled

teeth permit the helical gears to have multi-

ple teeth engaged at the same time, allowing

higher torque loading. Because of this differ-

ence, helical gears can have finer teeth

(more teeth per inch or centimeter) than

spur gears used to transmit the same torque.

Helical gears are used to transmit power

between two parallel shafts. Helical gears are

the most common type of gearing used to

Helical gears have teeth

machined in the form

of a helix, at an angle

to the gear body. This

allows multiple teeth to

be engaged simultane-

ously, providing higher

torque loading

Helical gear register

control between color

stations is achieved by

lateral movement of the

plate cylinder gear rela-

tive to the impression

cylinder gear.

Bevel gears are used to

transmit power between

two shafts that are at

right angles to each

other.

Helical Gear

Plate Cylinder

Helical Gear

134 FLEXOGRAPHY: PRINCIPLES & PRACTICES

drive flexo plate cylinders. They can have

smaller teeth than spur gears, require less

running clearance and therefore have less

backlash. The angled teeth allow for plate

cylinder color registration without using addi-

tional gears, such as in a differential gearbox.

The fewer gears there are in the drive chain,

the fewer the gear meshing points and, as

each mesh point must contain a degree of

backlash, the smaller the total amount of

backlash. The result of using helical gears is

improved multicolor registration tolerance.

tions is achieved by lateral movement of the

plate cylinder gear relative to the impression

cylinder gear (Figure

) The amount of

slope angle and the degree of sideways

movement of the gear. This phase shift is

limited to approximately two gear pitches in

either direction on each print station.

Bevel Gears. Bevel gears are used in con-

junction with spur gears as a register con-

trol device on the indirect (swing gear)

impression-drum drive system. They are

incorporated into a differential gearbox,

which can change the register of one plate

cylinder in relation to the other cylinders. A

differential register correction system has

an important advantage over the system

used on direct drive flexo presses in that it

has the ability to rotate the plate cylinder

360° relative to any other cylinder. The heli-

cal gear-color registration system used on

direct drive presses has a limited correction

distance.

Bevel gears are used to transmit power

between two shafts that are at right angles to

each other (Figure

). Many right-angle

gearboxes use two bevel gears, and more

sophisticated gearboxes use four bevel

gears. Bevel gears are almost never used to

directly transmit power to the plate or anilox

rollers, because the shafts of these rollers

are parallel to the impression-roller shafts.

Worm Gears. Worm-gear sets are used as

speed reducers and as right-angle gearbox-

es. A special type of worm-gear system

called the sliding-worm gearbox is used as a

registration device on some types of printing

presses. Worm gears are never used to pro-

pel the plate cylinder on flexo presses, but

they are used in other locations on the press

such as for print deck impression control.

Right-angle, speed-reducing gearboxes

use worm gears, which consist of a worm

shaft and a worm gear (Figure

). The

worm shaft is a cylindrical gear with a spe-

cial screw profile machined to mesh with the

round worm gear. The worm gear is a special

type of round gear with teeth that are con-

cave to match the profile of the teeth on the

worm shaft. More than one tooth may be

meshed together in a worm-gear assembly

and the number of meshed teeth is deter-

mined by the number of helical threads on

the worm. The multiple meshing of teeth

provides maximum torque transmission, as

compared to spur and bevel gears.

Gear Materials. Gears are made from many

different materials. The bull gear, which is

mounted on the impression drum, is usually

made from cast iron with surface-hardened

teeth. The anilox and fountain roller gears are

usually made from steel or hardened cast iron,

although some presses use synthetic gears for

these purposes. The plate-cylinder gears can

The multiple meshing of

teeth in a worm gear

provides maximum

torque transmission, as

compared to spur and

bevel gears.

PRESSES AND PRESS EQUIPMENT 135

be made from annealed steel or synthetic

composite materials.

The press manufacturer specifies all gears,

except those for the plate cylinder. The flexo-

grapher can select the type of gear material

for his plate cylinders. The plate-cylinder gear

must be softer than its drive gear, so that it

will wear out first. It is easier to replace a

plate-cylinder gear than a bull gear or other

internal machine gears. Many flexographic

printers prefer synthetic composite or plastic

plate cylinder gears because they help reduce

drive gear vibration and shock waves.

Gear Backlash. There must be a small amount

of space between meshing-gear teeth to

allow them to mesh correctly with sufficient

lubrication and to become engaged and dis-

engaged easily. However, gear wear will

increase this space, and the amount of back-

lash in any set of gears gradually increases.

Excessive backlash is a condition in which

there are larger-than-normal spaces between

successive gear teeth, normally caused by

incomplete meshing or tooth wear. Backlash

is detrimental to high quality printing and can

occur on all types of gears. Backlash allows

the position of the driven cylinder to change

in reference to the drive cylinder, which in

turn causes misregister. The number of gears

in the drive chain multiplies this register

error. Excessive backlash makes it impossi-

ble for the operator to maintain accurate mul-

ticolor register and therefore it is essential to

replace one or both gears in a set that has

developed excessive backlash. Vibrations

associated with backlash also increase plate

bounce and other print quality problems.

There are two types of gears that allow

maintenance personnel to remove all exces-

sive backlash without having to remove or

replace the gear set; these gears are collec-

tively known as anti-backlash gears. An anti-

backlash gear consists of two narrow width

gears whose angular position can be

changed so that there is less space between

each tooth. The screws on the side are loos-

ened, and one thin gear is shifted or offset,

as indicated in Figure , to remove excess

backlash. Another type of anti-backlash gear

is the sliding-worm type, which also allows

for the removal of backlash without removal

of the gears.

Gear Train. The impression cylinder (drum

or roller), the plate cylinder and the anilox

roll all must rotate with exactly the same

surface speed as the web speed in the flexo-

graphic printing process. If any of them is

turning at a slightly different rate, there will

be severe printing problems as well as

excessive plate wear. On a gear-driven press,

matching the speed of these three rollers

with the web is accomplished through the

use of a gear-train power transmission sys-

tem (Figure ). Typically, the power is

An anti-backlash

spur gear is actually

two gears. By changing

the position of one

relative to the other,

tooth spacing can be

altered, thus removing

excess backlash.

On a gear-driven press,

constant speed between

all rollers is ensured

since a main roller

delivers power to all

the rolls.

Counter

Impression

Roll

Driver

Driven

Plate

Cylinder

Anilox

Roll

Doctor Roll with

Separate Drive

Tooth Offset

136 FLEXOGRAPHY: PRINCIPLES & PRACTICES

delivered from the main motor to the

impression rollers or drum by a single drive

system. A separate gear-train transmission

on each printing deck then delivers the

power from the impression roller to the

printing and anilox rollers; the pitch of the

gears on all three rotating parts is the same.

A separate drive motor generally drives the

inking or fountain roller.

The diameters of all impression, printing

and inking rollers must match the pitch

diameter of the gear that propels them. Pitch

diameter is the diameter at which the gears

operate, not the internal or external diame-

ter of the teeth. The pitch diameter of a gear

is represented by the shaded portion of the

bottom gear in Figure .

It is extremely important that the gears

always operate on their pitch diameter,

because if they do not they can create a

speed mismatch and/or vibrations that will

cause print quality problems and excessive

plate wear. It requires correct printing-plate

mounting and a skillful press operator to set

all of the gears on the moveable anilox and

plate rollers so that they turn on their exact

pitch diameters. If one gear is set past the

pitch diameter, it will cause that roller to

rotate at a slower speed than desired. Con-

versely, if one gear is meshed before the

pitch diameter position, the roller that it dri-

ves will rotate faster than desired.

Repeat-length Increment

On a direct gear driven press, it is not pos-

sible to have a different gear pitch on the

The diameter at which

the gears operate (the

shaded portion on the

bottom gear) is the

pitch diameter of the

gear.

Root Circle

Pitch

Point

Base Circle

Whole Depth

Pitch

Circle

Diameter

Backlash

For a Spur

Gear

Tip Circle

Root Circle

Pitch Circle

Bottom

Clearance

impression drum or rollers than is used to

drive the plate and anilox rollers. Therefore,

the printing repeat-length of a flexo press

can only be changed in increments that are

equal to the pitch of the gearing driving the

plate cylinder.

Gear Measurement. Gear pitch is the distance

measured from the same point on each tooth

of adjacent teeth on a gear, and is measured

along the pitch diameter of the gear.

Incremental increases in printing repeats on

gear-driven presses is restricted to the dis-

tance between the teeth, or gear pitch.

The three methods for specifying gears are:

• Circumferential Pitch. The circumferential

pitch (CP) of a gear is obtained by dividing

the pitch-circle circumference of the gear

by the number of teeth on the gear.

CP PITCH CIRCLE DIAMETER

NUMBER OF TEETH

• Diametral Pitch.The diametral pitch (DP)

of a gear is obtained by dividing the num-

ber of teeth on the gear by the pitch-circle

diameter of the gear in inches (in).

DP  NUMBER OF TEETH

PITCH CIRCLE DIAMETER (IN)

• Module Pitch. The module pitch of a gear

is obtained by dividing the pitch circle

diameter of the gear in millimeters (mm)

by the number of teeth on the gear.

MODULE  PITCH CIRCLE DIAMETER (MM)

PITCH

NUMBER OF TEETH

The most common gear used on a flexo

press is a 0.25" CP. This gear has spaces of

0.25" between adjacent teeth, measured

along the circumference of the pitch diame-

ter of the gear. The next most common pitch

for flexo gearing has increments between

the teeth of 0.125". The most common

diametral pitch gear is 10. This means there

are 10 teeth for each 1" of diameter and

increments of 0.314" (10/in DP) between

adjacent teeth, as measured along the cir-

cumference of the gear-pitch diameter.

From the above formulae, given the speci-

fication of the gear (Module, DP or CP) and

the number of teeth on the gear, it is a sim-

ple calculation to find the rolling diameter of

the associated cylinder or roll. Tables are

given for reference in the Appendix.

Pitch and Bare Cylinder Diameters. When a

gear is mounted on the shaft of a plate cylin-

der (Figure ), the printing diameter of

the combined plate, mounting system and

bare cylinder must match the pitch diameter

of the gear. Therefore, the printing diameter

is the diameter of the bare plate cylinder

plus twice the thickness of the mounting sys-

tem and plate. For example:

A customer requires an image on a 16"

printed repeat;

• This repeat length would require a 64-

tooth gear with 0.25" CP;

• The pitch diameter of the gear would be

5.093";

• The press uses 0.107" plates mounted

with 0.020" foam tape for a total thick-

ness of 0.127";

• The bare plate cylinder diameter would

be 5.093"(2  0.127")4.839";

• Include 0.002" for plate squeeze (this

will increase the diameter), for a final

diameter of 4.839”  (2  0.002") 

4.843".

Plate-squeeze Allowance. In order to transfer

ink from one cylinder to another in the flexo

process there must be some squeeze

between the plate and the anilox or impres-

sion roll. If the mounted plates and the plate

cylinder were exactly the same size as the

pitch diameter of the gear there would be

zero printing pressure (plate squeeze) when

the gears were correctly set to turn on their

pitch diameters. The operator would not

obtain good printing across and around the

cylinder and would need to increase the

printing and inking pressure by moving the

PRESSES AND PRESS EQUIPMENT 137

138 FLEXOGRAPHY: PRINCIPLES & PRACTICES

plate cylinder closer to the impression drum

and anilox roller. This move would force the

plate-cylinder gear to over-mesh with the

other gears (running under the pitch diame-

ter), causing vibrations and typically “gear

striped” printing.

To overcome this problem, flexographic

printers allow for plate squeeze, by either

increasing the diameter of their plate cylin-

ders, or by increasing the overall plate

mounting height. The typical plate-squeeze

allowance is approximately 2% of the overall

plate height, including the mounting tape.

Gear Mounting

The gears that drive the plate cylinder and

the anilox roller are mounted on their respec-

tive shafts. The gear driving the anilox roller is

mounted permanently, on a one-way (Sprag)

clutch to allow for continuous rotation of the

anilox when the print cylinder is stationary.

The gear on the plate cylinder shaft can be

mounted permanently or temporarily.

Gears are precise parts of the flexographic

printing system, and if they become damaged

or worn they will affect print quality and reg-

ister. Many presses are equipped with remov-

able plate cylinders and when several sets of

cylinders have the same diameter, to save

money on gear sets, some flexographic print-

ers mount the gear temporarily on the plate

cylinder shaft and then remove it when the

job is finished. However, temporary mount-

ing of plate cylinder gears creates faster

tooth wear (because the gears are used more

frequently) and increases the possibility of

damaging the gears during the transfer from

shaft to shaft.

It is common to see temporarily mounted

gears with excessive wear cause backlash

and poor register, and succumb to broken

teeth, nicks and other damage marks. Any

faults in the plate cylinder gear are transmit-

ted to a second gear on the opposite side of

the plate cylinder, which delivers power to

the anilox roller. For these reasons, it is rec-

ommended that the gear that drives the plate

cylinder be permanently mounted.

A minority of flexographic printers mount

the gear permanently on the plate cylinder

shaft; that gear is used only when the plate

cylinder is printing a job. This method

reduces the wear and potential damage to

the gears. While it may be more expensive to

purchase a gear for each plate cylinder, it

saves money in the long run by reducing

makeready time, color register and generally

improving the print quality.

Gears and In-line Processing

The vast majority of flexographic presses

with diametral pitch (DP) gearing use 10/in

When a gear is

mounted on the shaft

of a plate cylinder, the

printing diameter of

the combined plate,

mounting system and

bare cylinder must

match the pitch

diameter of the gear.

Notice the horizontl

lines extending across

the printing plate align

with the pitch diameter.

Printing

Plate

Bare

Cylinder

Diameter

Cylinder

Undercut

Pitch

Diameter

Mounting

Tape

DP in favor of a metric gear scale module,

even though the press may have been made

or used in countries that use the metric sys-

tem. The reason for this choice is that the U.S.

standard of 10/in DP has been universally

accepted in the paper bag industry. Paper

bags and related products that employ con-

tinuous web in-line processing are made

almost exclusively with diametral-pitch flexo

presses. With the in-line process, when the

printed web is processed in a continuous

motion, the pitch of the gears on a flexo press

must match the gear pitch of the secondary

machine. If the secondary machine has an

intermittent web flow, the gearing on the two

machines does not have to match. Almost all

intermittently fed secondary machines use a

photoelectric detection system to register the

printed web with the secondary finishing.

Because of the difference between the cir-

cular pitch of the respective gears it is

impossible for a flexo press with circumfer-

ential pitch gears to match the printed image

length of another flexo press with diametral

pitch gears, and vice versa. In addition,

diametral pitch gears and circumferential

pitch gears are not interchangeable.

Dual-gear Systems

One of the most difficult decisions to

make when purchasing a flexo press is the

selection of the gear pitch, or the repeat

increments, of the press. Most flexo presses

can only be purchased with one gear system,

for example 0.25" CP, 10/in DP or 5 mm mod-

ule. A few common impression flexo presses

are built with dual gearing; that is, they can

print in either of two gear pitch (increment)

systems, such as 0.25" and 10/in DP, or 5 mm

module and 4/cm pitch. The impression

cylinder must have a diameter that equals

the pitch diameter of the gears used to turn

them. If a dual pitch of 0.25" and 10/in DP

was desired on a direct-drive press, the

impression cylinder might have a 60.001"

diameter and a 188.5" circumference. The

bull gear used to turn the impression cylin-

der would have 754 teeth .025" CP. The cy-

linder would have a second bull gear 600

tooth 10/in DP with a 188.52" circumference,

attached to the first one. Both of these gears

would have approximately the same pitch

diameter as that of the impression cylinder

(CP gear = 60.001" diameter; DP gear =

60.008" diameter). With dual-gear systems,

the choice of impression cylinder diameter

must always be a compromise between the

combination of CP and DP gear diameters.

Central-Impression Press Drives

For CI presses there are essentially two

types of gear drives, the direct drive and

the quadrant drive, sometimes called a

swing gear.

Direct Drive. In the direct drive, a large bull

gear is usually mounted on the outside of the

frame, with a pitch diameter equal to the

diameter of the central impression cylinder.

Typically the bull gear is a helical gear. The

plate cylinder gear, also a helical gear, is fas-

tened to a female spline and the assembly is

attached to a circumferential register mecha-

nism. A male spline fixed to the plate cylin-

der journal will fit inside the female spline.

With this arrangement, the plate-cylinder

gear can slide back and forth on the male

spline to effect the circumferential register

without affecting the plate cylinder’s side reg-

ister. The opposite end of the plate cylinder is

fastened to the side register mechanism.

A direct-geared press maintains good print

istration accuracy is possible because there

are fewer gears in the gear train and any

inaccuracies machined into the gear train

will repeat themselves on the same tooth of

each color station.

Direct-geared presses also have several

disadvantages. Since the central-impression

cylinder diameter is usually locked into the

pitch diameter of the bull gear, any damage

to the surface of the cylinder once meant

PRESSES AND PRESS EQUIPMENT 139

140 FLEXOGRAPHY: PRINCIPLES & PRACTICES

serious modifications to the gear system.

Today, there are ways to repair the drum sur-

face quite satisfactorily.

The plate cylinder’s impression setting is

made to accommodate the web thickness

and any plate variation. Once the ultimate

impression setting has been achieved, the

amount of backlash between the plate cylin-

der and the bull gear is a factor that must be

accepted. There are times when this excess

backlash can cause bounce problems.

Because of the amount of hardware

involved in driving the plate cylinder, on

some press designs the job changeover time

could be slightly longer on a direct-geared

press. To overcome these productivity prob-

lems, spare plate-cylinder gears are recom-

mended. Mounting them on the cylinder in a

pre-makeready area eliminates setup times.

Quadrant-geared Press. On a quadrant- or

swing-geared press, the bull gear does not

come directly in mesh with the plate cylinder

gear. The bull gear is still a helical gear but, in

this case, a pinion gear mounted on a cross-

drive shaft (usually inside the frame) drives a

compound spur gear on the outside of the

frame. A spur-type swing gear then meshes

with the compound gear after the impression

of the plate cylinder has been made.

Circumferential register is achieved by lat-

eral adjustment of the cross-drive shaft. The

pinion helical gear will be forced to rotate on

the bull helical gear, while the compound

spur gear will simply slide back and forth on

the swing gear. The plate cylinder gear is

clamped directly to the plate cylinder jour-

nal. Very few of this type of CI press are

being manufactured today.

There are a couple of advantages to this

type of press. The backlash between the plate

cylinder and the swing gear can be mini-

mized. The impression cylinder diameter is

not locked in as tightly to the bull gear pitch

diameter in the event of drum damage. The

positioning of the gears eliminates the need

for modifying the gear train for each setup.

Also, the elimination of the circumferential

geared press streamlines plate cylinder instal-

lation. These advantages can result in shorter

setup time on a quadrant-geared press.

The quadrant-geared press has some

notable disadvantages as well. The register

between the groups of print stations on

either side of the press cannot be maintained

as closely as on a direct-geared press due to

the location of the pivot point of the swing

gear assembly. But register between the indi-

vidual print stations within the group can be

maintained with greater register accuracy.

Line-shaft Drives

A line-shaft driven press has a distinct

advantage over straight gear train-driven

machines in that gear backlash from print

unit to print unit is not cumulative. A rigid

line shaft drives each printing cylinder

through a worm or bevel type right angle

gearbox. If one discounts some slight tor-

sion forces on the line shaft, it becomes

apparent that only one gear backlash per

printing unit enters into account. In theory

this means that the gear backlash of only

one printing unit determines the overall

print register tolerances.

Digital-servo Drive

The newest approach in drive technology

on printing machines is the servo drive,

whereby each printing unit has a separate

drive motor, or motors, that are controlled

electronically to rotate at a synchronized

speed with each other. With this technology,

the angular position of the print cylinders

can be controlled with incredible accuracy.

The scale of register accuracy obtained with

these digitally controlled systems is in the

region of thousandths of an inch.

Some manufacturers go as far as driving

every shaft on each printing unit with a sep-

arate motor, while others have one motor

per printing unit.

Servo-drive technology has truly opened

new horizons in the flexibility of flexograph-

ic printing presses. For example, digitally

controlled automatic print registration is

common and, theoretically, repeat lengths

are infinitely variable.

BEARINGS

Webster’s defines a bearing as an object,

surface or point that supports, or a machine

part in which (as a journal or pin) turns

another part. From an engineering stand-

point, a bearing is a device that accepts

motion, combats friction and wear, and sup-

ports the load.

Bearings are of two design groups: plain

(sleeve type) and rolling-element (anti-fric-

tion), with many variations in each group.

A plain or sleeve bearing accepts motion

directly on the stationary support. Friction

occurs between the two bearing surfaces or

within a lubricating film between surfaces.

The rolling element bearing is one in which

motion and load are accepted by rolling ele-

ments located between a moving element

and a stationary support element. Rolling ele-

ment bearings generally offer a wide range of

features and performance characteristics.

Plain-sleeve Bearings

A plain bearing is a simple device for pro-

viding support and radial positioning, while

permitting rotation of a shaft. It is the oldest

bearing device known to man. A plain bear-

ing provides simplicity, low cost and short

distance from shaft centerline to bearing

base. It is usually limited to heavy radial

loads and slow-to-moderate speeds. When

properly installed and with correct lubrica-

tion, excellent bearing life will result.

Plain bearings are generally made of a

bronze alloy, and selection of a proper alloy

for a given purpose is very important. In the

selection of the bearing alloy, it is necessary

that the service conditions and operating

factors, such as speeds, loads, operating

temperatures, lubrication and the presence

of foreign materials, first be analyzed. Only

then should the bearing alloy be selected, so

that its outstanding properties will be to the

greatest advantage.

A plain-sleeve bearing is inherently quiet in

operation because it has no moving parts.

With proper selection, installation and lubri-

cation, it does not fail suddenly. Wear is grad-

ual, and replacement of worn bearings can

be scheduled when equipment is normally

idle. Sleeve bearings, like all other bearings,

must be properly selected and used with con-

trol of shaft tolerances, housings, mounting

or installation procedures and lubrication.

Bearing alloy selection is generally a com-

promise of the following essential character-

istics:

• conformability;

• wear resistance;

• coefficient of friction;

• load carrying capacity;

• resistance to pounding; and

• fatigue resistance.

Plain-sleeve bearings find limited use on

modern flexographic presses. They are com-

monly used as plate cylinder bearings,

although the recent trend has been toward

anti-friction needle bearings. Some central

impression presses use plain bearings to

support the impression cylinder. Plain bear-

ings are commonly used in the bore of idler

gears and gear trains where limited space

restricts the use of anti-friction bearings.

The sleeve bearing, when used for plate

cylinders or central impression cylinders,

must be fitted with precision accuracy. As

with all bearings, the outside diameter must

be concentric with the inside diameter, oth-

erwise, cylinder run-out will occur. Very

careful bearing and shaft alignment is an

absolute necessity. Running clearance must

be proper for trouble-free operation.

Running clearance in sleeve bearings is

PRESSES AND PRESS EQUIPMENT 141

142 FLEXOGRAPHY: PRINCIPLES & PRACTICES

the amount of space between the shaft and

the inside dimension of the bearing. This

space accommodates the lubricant, permits

the formation of the protected oil film pre-

venting metal-to-metal contact when in

operation, and allows for expansion due to

temperature rise. The running clearance for

most cylinder applications is 0.001" to 0.002"

(25  to 50 ).

Assuming a proper bearing selection has

been made, the installation and lubrication

of the bearing are the most critical factors in

its serviceability. Adequate lubrication,

preferably on an automatic program basis,

must be provided. Operating environment,

and to a great degree, common sense and

consideration of the way a bearing functions

will influence the service life of precision

sleeve bearings.

Rolling Bearing

A rolling-element bearing consists of four

basic parts:

• inner ring (inner race);

• outer ring (outer race);

• rollers or balls; and

• retainer or separator.

Three of these parts, the inner ring, the

outer ring, and the rollers or balls, support

the bearing load. The fourth part, the bearing

retainer, serves to position the rolling ele-

ments.

Rolling element bearings can be grouped

into five standard forms, each having a num-

ber of variations. These forms are:

• ball bearing;

• tapered-roller bearing;

• straight-roller bearing;

• ball-thrust bearing; and

• needle-roller bearing.

All of the components and roller element

bearings are made of hardened steel.

Selection of material and control of material

quality are critical in the manufacture of

roller-element bearings. The theoretical

stress obtained in a rolling contact may

exceed 400,000 lb/in

. Bearing steel must be

the finest available through current metal-

lurgical technology. It must possess high

strength, toughness, wear resistance and

dimensional stability, and be free from inter-

nal defects.

A modern flexographic press uses many

rolling element bearings. Bearings of all five

standard forms are used. Some press manu-

facturers mount their central impression

cylinder on precision-selected taper roller

bearings that have a total indicated run-out

of less than 0.00017". A tapered-roller bear-

ing is ideally suited to carry all types of load-

ing: radial, thrust and any combination of

both. The rollers and races of a tapered-

roller bearing are built on the principle of a

cone. All lines coincident with the working

surfaces of rollers and races meet at a com-

mon point on the bearing axis. Thus, the

tapered roller bearing handles all loads: radi-

al, thrust and any combination of both.

Most idler rollers rotate on roller bearings.

Depending on use and location, the bearings

may be of open-type construction or sealed

and permanently lubricated. Roller bearings,

for some applications, may be mounted in

housings or pillow blocks.

The idler rolls located in the press dryer

usually require a special type of roller bear-

ing, since, because of their location, they

will be subject to temperatures above their

heat-stabilized range. Standard bearings are

usually heat-stabilized within a range of

275° F to 325° F to minimize growth of the

bearing over a period of time. If bearings are

to operate above this range, then bearings

that have been heat-stabilized at a higher

temperature should be used. Heat stabiliza-

tion will prevent bearing expansion and pos-

sible loosening of the bearing on the shaft.

All motors are fitted with roller element

bearings. These motor bearings generally

have special characteristics, since they are