United States

Department of

Agriculture Powered Back-up

Forest Service

Forest

Products

Laboratory

Research

Paper

Roll-New

FPL 428

Technology for

Peeling Veneer
Abstract

A powered back-up roll was designed and built for the

purpose of providing auxiliary torque to a veneer bolt.

Initial testing indicates that a substantial percentage of

the required torque can be supplied by the powered

back-up roll. This reduces the likelihood of spin-out

occurring and allows a reduction in the final core size

when used in conjunction with smaller chucks.

Note

This paper is the second in a series of four papers

describing the FPL powered back-up roll. The other

Research Papers are:

FPL 427 Influence of Chuck Design on Spin-Out Torque

in Softwood Veneer Peeling Blocks

FPL 429 Laboratory Performance of a Powered Back-Up

Roll for Peeling Veneer

FPL 430 Industrial Performance of Powered Back-Up

Roll for Peeling Veneer.

The use of trade, firm, or corporation names in this

publication is for the information and convenience of

the reader. Such use does not constitute an official

endorsement or approval by the U.S. Department of

Agriculture of any product or service to the exclusion of

others which may be suitable.
United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory1

Research

Paper

FPL 428

October 1982

Powered Back-up

Roll-New

Technology for

Peeling Veneer

By

FRANK J. FRONCZAK, Engineer

and

STEPHEN P. LOEHNERTZ, Engineer

Introduction

In order to develop a practical and efficient means of

providing auxiliary torque to veneer bolts, a powered

back-up roll (PBR) was designed and built at the Forest

Products Laboratory (FPL).

This report describes the design of the PBR and

discusses the test program and results obtained thus

far. Initial testing demonstrates the feasibility of the

PBR concept. The torque provided by the PBR was

measured while peeling Douglas-fir veneer of different

thicknesses. Veneer quality was not adversely affected

by the new process. This new technology promises

increased resource recovery and subsequent savings.

Additional test results will be reported as they become

available.

Background Information

The Role of Veneer in the

Forest Products Industry

Veneer is considered to be one of the Nation’s most

valuable forest products. An estimated 700 million

cubic feet of wood are consumed annually to produce

veneer, most of which is used in plywood. This is

equivalent to a 4,000-mile-high stack of 4 by 8 sheets.

1

Maintained at Madison, Wis., in cooperation with the University of

Wisconsin.

In 1979, plywood accounted for 13 percent of the total

volume of all wood products, and an even more

impressive 33 percent of the total value. By

comparison, lumber accounted for 76 percent of the

total volume, and 60 percent of the total value of all

wood products. While lumber has almost six times the

volume of plywood, it has only twice the value (fig. 1).

When considering the U.S. panel products industry,

plywood accounts for approximately 65 percent of the

volume, and 80 percent of the value. Particleboard,

medium density fiberboard (MDF), and hardboard

combined account for 28 percent of the volume, but

only 18 percent of the value (fig. 2). This indicates the

relative value of veneer and the importance of

increasing the recovery of veneer from the timber

resource.

Problems Associated with

Veneer Peeling

Historically, the major problems which cause reduction

in veneer yield have been spin-out, large core size, and

unpeelable logs.

Spin-out

This occurs when the torque required to peel a log

exceeds the amount which can be transmitted by the

chucks through the ends of the log. The chucks eat

through the wood and spin freely without turning the

log, making it unusable for further peeling. Spin-out
Figure 1.–Total U.S. consumption of wood products, 1979 data

(includes veneered products).

Figure 2.–U.S. panel products distribution, 1979 data.

occurs in 5 to 8 percent of all logs which are

considered peelable.

There are several approaches which have been taken in

an effort to solve the spin-out problem. One is to reduce

the amount of torque required to peel veneer. This can

be done by proper log conditioning, good lathe

maintenance, and use of a modified pressure bar, such

as a roller bar (1,2). 2

 Another approach is to increase the

amount of torque which the chucks can safely transmit.

This has resulted in the use of dual chucks, consisting

of a large retractable outer chuck and a smaller inner

chuck. Peeling of a log begins with the outer chuck,

when the log is the largest and therefore requires the

most torque. When the log is peeled down to a smaller

diameter, the outer chuck retracts and the load is

transferred to the inner chuck. Depending upon the

relative size of the chucks, however, spin-out often

occurs at this transition stage.

Figure 3 can be used to show how much veneer is lost

when a bolt spins out. For example, if a 12-inch bolt

spins out at a diameter of 8 inches, 44 percent of the

wood volume remains in the core. Only 56 percent of

the wood is converted to veneer. If the same 12-inch

bolt is peeled down to 5 inches, only 17 percent of the

wood volume remains in the core, and 83 percent has

been converted to veneer.

Large Core Size

This second major problem is related to the spin-out

problem. The size of the core is essentially determined

by the size of the inner chuck. Core size can be

reduced by using smaller chucks, but this increases the

probability of a spin-out. The use of dual chucks is a

compromise solution to large core size and spin-out. It

reduces the chance of spin-out, while providing a

smaller core than would be attainable using a single

chuck. The increase in veneer yield by going to a

smaller core is also shown by figure 3. New technology

may make it possible to peel a bolt down to a 3-inch

core, versus the standard 5 inch. For a 12-inch bolt

peeled down to 3 inches, only 6 percent of the wood

volume remains in the core, with 94 percent conversion

into veneer. Compared to the 5-inch core, this is an

increase of 11 percent in the wood volume converted to

veneer. Of course, the decision to go to a smaller core

would depend on the economics involved in converting

a 5-inch core into additional veneer versus chips, studs,

landscaping ties, etc.

Unpeelable Logs

The third major problem with conventional veneer

peeling is that logs with bad centers are unpeelable or,

at best, have an unacceptably high spin-out rate. These

logs cannot be adequately gripped by the chucks; and

as a consequence, they are relegated to sawtimber or

to the chipper, or they are left in the woods. This

problem represents a serious loss of a potentially

valuable product.

New Technology for Peeling Veneer

The FPL has developed a PBR which promises to

minimize the preceding problems. The PBR consists of

a pair of hydraulically powered rollers which turn

against the surface of the log and assist the lathe

chucks in providing torque to the veneer bolt. A

2

 Italicized numbers in parentheses refer to literature cited at the end

of this report.

2
Figure 3.–Percent volume of wood remaining in peeler cores.

(M 151-018)

hydraulic cylinder holds the PBR in contact with the log

at a constant force (fig. 4). Since it can provide much of

the required torque, the PBR enables a log to be peeled

using very small chucks. This means core size can be

reduced, spinout minimized, and so-called unpeelable

logs can be peeled.

Powered Back-up Roll

The general considerations for a full-size PBR are

explained in the following paragraphs.

Performance Target

When work was initiated on the PBR, virtually no

background information was available related to

powering a veneer bolt from the surface. Some

preliminary work establishing the coefficient of friction

of rubber, steel, and emery cloth with green wood had

been published (2). There were significant differences

between conditions encountered in this previous work

and those expected in a full scale PBR. Because of

these differences the results of the preliminary work

could not reliably be used to estimate the amount of

torque which could be transmitted to a veneer bolt

through friction.

It was also difficult to determine the amount of torque

required to peel veneer. Because of these factors it was

difficult to assess the percentage of the required

torque that would be available from the PBR. Based on

conversations with veneer producers and lathe

manufacturers, it was felt that if 10 percent of the total

required torque could be provided by the PBR, then this

would be a worthwhile achievement. While no

theoretical limitations exist which limit the amount of

torque that could be delivered, consideration of

practical problems such as the size of the roller and

hydraulic motor performance seemed to indicate that a

contribution of 10 to 25 percent would be a reasonable

performance target.

It was necessary to strike several compromises when

establishing the design criteria for the powered back-up

roll. Some of these compromises were necessitated by

unique requirements imposed by the FPL veneer lathe,

a 4-foot Coe A-frame model. In spite of limitations due

to the use of an old lathe, the powered back-up roll

was, wherever possible, designed to be compatible with

modern industrial lathes and peripheral equipment. A

full-size, rather than experimental scale, back-up roll

was designed and built. It was felt that, by working

with a full-size piece of equipment, more reliable

information could be obtained about both the potential

benefits and problems associated with this approach.

Two important differences between the FPL back-up

roll and one intended for commercial use are the speed

of operation and the method of positioning the back-up

roll on the log.

Operational Speed

The FPL lathe can operate at a maximum constant

speed of approximately 40 revolutions per minute (rpm).

With a 36-inch-diameter log, this corresponds to a

peripheral speed of 375 feet per minute (ft/min). Since

the lathe operates at a constant rotational speed, the

peripheral speed of the log decreases linearly with the

log radius. Because commercial lathes normally

maintain a constant peripheral speed, the controls for

the PBR were designed to be compatible with either

constant or variable peripheral speed. While the FPL

lathe’s maximum peripheral speed is less than 400

ft/min, the PBR was designed to be compatible with

lathes operating up to 1,000 ft/min on a continuous

duty cycle. Even higher speeds could be obtained with

a higher capacity hydraulic supply pump.

Log Loading

The FPL lathe configuration also dictated the method

by which the PBR had to be mounted to position itself

on the log. The FPL PBR was mounted to a floor stand

rather than the lathe itself; however, this in no way

3
Figure 4.–FPL PBR with lathe.

(M 150 229-6)

affects the actual operation of the PBR. In industrial

practice it would be mounted the same way as a

conventional back-up roll so as to not interfere with

normal log loading procedures.

Design Features

Supporting Frame

The back-up roller consists of two basic components,

the supporting frame and roller head assembly. The

supporting frame for the roller head assembly does not

affect the function of the back-up roller but merely

positions it on the log. A hydraulic positioning cylinder

generates a normal force to provide traction between

the roller and the log (fig. 5). The stand was designed to

provide a 12,000-pound maximum normal force on the

log when the hydraulic positioning cylinder’s pressure

is at its maximum of 3,000 pounds per square inch

(psi). The stand is fabricated from 6-inch square tubing

with a 3/8-inch wall thickness. The normal force is

obtained with a 4-inch-diameter bore hydraulic cylinder,

4

and its magnitude is controlled by the cylinder

hydraulic pressure. In the experimental prototype, this

pressure is regulated by a manually operated pressure

relief valve.

Roller Head Assembly

Roller Coating

The auxiliary torque is transmitted to the log using two

hydraulically powered rollers, The rollers are 7-1/2-inch

outside diameter (o.d.) steel tubing with a high friction

coating. The thickness of the coating is 1/8 inch so that

the final diameter of the rollers is 7-3/4 inches (fig. 6).

The material selected for the coating was DuPont

brand “Adiprene”®–a urethane rubber material. This

material bridges the gap between plastics and rubbers,

and is resistant to water, oil, abrasion, and high

temperatures.

Common industrial applications of this material are

solid industrial tires and rolls, equipment linings, and

seals. It was chosen because of its relatively high
Figure 5.–FPL PBR–side view showing frame and hydraulic

cylinder.

(M 150 229-10)

Figure 6.ñFPL PBR rollers (from bottom).

(M 147 732-9)

values of strength, coefficient of friction, and abrasion

resistance, with specific properties determined by the

formulation. The compound used here was L42, with a

hardness of 80 durometer A. For comparison, this is

somewhat harder than a typical man’s shoe heel.

The resilience of the surface coating provides a

relatively large contact area or “footprint” between the

rollers and the log. As the normal force increases,

the contact area increases as well. The contact area

also increases with log diameter. With a normal force

of 3,000 pounds the total contact area ranges from

approximately 12 to 24 square inches depending upon

log diameter. This appears to be adequate to prevent

high stress concentrations with consequent localized

wood failure.

Drive Train

The rollers themselves were made from 7-5/8-inch o.d.

steel tubing machined to 7-1/2 inches o.d. The length of

each roller is 13-1/2 inches. For ease of removal and

replacement, the rollers were attached to the shaft with

locking assemblies. This allows the replacement of

rollers for maintenance without removal of the gears or

shafting. Both rollers are driven directly from the

hydraulic motor with a gear drive. The gears have 28

teeth on a 7.0 pitch diameter, with an o.d. of 7-1/2

inches and a width of 3-1/2 inches. The driver and the

driven gears are identical so that the speed ratio

between the motor and rollers is 1:1. Each gear

interface is capable of transmitting approximately 20

horsepower (hp) at 77 rpm (150 surface ft/min) or 40 hp

at 200 rpm (400 surface ft/min). This means that each

gear interface can transmit a torque of approximately

1,100 foot-pounds.

The hydraulic motor used in this application is an

industrial geroler type with a displacement of 20.34

cubic inches per revolution. This motor requires a flow

rate of approximately 20 gallons per minute (gpm) at

200 rpm. It is rated for continuous duty up to 2,500 psi

supply pressure and up to 3,750 psi when used at a 10

percent duty cycle. The running torque at 2,500 psi is

approximately 630 foot-pounds, and 950 foot-pounds at

3,750 psi (fig. 7). A standard hydraulic motor with

standard hydraulic motor with internal bearings was

used because of its versatility; however, similar motors

are available without shaft end bearings, and they

could also be used for similar installations where size

is a critical factor.

Control System

Due to the experimental nature of the powered back-up

roll, the system was designed to be compatible with

several different control schemes. These ranged in

sophistication from simple manual control of pressure

to automatic control incorporating torque and speed

feedback.

Manual pressure control

The simplest control system maintains the supply

pressure to the motor at a relatively constant value by

5
Figure 7.–Top view of motor, gears and rollers,

(M 150 134-5)

using a manually operated relief valve. This valve is

located at the pump and can be set to obtain a supply

pressure ranging between approximately 150 to 3,000

psi. Because of dynamic factors and rapid variations in

cutting forces, such as caused by knots in the log, the

actual pressure drop across the motor fluctuates

slightly. This observed fluctuation does not affect the

performance of the prototype system. While manual

pressure control is adequate for testing, it is not

capable of dealing with sudden occurrences, such as

the rollers slipping on the log. A more refined control

system is needed for this, and is discussed later.

Automatic System

In order to provide more precise control of the system

while operating, the motor torque can be controlled

with a closed loop servosystem (fig. 8a and 8b). The

torque is controlled by regulating the pressure drop

across the motor with a servo-valve and servocontroller.

Using the motor and rollers, a 200 gpm flow is needed

to peel veneer at 400 ft/min, which causes a pressure

drop of less than 200 psi across the valve. The pressure

drop across the motor, ∆Pm, is measured by a

differential pressure transducer, and is compared to the

desired pressure drop, ∆PD. The servocontroller

automatically controls the valve opening to maintain

the desired Pc across the motor (fig. 9). The advantages

of this method over the manual system are that it

responds rapidly to changes in the load, and the

desired torque level can be set to preprogramed values.

Slip Feedback Control

An additional refinement which can be added to this

servocontrol system is speed differential or slip

feedback. By comparing the peripheral speed of the

rollers to the peripheral speed of the log, any slippage

between the rollers and the log can be detected. Any

such slippage is undesirable, because of potential

damage to the veneer and rollers, and because of

reduced transmission of torque. The speed-differential

signal can be used to control the servo-valve opening,

and to correct the speed of the rollers in order to match

the log speed. This system would work in conjunction

with the servo-pressure controlled system. In normal

operation, with no slip occurring, no speed differential

signal is generated (fig. 10).

These control systems have undergone limited testing

at the present time. Additional testing and perhaps

modification may be needed to ensure their proper

functioning over the intended operating range.

Position control

The PBR will allow the use of smaller chucks, and will

also allow peeling down to smaller core size. This

means that a commercial PBR should be capable of

providing log position control, or be compatible with a

separate position control system. Position control

is necessary to prevent deflection of the log away from

the knife as the log is peeled down to a small diameter.

Position control was not incorporated into the

experimental PBR; however, the hydraulic cylinder

which holds the PBR in contact with the log could be

used to perform this function. It should be noted that

the torque required to peel veneer decreases as the log

diameter decreases, thus the primary function of the

PBR during peeling could change from providing torque

to providing position control.

Several methods of accomplishing this position control

exist. The most straightforward of these methods is to

control the volume of hydraulic fluid in the cylinder

with a servo-valve. The command signal would be

generated from a position transducer which would

indicate the position of the knife carriage. A feedback

signal would be provided by a transducer which would

give the location of the back-up roller. Thus a PBR

could be used with a high normal force to provide

maximum torque when needed, and could also be

programed to provide position control when needed.

Test Program

In order to evaluate the effectiveness of the PBR, a

comprehensive test program was established with

objectives to:

1. Measure the torque provided by the PBR.

2. Evaluate the quality of the veneer peeled using the

PBR.

3. Evaluate the roller coating material.

4. Develop criteria for optimum controls for the PBR.

These objectives are discussed in detail below. West

coast Douglas-fir was used in the testing done thus far.

Veneer thickness ranged from 1/32 inch to 1/10 inch.

6
Figure 8.–(a) Servo-valve and (b) control panel.

(M 150231-4, M 150229-3)

7
Figure 10.– Schematic of pressure control with slip controladded.

(M151-020)

Torque Measurement

Torque was measured on the lathe using strain gages

mounted on one of the spindles. Gages were installed

so that they responded only to the torsional load. The

log was driven by only one chuck mounted on the

spindle with the strain gages. The other chuck

freewheeled on bearings and provided no torque to the

log, but helped hold the log in position (fig. 11)

There were two reasons why one chuck was modified

for freewheeling. First, it was desirable to measure all

of the torque in one location for the sake of simplicity

and accuracy. In this way the output signal from the

gages would be larger and less sensitive to electrical

noise. Secondly, it was discovered during early peeling

trials that a certain amount of “windup” occurred

between the two spindles, which were not totally in

phase while driving the log. This was shown by a wave

pattern in the torque recording covering a period of one

revolution of the log. This wave pattern disappeared

when one chuck was freewheeling.

The measured torque was the torque actually provided

by the lathe. A value for the PBR torque was obtained

indirectly as the difference in torque readings between

regular peeling and peeling with the PBR. Torque was

recorded on an x- y plotter versus the radius of the log,

which was measured using a position transducer which

indicated the position of the knife (fig. 12).

Veneer Quality

The quality of the veneer peeled using the PBR was

compared to that peeled using conventional

technology. The comparison was based on uniformity

of veneer thickness and on an evaluation of surface

smoothness and finished appearance. The objective

was to determine if the PBR caused deflection of the

log and if the rollers indented or scuffed the veneer.

A sheet of veneer peeled during one revolution of the

log was saved for each 1/2 inch of radius peeled off.

The thickness of the sheet was measured in various

8
Figure 11.–Free wheel chuck with tapered roller bearing.

(M 150 111-1)

Figure 12.–Torque data recording for Douglas-fir log No. 5,

1/10-inch veneer. (X-axis is log size, increases from

right to left.)

(M 150 135-8)

strategic locations. Surface smoothness and finished

appearance were subjectively evaluated.

Roller Coating

During the course of testing, the roller coating

properties which were evaluated included coefficient of

friction, cleanability or need to be cleaned, and

durability.

Preliminary Results

The test results obtained from peeling Douglas-fir logs

with the PBR were very encouraging. Tests using

different species and additional thicknesses of veneer

are continuing and will be reported in subsequent

publications.

The powered backup roll provided 100 percent of the

torque required to peel 1/32-inch and 1/16-inch veneer

from 4-foot bolts, and approximately 85 percent of the

torque for 1/10-inch veneer. Logs were kept green but

were not heated. Veneer quality and thickness were not

affected by the PBR.

The rollers were held against the log with the minimum

normal force needed to prevent them from slipping on

the surface of the log. The total area of the rollers

in contact with the log ranged from 12 to 24 square

inches, depending on log size. The roller surfaces

showed no signs of wear. In several instances where

the rollers did slip, the coefficient of friction was

computed using the applied normal force and the

instantaneous PBR torque reading. This value ranged

from 0.5 to 0.8. It is believed that the lower value of the

coefficient of friction was caused by the log being

somewhat dried out on the surface. With a dry log, the

rollers tended to load up with slivers of wood and lose

their grip. This did not occur with a good green log. In

any case, a nominal increase in the normal force

compensated for the dried log and eliminated the

tendency to slip.

The old-growth Douglas-fir logs which were peeled

contained pitch pockets which occasionally fouled the

rollers. This did not affect the operation of the PBR, i.e.

no slip occurred, and the rollers tended to be self

cleaning; but pitch buildup does appear to be a

problem.

For each veneer thickness, the hydraulic pressure to

the motor was manually adjusted to provide the

maximum torque to the log without exceeding either

the required torque or the system capacity. It was not

originally believed that the PBR could provide all the

torque required to peel thin veneer, but in fact it did. At

some points during the peeling process, the PBR

tended to overdrive the lathe, which normally operates

at constant rpm, by providing excessive torque. This

would need to be considered when designing a controls

system for an industrial application. The results for

Douglas-fir are summarized in table 1.

Figure 12 shows a typical torque trace which was

obtained during testing. For any given log, the method

of peeling alternated back and forth between regular

and PBR assisted. The upper traces indicate the

required lathe torque for regular peeling, and the lower

traces show the required torque when using the PBR.

9
Table 1 .–Powered back-up roll performance, Douglas-fir,

unheated

Torque Hydraulic

Log Veneer provided Roller/force pressure

diameter thickness by PBR against log to motor

In. In. Pct Lb Psi

2 4 1/32 100 2,100 1,500

2 4 1/16 100 2,500 1,500

24 1/10 85 2,900 2,500

The required lathe torque was reduced by the amount

indicated by the distance between the upper and lower

traces. This is the contribution of the PBR. The traces

shown are not smooth curves. They have peaks caused

by knots which required more torque than clear wood.

These peak torque values as well as the average values

are reduced by the amount contributed by the PBR.

Future Work

When the current test program is completed, several

new studies involving innovative processing will be

undertaken. The first of these, and the most ambitious,

will explore the possibility of peeling veneer without

chucks driving the log by providing all of the torque

with the PBR. This would require two freewheeling

chucks, and an independent means of advancing the

knife according to the rotational speed of the log.

Control systems development will play a large part in

this study.

Another planned study would deal with peeling thick

veneer, from 3/16 inch and up. This would require

operating the PBR at maximum design pressure; and

therefore, the study would be done toward the end of

all planned work in the event of any operational

difficulties, A ready means of peeling thick veneer

could spur the development of new veneer products,

and thus reinforce the already important role that

veneer plays in the forest products industry.

Literature Cited

1. Lutz, John F., and Robert A. Patzer. 1978. Spin-out of

veneer blocks during rotary cutting of veneer.

USDA For. Serv. Res. Pap. FPL 278. For. Prod.

Lab., Madison, Wis.

2. Lutz, John F. 1974. Techniques for peeling, slicing,

and drying veneer. USDA For. Serv. Res. Pap.

FPL 228. For. Prod. Lab., Madison, Wis.

3.0-11-10/82

10
V2, V3-2

U.S. Forest Products Laboratory

Powered Back-up Roll-New Technology for Peeling Veneer, by

Frank J. Fronczak and Stephen P. Loehnertz, Madison, Wis., FPL 1982.

11 p. (USDA For. Serv. Research Paper FPL 428).

Describes the design of the powered back-up roll and discusses

the test program and results obtained. Initial testing demonstrates

feasibility of PBR concept.

Keywords: auxiliary torque, veneer bolts, veneer quality, resource

recovery, veneer yield, spin-out, log peeling, chucks, core size, rollers,

log position control.
U. S. GOVERNMENT PRINTING OFFICE: 1982-554-030/ 15