United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Laboratory

Performance of a

Paper

Powered Back-up

FPL 429 Roll for Peeling

Veneer
Abstract

The performance of a prototype powered back-up roll

(PBR) for peeling veneer was evaluated at the Forest

Products Laboratory. The torque which the PBR

supplied to a veneer bolt was measured while peeling

veneer of different species and thicknesses.

The PBR provided up to 100 percent of the required

torque for 4-foot bolts and has the potential to virtually

eliminate the spin-out problem. The PBR will also allow

peeling to smaller cores, and the peeling of logs once

thought “unpeelable.”

The net result of implementing this new technology will

be a significant increase in veneer yield from the timber

resource.

Note

This paper is the third 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 428 Powered Back-Up Roll-New Technology

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

Laboratory’

Research

Paper

FPL 429

October 1982

Laboratory

Performance of a

Powered Back-up

Roll for Peeling

Veneer

By

STEPHEN P. LOEHNERTZ, Research General Engineer

Introduction

Every year an estimated 700 million cubic feet of wood

are consumed to produce veneer. This amount is

equivalent to 10 million logs averaging 32 feet in length

and 20 inches in diameter. If all of the plywood that

could be produced from this veneer were stacked in

4-by 8-foot sheets, the stack would be over 4,000 miles

high. The magnitude of these numbers underscores the

importance of obtaining the maximum yield of veneer

from the available timber supply. Actual use of plywood

accounts for 13 percent of the total volume of all wood

products, and 33 percent of the total value.

In conventional veneer peeling, a log is rotated against

a fixed knife by means of chucks embedded in the ends

of the log. Three major problems that limit veneer yield

are associated with this conventional peeling method:

(1) spin-out, (2) large core size, (3) unpeelable logs.

Spin-out occurs when the torque required to turn the

log exceeds the amount that can be transmitted by the

chucks. The wood at the ends of the log fails, and the

chucks spin freely. With dual chucks, spin-out may

occur when the large outer chuck is retracted. It is

estimated that the spin-out rate is 5 to 8 percent at U.S.

veneer plants.

The size of the log core remaining is determined by the

size of the chucks. Core size can be reduced by using

smaller chucks, but only at the risk of increasing the

spin-out rate. The use of dual chucks is a compromise

solution to the problems of spin-out and core size. It

reduces the chance of a spin-out, while providing a

smaller core than would be attainable by using a single

chuck.

Logs with bad centers are considered unpeelable

because they cannot be adequately gripped by the

chucks. As a consequence, these logs are relegated to

sawtimber, or to the chipper, or are left in the woods. It

is estimated that 25 percent of all logs from the woods

now fall into this category.

New Technology for Peeling Veneer

The Forest Products Laboratory (FPL) powered back-up

roll (PBR) was designed to reduce or possibly eliminate

the preceding problems (fig. 1). It provides auxiliary

torque for peeling veneer by means of hydraulically

powered rollers in contact with the surface of the log

(figs. 2,3). The rollers assist the chucks in turning the

log and thus reduce the torque that the chucks must

provide. A hydraulic cylinder holds the PBR in contact

with the log at a constant force (fig. 1). Detailed

information on the design of the PBR is contained in a

previous report (1). 2

1Maintained at Madison, Wis., in cooperation with the University of

Wisconsin.

2

Italicized numbers in parentheses refer to literature cited at end of this

report.

Objective

The objective of this study was to determine the

effectiveness of the PBR in providing auxiliary torque
Figure 1.– FPL powered back-up roll-side view showing

frame and hydraulic positioning cylinder.

(M150 229-10)

to a veneer bolt. Specific goals were to:

1. Measure the torque provided by the PBR in peeling

veneer of different species and thicknesses. These

were Douglas-fir and red oak, 1/32 inch and 1/10 inch

each; and white fir, 1/8 inch and 3/16 inch.

2. Evaluate the quality of veneer peeled using the PBR

by examining uniformity of thickness, surface

smoothness, and surface appearance.

3. Evaluate roller coating material for friction

characteristics, wear, and compatibility with wood.

4. Develop optimum control criteria for the PBR to

provide detection and correction for roller slip and

deflection of the log.

Procedure

Measurements

Torque was measured on the lathe using strain gages

mounted on one of the spindles. Gages were installed

in such a way that they responded only to a torsional

load. The log was driven by one chuck only which was

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.

By providing all the torque from one spindle instead of

two, the output signal from the strain gages was larger

in magnitude and less affected by electrical noise.

Figure 2.–View of rollers from bottom.

(M150-231-7)

Figure 3.– View of FPL powered back-up roll

resting on log.

(M150 134-5)

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.

All logs were peeled both with and without the PBR, in

an alternating fashion. A typical procedure would be to

begin peeling a log without the PBR and get a torque

reading, then apply the PBR and get another reading,

2
alternating back and forth, until the log was used up.

The traces then showed the difference in spindle torque

for the two methods and this difference was the PBR

torque contribution. A position transducer was used to

indicate the location of the knife, and thus the log

radius at any point during the peeling process.

The pressure drop across the hydraulic motor powering

the rollers was measured using a pressure transducer

(fig. 4). The torque output of the hydraulic motor is very

nearly proportional to the pressure drop. Experimental

traces of the spindle torque and motor pressure were

produced on a two-channel x-y plotter. The input to the

x-axis was log radius. In this way graphs of torque and

pressure versus radius were obtained.

Log Preparation

Logs were cut to a standard length of 50 inches, with

the cutoffs used for ring counts and specific gravity

determination. Information on log properties is

summarized in table 1. A notch was made in the end of

each log to serve as a revolution marker.

All logs were stored in a cold water tank at about 50° F.

The Douglas-fir logs were peeled at this temperature.

The red oak and white fir logs were heated at a water

temperature of 140° F for 24 and 48 hours, respectively, l

prior to peeling.

Results

Torque Data

The test results demonstrated that the PBR is capable

of providing a substantial amount of the torque

required for peeling veneer. Theoretically, the required

torque is a linear function of radius, if the cutting

forces remain constant. Thus, the required torque at a

log radius of 10 inches is double that at a 5-inch radius.

Figure 5 shows the general idealized form of the torque

curve, measured at the spindle, for peeling with and

without the PBR.

Ideal logs with a homogeneous composition would give

the smooth line results as shown. Real logs have knots

and irregularities that cause the torque to fluctuate

about some average value. The torque contribution of

the PBR was computed from the change in the average

values. Some data for average required torque are given

in table 2.

The PBR provided 100 percent of the torque to peel

1/32-inch and 1/16-inch Douglas-fir veneer, and 85

percent for 1/10-inch Douglas-fir veneer. No change in

the required torque was observed with respect to the

transition from fast growth to slow growth wood.

The PBR contributed 70 percent of the required torque

for 1/10-inch red oak, less than required for Douglas-fir

but not surprising in view of the differences in density

for these species. The PBR easily contributed 100

percent of the torque for 1/32-inch veneer.

Figure 4.–Top view of motor, showing servovalve and

pressure transducer.

(M150-231-2)

Figure 5.–Generalized torque curves,

(M151 748-2)

PBR performance with white fir, however, was degraded

somewhat by roller slip. The percent of torque

contributed was approximately 35 to 40 percent for

both 1/8-inch and 3/16-inch veneer. The 3/16-inch veneer

was peeled looser than the 1/8-inch veneer so the

required torque was approximately the same for both.

Normally it increases for thicker veneer. PBR torque

results are summarized in table 3.

3
Roller Coating

The material used for the roller coating was DuPont

Adiprene®, a urethane rubber material.

Adiprene® bridges the gap between plastic and rubber,

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

temperature. This roller coating proved very compatible

with wood, and showed no signs of wear at the

conclusion of testing.

Contact area between the rollers and log ranged from

12 to 24 square inches, depending on log size and the

pressure between the rollers and log. The static

coefficient of friction of the rollers against the log was

computed by taking the ratio of the transmitted force

to the normal force. 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 dried out on the

surface. With a dry log the roller tended to load up with

slivers of wood, and slip. This did not occur with a

green log. In any case, a small increase in the normal

force compensated for the dried log and eliminated the

tendency to slip. In comparison, the static coefficient

of friction for steel aginst green wood is approximately

0.17.

Pitch pockets were a minor problem with the old

growth Douglas-fir. Pitch collected in patches on the

rollers, but did not cause slip. Some slivers adhered

to the pitch on the rollers, but no significant buildup

occurred. The pitch was easily removed with a soap

solution of trisodium phosphate. Solvents however,

were not used for cleaning because they would nave

harmed the coating material.

The roller coating performed very well with the red oak

logs, and the rollers stayed very clean during this part

of the testing. The computed coefficient of friction for

these logs was 0.55.

Roller coating performance was poorest with white fir

where slip occurred, at the settings used for Douglas-fir

and red oak, due to excess water given off by the logs.

This water release was apparently due to the method of

conditioning, a hot water soak at 140° F for 48 hours.

This soaking time is excessive by industry standards.

Plants visited by the author where this species is

peeled have relatively dry logs, with no excess water

given off during peeling. The coefficient of friction for

white fir was computed to range from 0.25 to 0.55. As

with the red oak, the rollers remained clean.

Controls

A manual control system was used for testing. The

force holding the rollers against the log, i.e., the normal

force, was controlled by adjusting the pressure in the

hydraulic positioning cylinder. The torque applied to

the Iog by the rollers was controlled by adjusting the

pressure to the motor powering the rollers.

Roller Slip

Due to the slow rotational speed of the lathe, slip was

detected visually. When it occurred, the pressure

4

measured at the hydraulic motor dropped

instantaneously to almost zero, since the rollers were

turning freely without resistance. This instantaneous

change in the motor pressure showed up dramatically

on the x-y plotter as a vertical line cutting across the

entire page, which is a secondary means of detecting

slip. In an industrial situation it would be desirable to

directly measure and compare the surface speeds of

the log and rollers in order to detect slip.

The control logic for dealing with slip is shown in

figure 6. This process applies whether the system is in

manual or automatic mode. If slip occurs, two

corrective actions can be taken. First, the position

cylinder pressure, or normal force, can be increased.

This change increases the traction between the rollers

and the log. Second, the pressure to the hydraulic

motor can be decreased, which reduces the roller

turning effort and makes them less likely to lose their

grip. Both corrections can be made automatically,

using a speed error (slip) signal to regulate a valve that

controls pressure.

Log Position

There are limits on the magnitude of normal force that

the rollers should apply to the log. Excessive force will

accelerate wear of the roller coating, and possibly

cause indentation of the log surface. More importantly,

excessive force could cause bending of the log toward

the knife, especially at small core diameters, resulting

in nonuniform veneer thickness and irregularly shaped

cores. In the same manner, insufficient normal force

would allow the log to bend away from the knife, with

similar results.

The allowable range for normal force with respect to

log deflection was not established during the test

program. Due to the experimental setup the minimum

core size attainable during testing was limited to a

7-inch diameter. A 7-inch core on a 4-foot bolt is

approximately 30 times as stiff as an industrial core 8

feet long and 5 inches in diameter. Thus, any deflection

that occurred was not observed. It should be noted that

PBR Control Logic

Figure 6.–PBR control logic process.

(M151 748-4)
because the torque required to peel veneer decreases

as the log diameter decreases, the primary function of

the PBR could change from providing torque to

providing position control. PBR operating variables are

given in table 4.

Veneer Quality

Veneer was evaluated for uniformity of thickness,

surface smoothness and appearance.

Thickness

A sheet of veneer cut from one revolution of the log

was saved for each segment of the peeling of a log.

(Each log was peeled with and without the PBR, in

alternating segments.) Each veneer sheet was

measured in nine locations by a pneumatic actuated

dial indicator: at the four corners, along the edges

midway between the corners, and at the center of the

sheet. Several readings were made in the general

vicinity of each location to get a representative value.

Veneer peeled with the PBR tended to be thicker than

regularly peeled veneer by 0.001 to 0.004 inch. This

difference is shown in figure 7 for two typical logs. In

both cases thickness is plotted as the difference

between the actual peel and the lathe feed. Apparently

the PBR displaced the log slightly and thus affected

the veneer thickness. Adjustments in lathe settings

could correct for such displacement. There was no

evidence of the type of log deflection (bending)

discussed previously.

Surface

Veneer peeled with the PBR was indistinguishable in

appearance from veneer peeled the regular way. The

rollers left no marks on the surface, except when

they occasionally slipped. Slip produced slight scuffing

and, infrequently, a skid mark. It was determined that

light sanding would easily remove any marks.

Discussion

The results of the test program show that the PBR is

very effective in providing auxiliary torque to veneer

bolts. This success implies that the PBR has the

-potential to greatly reduce the magnitude of the spinout problem, and to allow use of smaller chucks. The

PBR may also make it possible to peel so-called

unpeelable logs, such as those with soft, or partially

hollow centers.

The PBR developed by FPL has generated considerable

commercial interest. In February 1981 FPL entered into

an agreement with the Boise Cascade Corporation to

install an industrial version of the PBR in their Yakima,

Washington plant. This plant peels a high percentage

of white fir logs and spin-out has been a problem. As

noted earlier, the PBR provided 35 to 40 percent of the

required torque when white fir was peeled in the FPL

experiments. Because this work was affected by the

excess water released by the logs used, it is expected

that actual in-plant performance will surpass 40

Table 1. – Material properties

Roundup Ring

Species No. diameter,1

count,

of logs range range

in.

West Coast

Douglas-fir 12 23-27 230-300

Northern red oak 6 14-15 80-100

White fir 22 10-20 58-222

1Diameter after peeling off the irregular surface.

Specific

gravity

0.41

.53

.36

Table 2. – Average required torque (ft-lb)

Radius 1/10 in. 1/10 in. 1/8 in.

Douglas-fir red oak white fir

in.

4 630 760 650

6 1,000 1,380 1,020

8 1,380 1,600 1,380

10 1,760 (1) (1)

1

These logs were smaller than a 10-inch radius.

Table 3. – PBR performance for 4-foot logs

Species Veneer No. Total number PBR

thickness of logs torque readings1

torque

in. pct

Douglas-fir 1/32 3 38 100

1/10 9 40 85

Red oak 1/32 3 16 100

1/10 4 8 70

White fir 1/8 17 56 35-40

3/16 5 (2) (2)

1Readings taken as follows: regular peeling, PBR peeling,

regular, PBR, etc., for any log.

2Regular peeling only, continuous readings.

Table 4. – PBR operating variables

Species

Douglas-fir

Red oak

White fir

Veneer Normal Motor

thickness force pressure

in. lb 1b/in.2

1/32 2,100 1,500

1/10 2,900 2,500

1/32 2,100 1,800

1/10 2,700 2,500

1/8 2,900 1,500

5
percent. The Boise Cascade PBR will be designed and

built by Premier Gear and Machine Works, Portland,

Oreg. The controls system will be developed by Lloyd

Controls. Performance of this industrial prototype will

be closely monitored by both Boise Cascade and FPL

personnel. A joint test program will be conducted, and

a report published upon the completion of testing.

Literature Cited

1. Fronczak, Frank J.; Loehnertz, Stephen P. Powered

back-up roll-new technology for peeling veneer.

USDA For. Serv. Res. Pap. FPL 428, For. Prod. Lab.,

Madison, Wis.; 1982.

2. Lutz, John F.; Patzer, Robert A. Spin-out of Veneer

Blocks During Rotary Cutting of Veneer. USDA For.

Serv. Res. Pap. FPL 278. For. Prod. Lab., Madison,

Wis.; 1976.

Figure 7. – Veneer thickness for Douglas-fir log No. 3,

1/10-inch veneer and for white fir log No. 13,

1/8-inch veneer.

3.0-7-3/83

6
U.S. Forest Products Laboratory

Laboratory Performance of a Powered Back-up Roll for Peeling

Veneer, by Stephen P. Loehnertz, Madison, Wis., FPL 1982.

7 p. (USDA For. Serv. Research Paper FPL 429).

Evaluates performance of a prototype powered back-up roll at the

Forest Products Laboratory. The net result of implementing this new

technology will be a significant increase in veneer yield from the

timber resource.

Keywords: Powered back-up roll, veneer peeling, spin-out,

veneer yield, veneer quality, resource recovery, chucks, core size,

rollers, torque output.
U.S. GOVERNMENT PRINTING OFFICE: 1982-544-030/ 31