NBSIR 76-1022

Evaluation of Thrown Objects

Tests for Proposed Safety

Standard for Power Lawn

Mowers

Donald C. Robinson and Roger B. Clough

Institute for Basic Standards

National Bureau of Standards

Wasfciington, D. C. 20234

June 1976

Final

Prepared for Consumer Product Safety Commission

Washington, D. C. 20207

NBSIR 76-1022

EVALUATION OF THROWN OBJECTS

TESTS FOR PROPOSED SAFETY

STANDARD FOR POWER LAWN

MOWERS

Donald C. Robinson and Roger B. Clough

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

June 1976

Prepared for Consumer Product Safety Commission

Washington, D. C. 20207

U.S. DEPARTMENT OF COMMERCE. Elliot L. Richardson, Secretary

Dr. Betsy Andwr-Johnaon. Ataigtmtt S^erHaiy for Schnce ltd Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Acting Director

Final

LIST OF TABLES

Page

Table 1 - Lawn Mower Characteristics 3

Table 2 - Weighting Factors for Dispersion Tests for Walk-Behind

Lawn Mowers 6

Table 3 - Results of Penetration Tests 13

Table 4 - Thrown Object Penetrability Tests 16

LIST OF FIGURES

Figure la - Portable Target Device for Outdoor Dispersion Tests 4

Figure lb - Lawn Mower Arrangement within Portable Target 4

Figure 2 - Chamber for Evaluating Proposed Thrown Objects

Tests 8

Figure 3 - Test Score: Acceptance Ratio vs. Blade Tip Speed

for Various Gasoline Power Mowers 10

Figure 4 - Lawn Mower and Target Arrangement for Penetration

Tests 11

Figure 5 - Number of Penetration Failures vs. Blade Tip Speed

for Six Lawn Mowers 12

CONTENTS

Page

1 . INTRODUCTION 1

2. OUTDOOR DISPERSION TESTS 2

3. PROPOSED DISPERSION TESTS 7

4. PENETRATION TESTS 9

5. ANALYSIS OF PENETRATION 14

5.1 Introduction ^.U 5.2 Theory I4

5.3 Shape Effects

5.4 Size Effects I7

5.5 Velocity Effects 17

5.6 Humidity Effects 17

6. CONCLUSIONS AND RECOMMENDATIONS 18

7 . ACKNOWLEDGMENTS I9

8. REFERENCES 20

EVALUATION OF THROWN OBJECTS TESTS FOR

PROPOSED SAFETY STANDARD FOR POWER LAWN MOWERS

Donald C. Robinson and Roger B. Clough

ABSTRACT

An evaluation was made of thrown objects dispersion and penetration tests which have been developed

for a proposed safety standard for power lawn mowers.

To evaluate the proposed laboratory dispersion tests,

supplementary outdoor dispersion tests were conducted

on walk-behind lawn mowers in which actual grass cutting conditions were simulated. A description of these

outdoor tests, a comparison of the outdoor and laboratory dispersion tests for a sample of walk-behind lawn

mowers, and an evaluation of the proposed dispersion

and penetration tests are given. The evaluation includes a theoretical discussion of the penetration of

thrown objects which is related to experimental results

in terms of the shape, size and velocity of the thrown

proj ectiles.

Key Words: Dispersion tests; lawn mowers; penetration

tests; power lawn mowers; safety standard; thrown objects tests.

1 . INTRODUCTION

A Thrown Objects Test which has been developed for the Consumer

Product Safety Commission (CPSC) for the Proposed Safety Standard for

Power Lawn Mowers [1]* has been evaluated. The proposed test was developed by an Offeror for CPSC to evaluate the tendency of power rotary

mowers to throw struck objects. During the development of the Thrown

Objects Test, mower characteristics which were identified included:

1) dispersion, the spatial distribution of thrown objects; 2) penetration, a measure of kinetic energy which the mower imparts to a struck

object; and 3) pickup, the relative tendency of mowers to lift and strike

objects which are in their cutting path. Tests were only developed, however, to estimate the ability of a struck object to penetrate a specified

target and to evaluate the dispersion, but not to measure pickup.

*Figures in brackets refer to literature references given at the end

of this report.
As outlined in the Rationale for Proposed Safety Standard [2], the

test mower is suspended so that the horizontal plane of its blade is a few feet above a non-rebounding surface.* This procedure eliminates

ricochets of objects off the "ground". The objects, which are nails or steel balls depending on the test, are forcibly injected upward into

the path of the rotating blade. Since there is no supporting surface

close enough to the blade, "ground" ricochets are thereby eliminated.

This geometry also plainly eliminates from the test the "pickup" effect,

i.e., variations from mower to mower in their ability to pick up objects

placed on the "ground".

To evaluate the proposed laboratory dispersion tests, the National

Bureau of Standards (NBS) conducted outdoor dispersion tests on walkbehind lawn mowers in which actual grass cutting conditions were simulated. A description of these outdoor tests, a comparison of the outdoor

and laboratory dispersion tests for a sample of walk-behind mowers, and

an evaluation of the dispersion and penetration thrown objects tests proposed by the Offeror are given in this report. Included in the evaluation

is an analysis of the penetration by thrown objects which discusses such

effects as the shape, size and velocity of the thrown projectiles and the

effects of humidity on the target material.

It is assumed that the reader of this report is thoroughly familiar

with the contents of References 1 and 2 which formed the basis for the

investigation. Copies of these references may be obtained from the Con- sumer Product Safety Commission.

2. OUTDOOR DISPERSION TESTS

The general characteristics of the walk-behind mowers used in the

evaluation program are given in Table 1. These mowers were purchased

from normal retail stock. The models were chosen to represent many of

the existing lawn mower design variables which were available during

the late summer period of 1975. For the outdoor dispersion tests a portable target with wheels to move on rails along a grass surface was constructed. The test mower was centered in the target and attached to it

by thin cables, as shown in Figure 1. Initial tests were conducted with

the target stationary on the grass. Fifty sixpenny box nails were fed

head first into the rotating blades through a hole in the top of the

*For convenience, this surface is referred to as the "ground" in the

Rationale for the proposed laboratory tests [2].
Table 1 - Lawn Mower Characteristics

Mower Power

Measured Blade

Tip Speed

ft/min (m/s)

Blade Cutting Width

m (m)

WIA Electric 8 600 (43.7) 18.25 (0.46)

W2A*

W2B

Electric

El ectric

15 400 (78.3)

15 200 (77.3)

18.62

18.62

(0.47)

(0.47)

W3* Electric 18 600 (94.6) 18.00 (0.46)

W4A^

W4B

Gasoline

Gasoline

15 900 (80.8)

16 100 (81.8)

21.00

21.00

(0.53)

(0.53)

W5A Gasoline 16 300 (82.8) 21.50 (0.55)

W7A- Gasoline 17 800 (90.5) 23 . 75 ( 0 . 60)

Foreign Gasoline 17 300 (87.9) 19.00 (0.48)

W7B** Gasoline 12 700 (64.6)

(reduced throttle)

23.75 (0.60)

"Mowers tested on grass in addition to proposed dispersion tests.

'""Penetration test for W7B was conducted at a reduced throttle. All

other tests conducted with mowers at full throttle.

3
Figure la - PORTABLE TARGET DEVICE FOR OUTDOOR DISPERSION TESTS
mower deck.* This test was repeated, on a new grass surface, for each

of the five lowest blade height settings. It was apparent that these

tests were unrealistic, but they did indicate the relationship between

blade height setting and the number of target hits and gave a general

idea of the relative thrown object dispersion for the three gasolinepowered mowers tested.

A second set of dispersion tests was conducted in which the target

was towed along rails with the mower, so that the mower wheels were always in contact with the ground. A small hole was cut in the front of

the target and 50 nails were placed in line along the grass over which

the mower-target assembly was moved. This test was repeated for each

of the four lowest blade height settings for each of five lawn mowers.

To enhance the pickup of the nails, which are relatively heavy with low

surface area, small cubes of styrofoam plastic were attached by pushing

the nail shank through the cubes up to the head surface. No systematic

tests were made to optimize this technique, but this modification did

improve the nail pickup.

The results of these tests clarified the relationship between blade

height setting and the number of target hits which the initial tests

had indicated. The "worst" blade height, i.e., the height of the blade

giving the maximum number of target hits, is apparently determined by

the combination of the housing design and lift properties of each mower. Specifically, the number of nails picked up and the number of target hits

appeared to vary with the blade height setting differently for each of

the mowers.

Having determined the "worst" blade height setting, additional tests were run on each mower at that setting. One hundred nails with the attached styrofoam material were picked up as in the previous test, the target

hits were counted, and the test repeated. Most of the hits occurred in

elevations one to three, with relatively few in the fourth elevation below the blade plane, the elevations being designated in five angular

increments originating from the blade tip [1], as indicated in Table 2.

It should be noted that elevation four intersects the ground and elevation five is not applicable to these tests. Scoring of the mowers for

outdoor tests on grass was computed in the manner specified in Section

1205. 21(s) of the proposed standard [1], shown in Table 2. The equations

used were modified as required for the number of test projectiles used.

It may appear to the casual observer that the introduction of a grass

surface and the problems of nonuniform test surfaces, ricochets and other

*The rationale for selecting nails for the dispersion tests is their

resemblance to the most dangerous real-life thrown objects, pieces of

wire [2]

.

5
Table 2 - Weighting Factors for Dispersion Tests for Walk-Behind

Lawn Mowers

Zone*

Elevationt ^""^^

Front

90°

Right

105°

Operator

60°

Left

105°

1 (+45° to +30°)

i. ^

3.4 12.1

1 1 ^'"^

-LX

100 12.1

2 (+30° to +15°) 3.1

,

10.9 90

1 7

^-'''''^

10.9

3 (+15° to 0°)

^

2.4

8

^^^^

8.5 70

1 fi ^^'"''^

8.5

k

( 0° to -15°)

^

1.2

^

4.2

1 4

^'^'^'^

35

1 9

4.2

(-15° to -30°) \^r 0.2

^

0.6

IS/'^

5 20x^

0.6

*Zones defined with respect to center of rotating blade [1].

tElevations defined with respect to blade tip circle [1].

Note: The number in the corner of each box is the window number.

The equations for the dispersion test calculations for walk-behind mowers

are

Score for each test : 20

i = 1

where H.W. = number of hits times the

X X weighting factor in window "i"

Standard deviation for each test:

6
questions relating to actual grass cutting conditions could give misleading results. However, the results of this evaluation showed that:

1) none of the five tested lawn mowers unconditionally failed the ac- ceptance criteria of the proposed standard, X < (2-R)*, although one mower would have required retesting; and 2) the relative performance

of the mowers was similar for the outdoor tests and the proposed mower

tests. These results should not be construed as verifying the procedure

of the proposed tests since only a limited number of tests were conducted

on a small sample of mowers using one type of projectile, picked up at one location, and no replicate tests were performed on different samples

of the models tested. The results suggest rather that the introduction

of a grass or grass-like substrate which permits ricochets of objects

may not be "a source of great confusion and error in the interpretation

of test data" as stated in the Rationale of the Proposed Standard [2].

3. PROPOSED DISPERSION TESTS

For the evaluation of the proposed dispersion tests, a wooden weatherproof test chamber was constructed suitable for conducting the dispersion

and the penetration tests on any of the walk-behind and most of the riding

mowers in the test program. A photograph of the test chamber is shown in

Figure 2. The mower in the figure is set up for a dispersion test.

In addition to the five walk-behind mowers tested outdoors, two other

models were employed in the evaluation of the proposed dispersion tests

[1]. The mower to be tested was first centered in the test chamber so that the horizontal plane of the blade tip circle was the required minimum

distance of 32 in (81 cm) above the floor. The blade cutting height of

the mower was adjusted to the setting closest to 2 in (51 mm) and the trailing shield was supported so that its position was the same as if the mower

were placed on the ground. Energy absorbent material was placed under the

mower after adjustment of the location of the nail injection tube.

To avoid problems with the tearing of large areas of the proposed dispersion test target material of 50 Ib^'fkraft paper, which complicates the

*X is the average of the scores for two tests and R is the average of

the standard deviations S for two tests or 0.20, whichever is greater, for

walk-behind mowers. Equations for X and S are given in Table 2.

**Paper and cardboard specifications referred to in this report are trade designations which have not yet been issued in metric equivalents.

This is non-trivial since units vary from weight/ream, weight/standard

sheet, to force/area bursting strength, and so forth.

7
Figure 2 - CHUMBER FOR EVHIUATIN6 PROPOSED THROWN OBIECTS TESTS

(DISPERSION TEST ARRANGEMENT SHOWN)

8
scoring of nearby hits passing through the torn areas, a single layer of

350 lb cardboard specified for the penetration tests was used as the target. No comparative tests of localized damage between cardboard and kraft

paper targets were made.

Dispersion tests were run for each of the mowers listed in Table 1.

Tests were conducted with the grass bag attached as well as without the

grass bag for all but the foreign made mower, which could only be used_

with a grass bag. Each of the mowers tested had average test scores, X,

which fell below the specified criteria (2-R) for unconditional acceptance,

both with and without the grass bag [1]. Furthermore, all but mower W7A

had test scores which fall below the criteria (1.6-R) specified for un- conditional acceptance 24 months after the effective date of the proposed

standard [1]. An additional dispersion test was conducted for W7A and it

appears that this mower might fail the proposed retesting criteria for

24 months after effective date of the standard.

To compare the results of the proposed and outdoor dispersion tests,

the ratio of the average test score to the unconditional acceptance criteria, X/ (2-R) , was calculated. These values were plotted against the

mower blade tip speed, which was based on a measurement of the maximum

blade rotational speed. The resulting graph for the gasoline lawn mowers

is shown in Figure 3.

As can be observed from this figure each of the gasoline powered

mowers would be considered acceptable when evaluated by the proposed

dispersion tests since the ratio for unconditional acceptance was always

less than unity. The most obvious feature of the graph is the clear dependence of the test score to acceptance ratio on the mower blade tip

speed. The mower having the highest blade tip speed was relatively worse

in its dispersion characteristics than the other mowers tested. The same conclusions were reached when comparing the results of the electric powered

mowers with each other.

4. PENETRATION TESTS

Penetration tests to evaluate the tendency of a thrown object to

penetrate a target were conducted on six walk-behind mowers according

to the proposed standard [ 1] . The setup for these tests is shown in

Figure 4. The results of the tests are summarized in Table 3, where the

results are listed in the order of increasing mower blade tip speed, a plot of which is given in Figure 5. It is clear from Figure 5 that the

blade tip speed strongly influences the penetration characteristics for

the mowers. The significance of the blade speed in thrown objects technology is documented in various sources, silch as Reference 3 and in the

following analysis where additional background information and references

are given.

9
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/

/

/

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Without

Grass Catcher

Proposed Dispersion

Tests with Mowers

Susperrded

With Grass Catcher

15(76) 16(81) 17(86) 18 x10^(91)

BLADE TIP SPEED, ft/min(m/s)

Figure 3 -TEST SCORE : ACCEPTANCE RATIO VS. BLADE

TIP SPEED FOR VARIOUS GASOLINE POWER MOWERS

10
Figure 4 - LAWN MOWER AND TARGET ARRANGEMENT FOR PENETRATION TEST

11
4

o

e

e12(61) 14(71) 16(81)

BLADE TIP SPEED, ft/min (m/s;

18 X 10^(91)

Figure 5 - NUMBER OF PENETRATION FAILURES VS BLADE

TIP SPEED FOR SIX LAWN MOWERS

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13
5. ANALYSIS OF PENETRATION

5.1 Introduction

In this section, we develop a theory of penetration which provides

a simplified treatment of the major factors affecting depth of penetration. This theory is compared with available experimental results and

the agreement is good. The proposed penetration test is designed to measure the depth of a thrown object for an evaluation of its hazard to

people. Penetration depends not only on the kinetic energy of the object, but also on its shape and its orientation with respect to the target

surface. That is a nail is less likely to penetrate the target if it

strikes the target sideways instead of head-on. This shape variable is

eliminated in the proposed test by using spherical thrown objects, i.e.,

0.250 in (0.64 cm) diameter steel balls. This tends to give, for a number of randomly spinning objects, an average projected area and thus an average penetration. Undoubtedly, the nail having equal mass and velocity

as the sphere (i.e., having the same kinetic energy) will penetrate further than the sphere if the nail strikes point first.

5 . 2 Theory

Understandably, there is little data available on the penetrability

of human flesh as compared to other materials. In general, there is agreement that depth of penetration is a function of particle velocity. By as- suming that human tissue acts as a viscous medium, Sperrazza and Kokinakis

[4] solved a second-order differential equation, the result of which predicted that depth of penetration should be proportional to impact velocity.

In fact, their experimental data indicated that the relationship was linear,

but not strictly proportional. If d is the depth of penetration, m is the mass of the thrown object, A is its (cross-sectional) area, V-j^ is the impact velocity, and C-^ and are constants, Sperrazza and Kokinakis [4]

give, for tissue, the empirical result

d =^ CAV, - V)mA~^ (1) i 1 o

A similar result was obtained by Whiteford and Regan [5]. The tests were

performed using steel cubes fired into wallboard. Other tests, performed

by the Army, using spherical and cylindrical objects fired into wallboard

and similar media [6,7] gave results which can be expressed as

d = C2V^m^A"^ (2)

where u = 1.358, v = 0.925 and w = 0.867 for one brand of wallboard [6]

and u = 1.236, v = 1.160 and w = 1.121 for a second brand [7]. Tests

performed by Nestelroad and Sevart [8] on 350 pound cardboard using

0.176 in (0.45 cm) diameter, 350 mg balls, showed that

d = C^Y^''^^ (3)

14
We may derive a generalized form for these results by allowing for a nonlinear viscoelastic target material behavior. Upon striking the

target, the object exerts a dynamic force mV. This is resisted by a force of nonlinear viscosity, -C^AV'^, where and N are constants.

Then,

mV = -C,AV" (4) 4

where n = 1 is the linear viscous case [4] . By integration it can be

shown that this gives a maximum depth of penetration in the nonlinear

case of

d = C^V^mA"-*-, n 1 (5)

where y = 2(1 - n). This result neglects oscillatory damped rebound effects prior to stopping. The result agrees directly with the empirical

result (equation 2). Similarly, the linear viscous case (n = 1) gives

d = V.mA"^, n = 1 (6)

^4 ^

This is the rational basis for Sperrazza and Kokinakis' [4] empirical

equation (1).

This nonlinear viscosity model thus appears to be in quite good

agreement with all of the preceding results. Equations 5 and 6 give the

general form of the empirically determined equations (1-3) . In particular, the theory predicts the exponent for mass to be plus one, whereas

values of +1 [4,5], 0.925 [6], and 1.16 [7] were measured. The exponent

of area, w, is here predicted to be -1, and empirical values of -1 [4,5]

,

and -0.867 [6], and -1.121 [7] were measured. The flexibility of the model

(etc.) fit the exponent of velocity as accurately as we wish.

5.3 Shape Effects

Tests have been conducted [ 9] which demonstrated the penetrability

of 350 lb cardboard by a 0.50 in (1.27 cm) diameter by a 0.50 in (1.27 cm)

long steel cylinder. We have tabulated these and the Nestelroad and

Sevart [8] results in Table 4. There is some disagreement on the critical

values for impact velocities. The data reported in [9] shows that a 0.50

x 0.50 in (1.27 x 1.27 cm) steel cylinder will fully penetrate one layer

of 350 lb cardboard at about 4620 ft/min (23.5 m/s) . The data in [8] indicates, that for a 0.176 in (0.45 cm) diameter ball, the velocity to

penetrate one layer is about 8760 ft/min (44.5 m/s). This can be explained

15
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16
by the above model from equation (5) , for the case of the equal diameter

for the cylinder and sphere. From equation (5) , for the case of sphere

and the cylinder of equal diameters striking the target "end on", the

theoretical velocity ratio (V1/V2) = 0.470 where V]^ is the velocity of

the cylinder and V2 is the velocity of the sphere. The measured velocity

ratio is (V1/V2) = (4620/8760) = 0.527. This suggests the potential usefulness of the model in explaining and predicting relative penetration

behavior for various shapes.

5.4 Size Effects

Another application of the theory that is quite pertinent to the

development of the proposed standard is the effect of ball size on the

depth of penetration. Recall that the proposed standard which uses

0.250 in (0.64 cm) diameter balls was partly based on the 0.176 in

(0.45 cm) diameter ball tests [8].

We would like to know if direct comparison can be made between depth

of penetration in the 0.176 and 0.250 in (0.45 to 0.64 cm) diameter ball

tests at the same velocities. The empirical equation (3) obtained by

Nestelroad and Sevart [ 8] can be rewritten as

1 72 -1

d = C^mV^ A (7)

by combining equations (3) and (5). For a given spherical material, the

ratio of (m/A) = pV/A = p(4R/3), where R is the ball radius and p is the

material density. Thus d = C5RVi^'^^, or the depth of penetration at a given velocity should be directly proportional to the ball radius. Increasing the ball diameter from 0.176 to 0.250 in (0.45 to 0.64 cm) at

a given engine speed should increase the depth of penetration by about

42 percent. This is certainly a significant change representing a penetration of over four sheets of cardboard for a 0.250 in (0.64 cm) ball

versus about three sheets for a 0.176 in (0.45 cm) ball.

5.5 Velocity Effects

It should be pointed out that a small change in the velocity ex- ponent would correspond to a great variation in depth of penetration

according to equation (7). We can also ask the specific question, "What

effect should doubling the engine speed have on depth of penetration?".

From equation (7) we have (d2/d2) = (2)-'^*^^. That is, doubling the engine

speed should increase depth of penetration by a factor of 3.3.

5.6 Hvimidity Effects

It could be that the failure of some of the mowers tested was due

to the fact that the penetration tests were performed in dry midwinter.

17
On the other hand, the previous tests were performed elsewhere, in the

spring, when humidity was presumably higher.

Even without resorting to the model, we can intuitively rationalize

that the dry cardboard tends to fracture rather than yield viscously so that it acts as a relatively brittle material to high speed missiles.

Using the model, on the other hand, we see that in a higher humidity en- vironment the cardboard is less brittle and more viscous (large n, hence

small y, hence small penetration according to equation (5)). Thus, the

dry winter tests would be expected to have greater penetration and the

presumably more humid tests performed in the spring would be expected to

give lesser penetration. In short, mowers might well pass the penetration

tests in spring or summer when the humidity is high and fail the test in

winter.

The predicted maximum depth of flesh would be slightly over an inch

for a 0.25 in (0.64 cm) diameter steel ball missile. This corresponds

to penetration of five layers of cardboard, assuming an equal penetrability for 350 lb cardboard and human tissue. This is of the same order

of magnitude as maximum wound depths found in hospital accident reports

of about 4 to 5 in (10 to 12 cm) due to wire objects, which are more

sharply pointed. At these speeds, either the wire objects or steel balls

could kill a bystander if struck in a critical area, such as in the heart

or in the head. Indeed, this has happened, as is well known, and has been

a primary motivation to developing the standard. The scoring distribution

of the proposed test is designed to permit deeper penetration at low elevations, presumably where vulnerability to vital organs is less. However,

for a bystander at 22 ft (6.7 cm) from the mower, a penetration of about

an inch with a steel ball (or about 4 in (10.2 cm) with a properly oriented

wire) is permissible to a height of 6 ft (1.8 m) . This is sufficient to cause death. Therefore, it would appear that this whole situation requires

further investigation.

6. CONCLUSIONS AND RECOMMENDATIONS

It should be emphasized that the need to answer questions about how

realistically the suspended mower test position simulated actual grass

mowing was the principal reason for conducting the outdoor tests in which

grass cutting conditions were simulated. The single most useful result

from the outdoor tests was the observation that there exists a "worst"

blade height setting for which the target hits are most frequent, which

may depend on some variables not tested. The proposed suspended mower

dispersion tests cannot account for this least favorable mode of operation.

Based on the results obtained during the outdoor tests and the proposed dispersion tests, we concluded that the test acceptance criteria

may not be rigorous for many walk-behind mowers currently being manufactured.

18
If the influence of a test surface is to be considered, then we recommend

that it be included as an integral part of the retesting. To implement

this evaluation, a controlled substrate surface would be preferred to the

actual grass surfaces used in this study. Any such refinements to the

proposed dispersion test would require further test development.

With regard to the proposed projectiles for the dispersion tests,

it is necessary to specify the dimensions of the nails since there is no generally accepted, unique, sixpenny steel box nail configuration. During

these tests, three different types of "sixpenny box nails" were purchased

at one time or another varying in both head and shank dimensions. We also

recommend that either a single layer of double wall cardboard be used for

the dispersion test target or that the kraft paper be supported by cardboard, rather than the proposed 50 pound kraft paper alone. A single bent

nail can tear a large hole in the kraft paper, and nails which then hit

nearby will not be detected.

The limited penetration tests conducted suggest that this is a discriminating test for currently manufactured lawn mowers. To guard the

safety of test personnel, it is necessary to provide some additional barrier beyond the fifth target layer in the region where the steel balls

are ejected. The test chamber developed for these tests was provided

with sliding doors which offer complete protection from thrown projectiles

except for a narrow space where the projectile injection tube can protrude. An additional barrier to cover this narrow area is recommended

for complete protection of test personnel.

7. ACKNOWLEDGMENTS

The authors are indebted to D. E. Marlowe and R. E. Snyder for the

design and construction of test equipment, to F. Davis for assistance

with the experimental program, and to N. Halsey for photographic results.

19
8 . REFERENCES

[1] Consumers Union of United States, Inc., Proposed Safety Standard for

Power Lawn Mowers, 1205.10, 1205. 21(s), July 1975.

[2] Consumers Union of United States, Inc., Rationale for Proposed Safety

Standard for Power Lawn Mowers, July 1975.

[3] Ferguson, H. S. , "Thrown Objects Technology and The Low Tip Speed

Rotary Lawn Mower," presented to the Thrown Objects Subcommittee

of the Safety Standard for Power Lawn Mowers, April 1975.

[4] J. Sperrazza and W. Kokinakis, "Ballistic Limits of Tissue and

Clothing," Ann. of N.Y. Acad. Sci. , Vol. 152 (1968), p. 162-167.

[5] C. W. Whiteford and J. M. Regan, "The Determination of the Striking

Velocity of Steel Fragments by Their Mass and Penetration into Witness

Material," (April 1961), Ballistic Research Laboratory Aberdeen Proving Ground, Maryland, available from Defense Documentation Center,

Alexandria, Virginia, Report AD 258533.

[6] D. Malick, "The Calibration of a Collection Medium for the Determination of Particle Velocity," (July 1962), Ballistic Research Laboratory, Aberdeen Proving Ground, Maryland, available from Defense

Documentation Center, Alexandria, Virginia, Report AD 278545.

[7] D. Malick, "The Calibration of a Wallboard for the Determination of

Particle Speed," (May 1966), Ballistic Research Laboratory, Aberdeen

Proving Ground, Maryland, available from Defense Documentation Center, Alexandria, Virginia, Report AD 485059.

[8] R. Nestelroad and J. Sevart, "Results of 350 Pound Cardboard Penetration Calibration Test," (May 1975) Wichita State University, Department of Mechanical Engineering, unpublished senior project.

[9] James F. Efflant, letter to R. Heidner and S. Heth, November 4, 1966,

[10] Guy E. Braughler of Daisy Air Rifle Co., quoted in [7] above.

USCOMM-NBS-DC 20
<BS-114A (REV. 7-73)

' U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 76-1022

2i Gov't Accession

No.

3. Recipient's Accession No.

TITLE AND SUBTITLE

EVALUATION OF THROWN OBJECTS TESTS FOR

PROPOSED SAFETY STANDARD FOR POWER LAWN ^£)WERS

5. Publication Date June 1976

6. Performing Organization Code

k AUTHOR(S)

j Donald C. Robinson and Roger B. Clough

8. Performing Organ. Report No. NBSIR 76 1022

PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project ' Task 'Work Unit No. 2130541

11. Contract /Grant No.

2. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Consumer Product Safety Commission

Washington, D. C. 20207

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

6. ABSTRACT (A 200-word or less (actual summary ol most significant inlormation. U document includes a significant

bibliography or literature survey, mention it here.)

An evaluation was made of thrown objects dispersion and penetration tests

which have been developed for a proposed safety standard for power lawn mowers.

To evaluate the proposed laboratory dispersion tests, supplementary outdoor

dispersion tests were conducted on walk-behind lawn mowers in which actual grass

cutting conditions were simulated. A description of these outdoor tests, a com- parison of the outdoor and laboratory dispersion tests for a sample of walk-behind

lawn mowers, and an evaluation of the proposed dispersion and penetration tests is

given. The evaluation includes a theoretical discussion of the penetration of

thrown objects which is related to experimental results in terms of the shape,

size and velocity of the thrown projectiles.

7. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

I Dispersion tests', lawn mowers', penetration tests^ power lawn mowersj safety

standard; thrown objects tests.

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