NAT'L INST, OF STAND & TECH NBS

Publications

NBSIR 77-1299

POWER LAWN MOWERS:

Time-to-Blade Access

A. M. Ramey

J. J. Persenky

Human Factors Section

Product Systems Analysis Division

Center for Consumer Product Technology

Institute for Applied Technology

National Bureau of Standards

May 1 977

Final Report /—QC100

.U56

77-1299

Prepared for U.S. Consumer Product Safety Commission

DEC \ 2 1979

NBSIR 77-1299

POWER LAWN MOWERS:

TIME-TO-BLADE ACCESS

A. M. Ramey

J. J. Persensky

Human Factors Section

Product Systems Analysis Division

Center for Consumer Product Technology

Institute for Applied Technology

National Bureau of Standards

May 1 977

Final Report

Prepared for U.S. Consumer Product Safety Commission

U.S. DEPARTMENT OF COMMERCE. Juanita M. Kreps. Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Table of Contents

Page

List of Figures iii

List of Tables iii

Acknowledgements v Abstract vii

1.0 Introduction , 1

2.0 Methodology 2

2.1 Participants 2 2.2 Apparatus 2 2.3 Procedure 5

3.0 Results 6

4.0 Discussion 9

References ...» 11

Appendix A- -Wiring Diagram 13

Appendix B-j^- -Research Participant Agreement 15

Appendix B2 -

- Ins tructions 17

Appendix B,--Data Sheet 19

i

List of Figures

Number Title Page

1 Comparison of Lawn Mower to Simulator .... 3

2 Test Fixture 4

List of Tables

Number Title Page

1 Reaction and Movement Times 7

2 Analysis of Variance, Movement Time (T^) . . . 8

3 Duncan Mult iple -Range Test, Movement 8

Time (T^)

4 Percentile Data, Movement Time 10

iii

Acknowledgements

The authors wish to acknowledge the help of Michael

Fulcomer, Harry Appleby, and James Huckeba in instrument

construction, Barbara Stanton and Addie Stewart in data

collection, and Kim Leaman and Bonnie Matteson in secretarial support.

ABSTRACT

This document is the final report for the Consumer

Product Safety Commission of an investigation of time-toblade access for walk-behind power mowers. The problem

studied was: "How long after the dead -man control is released

should a lawn mower blade be allowed to rotate?" The Human

Factors Section at NBS performed this empirical study of

operator movement time to determine an ergonomically sound

recommendation for blade stopping time.

One hundred participants were tested using a reaction

time device designed to measure mower users' approach times

to various areas of potential blade contact. The test

apparatus permitted measurement of both reaction time--

or time to release a simulated dead-man control at the

onset of a cue light and, more importantly, movement time--

or the time from the release of the dead-man control to

activation of one of five switches in the blade access

area

.

Analysis of the data revealed no statistically significant differences in reaction time as a function of movement

distance. As expected, however, movement time does increase

linearly as movement distance increases. The range of

average movement times observed was from 0.6 to 3.3 seconds.

The median movement time at the shortest and longest distances

were 1.4 and 2.2 seconds respectively. An inspection of

percentile information for movement time suggests a bladestopping time of 0.7 seconds in order to protect the maximum

reasonable number of people.

Recommendations for needed further research are discussed.

vii

Power Lawn Mowers

Time -to -Blade -Access

1.0 Introduction

The Product Profile , "Power Mowers" prepared by the

U.S. Consumer Product Safety Commission (CPSC) and dated

October 1976 (1) , indicates that there were an estimated

178 000 power mower -related injuries in 1975. The associated

costs were estimated to be in excess of 73 million dollars.

The National Electronic Injury Surveillance System estimates

that over half of these injuries are from contact with

the blade. In an attempt to reduce the number of blade

contact injuries, the CPSC has accepted Consumers Union's

(CU) proposal (under Section 7 of the Consumer Product

Safety Act) to include a requirement for a dead-man control

on power lawn mowers. This dead-man control must be continuously

activated by the lawn mower operator in order for the blade

to operate. If the control is released, the blade must

stop

.

The problem raised by this requirement is: "How long

after the dead-man control is released should the blade

be allowed to rotate?" An alternative question is: "How

long does it take an operator to move from the handle

location to an area of potential contact with a moving

blade?" Several blade stopping times have been suggested.

The power mower industry, through the Outdoor Power Equipment

Institute, suggested a stopping time of seven seconds

(2) . Consumers Union suggested a three or four second

stopping time based on a brief study of college students

using mowers. (3). In December 1974, the National Bureau

of Standards (NBS) analyzed data from the University of

Iowa and recommended a blade stopping time of one second

as a safest case (4)

.

In view of the inconsistencies among these recommendations,

the Office of the Medical Director at CPSC requested that

the NBS Human Factors Section perform an empirical study

of operator movement time to determine an ergonomical ly

sound recommendation for blade stopping time for walkbehind power mowers.

This report describes the results of Phase I of a proposed three-phase effort. It is anticipated that Phase II

will be designed to determine how operators approach the

blade area in a naturalistic setting. Phase III will then

make use of the information collected in Phase II so that a high fidelity simulation of the approach-to-blade area can be

designed for laboratory measurement of time-to-blade access.
The current study (Phase I) was a basic laboratory

simulation of operator approach-to -blade -area patterns

where the data were time -to -blade -area access. The 100

participants were tested using a reaction time device

designed to simulate a power lawn mower (Figure 1) . Both

simple reaction time (time from cue activation to release

of the lever simulating a dead-man control) and movement

time (time from lever release to reaching various lawn

mower blade-access areas) were measured.

" 2.0 Methodology

2.1 Participants

Sixty-four males and 36 females participated in the

study. They ranged in age from 16 to 62 years with a mean

age of approximately 35 years (S.D. = 12.25 years). Mean

height was 174.0 cm (S.D. = 8.7 cm), with a range of 154.6

to 191.3 cm. Weight averaged 72.3 kg (S.D. = 15.8 kg),

with a range of 45.9 kg to 138.6 kg. Of the participants

88 were right hand preferred and 12 were left hand preferred.

2 . 2 Apparatus

A sketch of the simulator used in this study is presented

in Figure 2. The test apparatus included: a dead-man

control located at the handle, five location - indicator

lights and corresponding switches, and a cue light. All

parts were mounted on a plywood platform equipped with

wheels for ease of transport.

Five switches were positioned behind metal plates

on the right side of the apparatus at distances which

represented areas of potential blade contact. Distance

data were collected from a sample of typical lawn mowers

and included the horizontal distance from the handle to

the right rear wheels, discharge chutes, and right front

wheels. The closest back wheel was found to be 50.0 cm

from the handle, whereas the farthest front wheel was

150.0 cm. Switches One through Five on the test fixture

were situated at 50.0, 75.0, 100.0, 125.0, and 150.0 cm

from the handle, respectively. Five orange locat ion - indica

tor lights were positioned along the top of the right

side of the platform at distances corresponding to the

switch locations.

A red cue light was located at the far end of the

platform, 180.0 cm from the handle. This light cued the

participants to release the handle and move to and depress

the switch corresponding to the orange light which was

lit

.

2
3
CO
The control box included three timers and switches

for activating the various location-indicator and cue

lights. The timers measured:

Tl - Simple Reaction Time; that is, time between cue

light presentation and handle release.

T2 - Movement Time discounting errors; that is, time

from handle release to striking any of the switches.

T3 - Movement Time to correct switch; that is, time

from handle release to striking the correct switch.

Reaction times were recorded to 0.1 of a second. Timer

Ti was activated by the experimenter and timers T2 and T3

were activated by the release of the dead-man control.

In addition to the three timers and the light controls,

the experimenter operated a separate timer to set the

inter-trial (time between complete trials) and inter -stimulus

(time between onset of location- indicator light and cue

light) intervals.

The wiring diagram for the test fixture is presented

in Appendix A.

2.3 Procedure

Once the required consent form was signed, each participant's age, sex, weight, height, and handedness were recorded.

Participants were then instructed to:

- Stand behind the handle.

- Grip the dead-man control when a locationindicator light was activated.

- When the cue light was activated, release

the handle, move to and depress the switch

corresponding to the location-indicator light.

- Return to the original position behind the panel

and await the next trial.

In addition the participants were instructed to move at

a normal rate of speed and not to rush.

The total number of trials per participant was 25,

five trials at each of the five positions. The presentation

sequence was randomized for each subject.

5
The inter-trial interval was held constant at 15

seconds. However, the inter - st imulus interval varied from

5 to 15 seconds with a mean of 10 seconds. The time between

presentation of the location - indicator lights and the

cue light was randomized.

A copy of the consent form, instructions, and data

sheet are included in Appendix B.

.

" 3.0 Results

Blade-area-access -time data are summarized in Table

1. Simple reaction time (T]^) and movement time to correct

switch (T3) are presented separately, as are data for

left- and right-handed participants. In addition, data

for all participants, combining left- and right-handed

individuals, are presented. Time for errors (T2) is not

presented since only one error was committed in the 2500

trials.

No mean difference (t-test) was found between the

left and right-handed group for simple reaction time (Ti)

.

Also, a t^-test for independent samples revealed that the

differences between the right- and left-handed groups

in movement time CT3) were not signif icant^ . Therefore,

data for both groups were combined for further analysis.

A Treatment by Subjects Analysis of Variance was

performed for reaction time (Ti) . No significant treatment

effect was f ound^ . A similar analysis of the movement

time (T3) data yielded a significant treatment effect^.

A summary of the analysis of variance for movement time

is presented in Table 2.

The Duncan Mult iple -Range Test was used to further

analyze the significant treatment effect found for movement

time (See Table 3) . The results indicate that a positive

relationship exists between distance and movement time.

As would be expected, this relationship is an increasing

linear function; specifically, for every 25.0 cm that

distance increased, movement time increased by 0.2 seconds.

l(t(98) = 1.13, p > .05)

2[F(4,396) = 2, £ > .05]

5[F(4,396) = 339.16, p < .01]

6
Table 1 Reaction Time and Time to

Move from Lawn Mower Handle

to Various Locations on the Mousing

(Based on Participants' Averages for 5 Trials)

Switch Position

Time Statistic 5 0.0 cm 75.0 cm 100.0 cm 125 . 0 cm 150.0 cr Seconds C t~\ n r\ o e c ona s oeconc-s Seconds Seconds

Right iianaea rarricipants

- (N =• 88)

Reac - Med 0.5 0.5 0.6 0.6 0.6

t ion

Range 0.3-1.2 0 3-2 0 0 3-1 1 0.3-1.3 0.3-2.0

Move - Med 1.4 1.5 1.8 2 . 0 2 . 2 ment

Range 0.6-2.7 n 7-7 s n 7 - n 0.8-3.3 0.9-3.8

Left Handed Participants

(N = 12)

Reac - Med 0.5 0.6 0.5 0.6 0.6

t ion

Range 0.3-0.9 0.3-0. 8 0.3-0.9 0.3-1.1 0.3-1.1

Move - Med 1.4 1.4 1.6 1.8 1.9

ment

Range 0.7-2.1 0.6-2. 0 0.9-2.1 1.1-2.5 1.2-3.0

Combined Data

(N = 100)

Reac - Med 0.5 0.5 0.6 0.6 0.6

tion

Range 0.3-1.2 0.3-2 . 0 0.3-1.1 0.3-1.3 0.3-2.0

Move - Med 1.4 1 . 5 1.8 2.0 2 . 2 ment

Range 0.6-2.7 0.6-2 . 8 0.7-3.0 0.8-3.3 0.9-3.0

7
Table 2 Analysis of Variance

Movement Time (T,-)

Source SS d£ ms F P

Total 190. 0 499

Sub j ects 136. 7 99

Treatments 40. 7 4 10.17 339.16 <.01

Error 12.6 396 0.03

Position

50. Ocm

75. 0 an

100. Ocm

125.0 an

150.0 cm

Table 3

Duncan Multiple -Range Test

Movement Time (T^)

Position

50.0 an 75. 0 on 100. Oon 125.0 cm 150.0 cm

1.4

1.4

1.6

1.8

2.0

2.2

.2 r Critical

Range

.8* 5 .03

.6* 4 .03

.4* 3 .02

.2* 2 .02

p < .05

8
4 . 0 Discussion

The major conclusion of the present study is that

a blade stopping time in excess of 2.2 seconds may be

too long for a majority of the population. This is based

on the 2.2 second median for movement time at the longest

distance and the even shorter times for the other distances

tested. It is recommended that a "worst case" approach

be adopted when establishing a blade stopping time for

regulatory purposes. That is to say, the 5th percentile

time is recommended, since this is the fastest time that

could be achieved. Therefore, a greater percentage of

the population, approximately 95 percent of the sample,

would be protected if this time were adopted for the regulation. Also, the time selected should be based on position

one, the closest position, since some of the mower adjustments

which have resulted in accidents have been at this position.

Table 4 presents percentile information for movement

time for each of the five simulated blade -contact locations.

It can be seen again that if a "worst case" approach is

taken, it is the 5th percentile time, rather than the

95th, which would protect 95 percent of the population.

Conversely, only five percent of the population would

be protected if the 95th percentile time were adopted.

It is, of course, CPSC's responsibility to establish the

blade-stopping time. As seen in Table 4, solely in terms

of basic Movement Time, 0.7 seconds (the 5th percentile

time for the closest position) would be the appropriate

conservative choice.

In lieu of any data to the contrary, it must be assumed

that lawn mower operators move directly from the position

behind the mower handle to an area of potential blade

contact. For example, they may go immediately from the

handle to the wheels to make height adjustments or to

discharge chute to remove debris. In the original lawn

mower research proposal to CPSC, the NBS proposed to conduct

field observations of lawn mower operators so that realistic

movement scenarios could be developed (Phase II). Further,

these scenarios would then be simulated in a laboratory

setting to determine actual movement times. Until these

studies are performed, more valid time criteria cannot

be determined.

9
Median

5th Percentile

95th Percentile

Table 4

Percentile Data

Movement Time (T3) in Seconds

Switch Position

50.0 cm 75.0 cm 100.0 cm 125.0 cm 150.0 cm

1.4

0.7

2.2

1.5

0.8

2 . 5

1.8

0.9

2.8

2.0

1.1

3 . 0

2.2

1.2

3.2

10
References

1. U.S. Consumer Product Safety Commission, Product Profiles : Power Mowers , October 1976.

2. Outdoor Power Equipment Institute, Technical Comments

and Recommendations on Proposed Lawn Mower Safety

Sta.ndard , Submitted to the U.S. Consumer Product

Safety Commission, September 1975.

3. Consumers Union of United States, Inc., Proposed Safety

Standard for Power Lawn Mowers , Submitted to the

U.S. Consumer Product Safety Commission, July 1975.

4. Porter, L.G., Jones, C.E., and Persensky, J.J. Some

Aspects of Lawn Mower Safety , Status Report to

U.S. Consumer Product Safety Commission, December 1974.

11

Appendix A- -Wiring Diagram

EXPERIMENTERS

SWITCHES

OPST

201 2

—CHASSIS PLUS MOUNTED ON SIMULATED MOWER

13

APPENDIX

MBS.783 U.S. DEPARTMENT OF COMMERCE

12.75, NATIONAL BUREAU OF STANDARDS

RESEARCH PARTICIPANT AGREEMENT

3. Li>si LiMiiiM N...

1

i

i

.

i 1. Pnnt iu.tl lnvet.lip.itcii

Dr. J. J. Persensky

2. D 1 VIS ion /Seel ion 441.02

4. Location 1

i ^ i Gatlhershurii I

j

; Other ilvi i

j b. txperrnient Name Cod(

Tims to Mower Blade Access. Phase I: Preliminary Similation

I 6. Description of Exoerinient The purpose of this research is to determine liow long it takes a lawn mower

* operator to move from the handle to different locations on the mower housing.

The equipment used is not an actual lawn mower but is designed to simulate

actual distances. The participant is asked to stand at the handle until a signal

light is presented, then to move to and touch a panel near the front of the

simulated mower. This procedure will be repeated several times. The test will

1 help to define safe stopping times for lawn mower blades.

! 7. Risks to ParticiDant

j There is a slight possibility that when moving around the equipment, the

I participant might fall. To avoid such accidents a slip resistant surface will

I be used.

8. Responsibilities of Participant The participant, with informed consent, will follow instructions and answer

questions regarding age, sex, weight, and handedness.

9. Responsibilities of Investigator is The investigator will (1) fully explain the test procedure, (2) explain

the purpose of the test, (3) provide safe test conditions, and (4) keep personal

information confidential.

10. IT IS UNDERSTOOD THAT EITHER THE PRINCIPAL INVESTIGATOR. THE PARTICIPANT, OR THE PARTICIPANT'S PARENT OR GUARDIAN MAN

TERMINATE THE PARTICIPANT'S INVOLVEMENT IN THE RESEARCH AT ANY TIME WITHOUT INCURRING LEGAL LIABILITY FOR SUCH

TERMINATION.

11.1 hereby certitv that my participation is voluntary and that I have read and accept the terms of this aereement.

! Participant, or Parent or Guardian I Sninat uif

,

Datt

12. Principal Investigaioi ISiQnaturr DfiU

13. Early Termination by I SignBturc Date

15

APPENDIX B2

Instructions

Time -to- Blade -Access

I want you to stand behind this handle. When you see

one of the orange lights come on, please grip the handle.

Then, when you see the red light at the far end of the box

come on, release the handle and depress the metal plate under

the orange light that is lit. When you hit the metal plate,

the orange light will go out. Then return to the handle

area and wait for one of the orange lights to come on again.

Please don't hurry to strike the metal plates, but rather

walk at a normal rate of speed.

Do you have any questions?

17

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19
NBS-n4A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICA riON OR RI-PORT NO. NBSIR 77-1299

2. Gov't Accession

No.

3. Rccipitnt's Accession No.

4. TITLE AND SUBTITLE

Power Lawn Mowers: Time-to -Blade -Access

5. Publication Date May 1977

6. Performing Organization Code

/. AIM H(m( ^ ) A. M. Ramey, J. J. Persensky

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Projett/Task/Work Unit No. 441-1424

11. Contract/Grant No.

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

Consumer Product Safety Commission

5401 Westbard Avenue

Washington, D. C. 20207

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.) This document is the final report for the Consumer Product Safety Commission of

an investigation of time-to-blade access for walk-behind power mowers. The problem

studied was: "How long after the dead-man control is released should a lawn mower

blade be allowed to rotate?" The Human Factors Section at NBS performed this empirica]

study of operator movement time to determine an ergonomically sound recommendation for

blade stopping time.

One hundred participants were tested using a reaction time device designed to measure mower users' approach times to various areas of potential blade contact. The

test apparatus permitted measurement of both reaction time- -or time to release a simulated dead-man control at the onset of a cue light and, more importantly, movement

time- -or the time from the release of the dead-man control to activation of one of five

switches in the blade access area. Analysis of the data reveal no statistically significant differences in reaction

time as a function of movement distance. As expected, however, movement time does

increase linearly as movement distance increases. The range of average movement times

observed was from 0.6 to 3.3 seconds. The median movement time at the shortest and

longest distances were 1.4 and 2.2 seconds respectively. An inspection of percentile

information for movement time suggests a blade -stopping time of 0.7 seconds in order

to protect the maximum number of people.

Recommendations for needed ftirl^er research are discussed.

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

)

Consumer product; human factors; lawn mowers; reaction time; safety; standards

18. AVAILABILITY [J^ Unlimited

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^ Order I-'rom National T echnical Information Service (NT IS) Springfii-lJ, Virginia 221^1

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UNCL ASSIFIED

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