NAFL INST OF STAND & TECH

iMCis>iK /7-1298

Power Lawn Mowers: Ease of

Pull

V. J. Pezoldt

J. J. Persensky

Human Factors Section

Product Systems Analysis Division

Center for Consumer Product Technology

Institute for Applied Technology

National Bureau of Standards

June 1977

Final Report

Prepared for U.S. Consumer Product Safety Commission

—QC-

100

U56

77-1298

POWER LAWN MOWERS: EASE OF

PULL

NBSIR 77-1298

"dec Y2 1979

Use

V. J. Pezoldt

J. J. Persensky

Human Factors Section

Product Systems Analysis Division

Center for Consumer Product Technology

Institute for Applied Technology

National Bureau of Standards

June 1977

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

1.0 Introduction 1

2.0 Methodology 2

2.1 Subjects 2

2.2 Apparatus 2 2.3 Procedure 4

3.0 Results 7

3.1 Data Reduction 7

3.2 Data Analysis 7

4.0 Discussion 15

Appendix A- -Research Participant Agreement 19

Appendix B- -Number of Subjects and Number of Pulls

Executed at Each Pull Distance 21

Appendix CI --Minimum Force Judged Hard and Maximum

Force Judged Easy for Subjects Using Handle Simulator 23

Appendix CI I --Minimum Force Judged Hard and Maximum

Force Judged Easy for Subjects Using Housing Simulator 25

Appendix D- -Analyses of Variance: Handle 27

Appendix E- -Analyses of Variance: Housing 29

Appendix F- -Duncan Multiple Range Test: Handle

Maximum Force Judged Easy 31

Appendix G

- -Correlations Between Hardu^in, Easyjn^ax ^.nd

Subject Variables 33

Appendix H- -Percentage of Double and Incomplete Pulls 35

i

List of Figures

Figure Page

1 Ease of Pull Test Fixtures 3

2 Housing Simulator 3

3 Handle Simulator . 3

' 4 Test Apparatus Pull Handle 5

5 Cumulative Distribution of Hardjnin and

Easymaxj Handle Simulator, Female Subjects 11

6 Cumulative Distribution of Hard^ij^ and

Easyjj^ax» Handle Simulator, Male Subjects . 12

7 Cumulative Distribution of Harduj^^n and

Easyjjiaxj Housing Simulator, Female Subjects 13

8 Cumulative Distribution of Hardj^in

Easyjnax» Housing Simulator, Male Subjects. 14

List of Tables

Table Page

I Cumulative Distribution of Hardmin and

E3.syniax> Handle Simulator, All Pull

Distances Combined 9

II Cumulative Distribution of Hardmin and

Easyji^ax* Housing Simulator, All Pull

Distances Combined 10

iii

Acknowledgements

The authors wish to acknowledge the contributions of

many individuals toward the completion of this project.

Thanks are due to: Marie Hedman, Ann RameySmith, Barbara

Stanton, Addie Stewart and Martha Willard for many hours of

data collection and reduction; to Don Robinson and Michael

Fulcomer for design and construction of the test apparatus;

to Kim Leaman and Bonnie Matteson for secretarial support;

and to John McGahan and Harold Van Cott for major contributions to the conception, planning and execution of this

research.

V

Power Lawn Mowers: Ease of Pull

1.0 Introduction

The Consumer Product Safety Commission (CPSC) estimates

that there were 178,000 power lawn mower -related injuries

in calendar year 1975. Costs associated with these injuries

are estimated to be in excess of 73 million dollars. Over

half of the total lawn mower injuries resulted from contact

with the mower blade (1) . In an effort to reduce the number

of these injuries CPSC has accepted the proposal made

by Consumers Union (2) , the offeror of a proposed lawn

mower safety standard, to include a requirement for a "dead-man" control in a mandatory safety regulation (3)

The dead-man control must be continuously activated by

the mower user in order for the blade to operate. Release

of the control stops the blade within a specified time.

The blade may be stopped either independently of the mower

engine by means of a blade clutch/brake system or in conjunction

with engine shut-down. If this latter method is employed

to stop the blade, lawn mower users would likely be required

to restart their mowers repeatedly throughout a period

• of use. The CPSC has considered that this inconvenience

may encourage consumers to defeat the dead -man control,

unless the mower is "easy to restart." This study was

designed to provide CPSC with data based on subjective

judgments which can be used to objectively define "easyto-pull" as it relates to restarting power lawn mowers.

Many factors are involved in making a judgment about

whether a given lawn mower is easy to restart. Among the

more important of these are:

• the force of pull required to start the engine;

• the distance through which a pull must be made;

• the average number of pulls required per engine

restart

• the number of times the engine must be restarted

during the mowing period; and

• the time interval between restarts.

• In addition to these factors, which are all external to

the individual starting the mower, a number of human characteristics are of at least equal importance. To the obvious

factors of age, sex and physical condition must be added

the kinesthetic and proprioceptive feedback cues experienced

when pulling the starting cord and the connotations placed

upon the terms "easy" and "hard".
This study did not attempt to explore all of these factors.

The data generated by this effort, therefore, cannot be construed

as providing a definitive answer to the question "What is

easy to restart?" Rather, the present study attempts to

define easy to pull under laboratory conditions. In psychological terms , tEe problem becomes one of determining the relationship between physical stimuli and the psychological responses

to such stimuli. In this case the stimuli are the forces

which are exerted on a simulated pull -start mechanism and

the responses are subjective judgements about the ease or

difficulty involved in applying these forces.

2.0 Methodology

2 . 1 Subj ects

Seventy-four paid volunteers, 38 females and 36 males,

participated in the study. The participants ranged in age

from 12 to 63 years with a mean age of 33 years (S.D. = 17.7

yr) . Subjects ranged in height from 152 to 190 cm (Mean

= 169.1 cm, S.D. = 9.1 cm) and in weight from 40.0 to 97.2

kg (Mean = 65.5 kg, S.D. = 13.8 kg). All but three subjects

were right-handed. Requirements for subject participation

included general good health (no history of cardiovascular

trauma or chronic muscle -related incapacity) and experience

in pull -starting power lawn mowers. "Experience" was defined

as having operated a pull -start lawn mower at least 10 times

during the most recent mowing season. Each subject participated

in the study for approximately 1.5 hours a day on four

consecutive days. Since the distribution characteristics

of the lawn mowing population are not known, the sample of

subjects tested were chosen to reflect a broad range of

mower users.

2.2 Apparatus

Two simulators were designed and built for use in this

study. The simulators were operationally identical with

the exception of the locus of pull. Pulls on Simulator I (hereafter referred to as the "housing" simulator) were executed

from a height typical of several current lawn mower designs,

i.e., from a height of 38 cm (15 in). Simulator II ("handle"

simulator) was designed to be pulled from a position on the

handle, 83 cm (33 in) above the ground. Although most current

lawn mowers are similar to the housing simulator, lawn mowers

incorporating a dead-man control may have the pull handle

located in an area similar to that on the handle simulator.

Photographs of the two simulators appear in Figures 1-3.

2
3
The heart of each simulator was a modified, general

purpose five horsepower, gasoline engine. The automatic

compression release mechanism was defeated, increasing

the force required to overcome the first compression stroke.

The cyl inder -head was removed and replaced with a variable

displacement cylinder. The piston in this cylinder could

be positioned by the investigators in incremental steps,

providing a means to vary the compression ratio and, theoretically, the force required to overcome the compression.

The pulleys for winding the pull cords were notched

to accept cords of 46, 61, 76, and 91 cm (18, 24, 30 and

36 in) lengths in such a manner that regardless of cord

length, and therefore distance of pull, a pull would be

initiated from the same place in the compression cycle.

Custom-made pull handles, attachable to any of the

cords used, were employed (Figure 4). Measurement of the

forces applied when pulling the start cord was accomplished

by incorporating a load cell in the pull handles. These

load cells were interfaced with a digital readout peak

load indicator. Peak force, i.e., the maximum force exerted

at any point in the pull was measured to the nearest pound

(1 Ibf = 4.45 N)

In addition, photocells interfaced with a digital

interval timer were mounted on the shaft to permit timing

of a pull from initiation through completion of the specified

pull distance.

In summary, the simulators permitted pulls similar

to those experienced when pull -start ing a power lawn mower.

Pulls could be made through four distances and the difficulty

(resistance) of pulls could be manipulated by the experimenter.

Difficulty of pull could only be controlled in a very gross

sense, the actual force applied was determined by the participant.

The peak force and elapsed time of each pull was measured.

Forces required to actually start the engine could not

be determined since no gasoline nor spark plugs were present

in the simulators.

2 . 3 Procedure

After the participants signed the required informed

consent document (Appendix A) their age, sex, weight, and

height were recorded. Each subject was assigned to either

the handle or housing simulator for the duration of the

study. Assignment to simulator type was random with the

constraint that age and sex were distributed approximately

equally between the two simulators. Subjects were then

instructed as to the specific task they were to perform

and the apparatus was demonstrated.

4
Figure 4

Test Apparatus Pull Handle

Incorporating Load Cell
The subjects' task consisted o£ making 36 pulls at

the appropriate simulator on each o£ four consecutive days.

Each day the subjects were provided with a different length

cord which determined the distance through which they were

to pull. The distances through which they pulled were

approximately 46, 61, 76, and 91 cm (18, 24, 30, and 36

in). The order in which subjects pulled through the various

distances was randomly determined. Subjects were instructed

to pull the "starting" cord in the same manner they would

to start a lawn mower. Immediately following each pull,

subjects were requested to judge whether that pull was

easy or hard. A forced choice paradigm was employed requiring

subjects to judge each pull as either "easy" or "hard".

No intermediate responses were allowed. Participants were

instructed that the simulators were "perfect lawn mowers",

that is, they would start on every pull. In fact, of course,

the engine never started. Subjects made six pulls in a five minute test period followed by a rest period of approximately 10 minutes. Six test periods were accomplished each

day by each subject.

A modified staircase method was employed to determine

compression ratio adjustments to the simulator after each

pull. Generally, if a subject judged a pull "easy" the

compression ratio of the simulator engine was increased

one unit for the next pull. If a pull was judged "hard",

the compression ratio was decreased. Some variation in

this procedure occurred in an attempt to assure that subjects

pulled over a broad range of forces.

After each pull, three measures were recorded: the

judgment about the pull, either "easy" or "hard;" the peak

force exerted during the pull; and the time from initiation

of the pull to completion through the specified pull distance.

Peak force, rather than any measure of force over time,

was employed for several reasons. First, peak force proved

simple to measure. Second, pilot tests showed this measure

to be capable of discriminating between pulls judged easy

and pulls judged hard. Finally, peak force is the most

easily adaptable to test method development.

The experimental plan called for each of 80 subjects

(40 using the handle simulator, 40 using the housing simulator)

to execute 36 pulls through each of four distances. This

array would have resulted in a total of 11,520 pulls.

Due to the exigencies of testing (equipment failure, subject

availability, etc.) only 74 subjects were tested. Most

subjects completed the full complement of 36 pulls at each

distance, however, a few subjects did not complete the

entire test as scheduled.

6
3.0 Results

3.1 Data Reduction

The total number of subjects of each sex who pulled

on each simulator and the total number of pulls executed

at each distance are shown in Appendix B. A total of

5372 pulls were made from the handle position and 5009

pulls from the housing.

The data for each subject consists of a series of

approximately 36 judgments (easy or hard) at each pull

distance. Each judgment is associated with a peak force

and an elapsed time. The peak forces were grouped in

22 N (5 Ibf) intervals. All analyses, with the exception

of correlations between peak force and elapsed time, were

performed on the mid-points of these categories. Thus,

for analyses all pulls resulting in a peak force range

of 176 - 198 N (39.5-44.5 Ibf), for example, were treated

as being peak forces of 187 N (42 Ibf)

For most analyses, again with the exception of some

correlation coefficients, the measures of peak force of

primary interest were the maximum forces judged easy (Easyjjj^^^)

and the minimum forces judged hard (Hardjj^j^j^) for each

subject. Hardji^in ^^^Xmax <i3.ta are presented in Appendices

CI and CII for each subject at each pull distance for

the handle and housing simulator respectively.

3.2 Data Analysis

The data presented in Appendices CI and CII were

subjected to four two-factor mixed design Analyses of

Variance to determine the effects of sex and distance

of pull on Hardjnin and Easyjj^ax- Separate analyses were

performed for the handle and housing data. The analyses

are summarized in Appendices D and E.

Analysis of the handle data indicate a significant

difference in mean peak force as a function of sex for

both Hardjixinl and Easy^i^ax^ • In both cases the peak forces

applied by males are greater than those applied by females.

Differences among the mean peak forces as a function of

distance of pull are significant for Easyj^^g^-^c-^

for Hard^in4.

1[F(1,36) = 11.04, p < .01]

2[F(1,36) = 13.99, p < .011

3[F(3,108) = 3.01, p < .05] 7

4[F(3,108) < 1, p > .05]
The mean Easy^jj^x P^ak forces at each pull distance

were subjected to a Duncan Multiple Range Test to determine

the source of the differences among these means. As indicat(

in Appendix F, the mean peak force judged easy for the

91 cm (36 in) pull is significantly (p < .05) greater

than that for any other pull distance. No other significant

differences are indicated.

Analysis of the housing simulator data indicates

a significant difference in peak force of pull as a function

of sex for EasymaxS but not for Hardjy^in6 . No significant

differences among the means as a function of distance

of pull are evident for either Easy^^ax^ ^^^^min^ • Tables I and II show the cumulative distributions

(number and percent) of Hardjj^in ^^^Xmax males

and females using the handle and housing simulators respectively for all pull distances combined. These data were

employed in constructing Figures 5 through 8. These figures

display the cumulative percentages of the peak forces

for Hardjjiin ^^^^ Easyjj^a.x separately for male and females

using the handle and housing simulators.

A series of correlation coefficients (Pearson ProductMoment Correlations, r) were computed to determine the

presence of any predictive relationships between peak

force of pull and subject variables. Appendix G shows

the correlation coefficients between both Hardji^in ^".d

Easynax and subject age, weight and height. Only one

of these, the correlation between Hard^in. and weight of

male subjects using the housing simulator (r = .68, p < .05)

is statistically significant. Correlation coefficients

were also computed between the measures of Hardj^i^ ^^'^ Easymax' These correlations were computed separately for

male and female subjects using the handle and housing

simulators. All of these coefficients, reported in Appendix

G, are significant (p < .05) and positive.

A final series of correlations was computed between

measured peak force (disregarding judgments of easy or

hard) and the time from initiation to completion of pulls

through the four distances. Although these correlations

were actually computed for only a sample of the subjects,

it appears that there is no consistent relationship between

these measures in the present data.

5[F(1,34) = 6.23, p < .05]

5[F(1,34) = 3.44, p > .05]

/[F(3,102) < 1, p > .05]

«[F(3,102) < 1, p> .05]
Table I Cumulative Distributions of Hard . and Easy mm 'max

Handle Simulator, All Pull Distances Combined

Female Subjects

Hard . min Easy,max

r eaK

N

Force

Ibf

Cumulative Cumulative

Number Percent

Cumulative

Number

L. UIIIU X d L X V C Percent

98 22 20 100

120 27 19 95

142 32 15 75

165 37 8 40 20 100

187 42 3 15 18 90

209 47 16 80

231 52 14 70

254 57 11 55

276 62 8 40

298 67 5 25

320 72 343 77 4 20

365 O 0 0 L 387 87

Male Subjects

1 5

Peak

N

Force

Ibf

Cumulative Cumulative

Number Percent

Cumulative

Numb e r CumulativePercent

120 27 18 100

142 32 15 83

165 37 12 67

187 42 11 61

209 47 8 44

231 52 7 39 18 100

254 57 4 22 14 78

276 62 3 17

298 67

320 11 13 72 343 11 2 11 10 56

365 82

387 87

409 92 0 50

432 97 1 fi o 33

454 102 c 28

476 107

498 1.1121. t* 521 117 3 17

343 122

565 127 2 11

587 132 1 6

9
Table II

Cumulative Distributions of Hard . and Easy

I, . mm -^max Housing Simulator, All Pull Distances Combined

N lb£

120 27 142 32 165 37 187 42 209 47 231 52 254 57 276 62 298 67

320 72 343 77 365 82 387 87 409 92 432 97 454 102

476 107

498 112

521 117

543 122

565 127

587 132

610 137

632 142 654 147

Female Subjects

Hard . Peak

N ForceIbf

Cumulative

Numb e r Cumulative

Percent

120 2 7 18 100

142 32 17 94 iOb 37 16 89 187 42 IS 83 209 4 7 12 67

2 31 5 2 9 50

O C ^ 5 7 5 28

"y n a lib 62 4 22 298 6 7 320 72 343 77 2 11 365 82 1 6 387 87 409 92 432 97

454 102

476 107

498 112

521 117

Easy' max Cumulative Cumulative

Number Percent

18 100

17 94

IS 83

14 78 11 61

9 50

7 39

4 22

2 11

1 6

Male Subjects

Peak Force Cumulative Cumulative Cumulative Cumulative Number Percent Number Percent

18 100

16 89

15 83

10 56

9 50

7 39 18

4 22 100

16 89

11 15 83

14 79

6 13 72

12 67

11 61

8 44

7 39

6 33

4 22

2 11

10

3idwvs JO iN3oii3d aAiivinwno

12


Appendix H shows the percentage of "double" and "incomplete" pulls at each of the four pull distances. A "double

pull" is defined as any pull during which a noticeable

pause was observed in the execution of the pull. An "incomplete

pull" is any pull in which the pull cord did not come

completely off the rewind pulley, i.e., a pull shorter

than the specified nominal distance. Many more of these

pulls were observed on pulls through 91 cm than any of

the shorter pulls.

4.0 Discussion

The results of the analyses of variance suggest that

the peak forces for both Hardu^in. and Easyn^^x be collapsed

over distance of pull for both housing and handle data

since, with one exception, there were no statistical differences among these forces as a function of distance of

pull. Although analysis of the handle data indicated

a statistically significant difference between pulls of

91 cm and the other pull distances for Easymax? this difference is insignificant in practical terms. The largest

difference between the mean Easy^ax ^1 pulls

and that for the other pull distances is 20.9 N. This

is less than the size of the intervals into which peak

forces were grouped for analysis. Although the maximum

forces judged easy for pulls through 91 cm were as great

or greater than Easyj^^^x ^he other pull distances,

there is some evidence that 91 cm is too long a pull for

starting a lawn mower. This is suggested by the much

greater percentage of double and incomplete pulls at this

distance than any other distance.

Figures 5 through 8 provide the basis for defining

"easy to pull" under the conditions of this study. The

cumulative percentages of Hardjj^j^j^ and Easyjj^g^^ displayed

in these figures define the lower limit of "hard to pull"

and the upper limit of "easy to pull" respectively. For

any given percentage of the sample, the Hardji^i^ line defines

the minimum peak force which was judged hard. Similarly,

the Easymax line defines the maximum peak force which

was judged easy for any specified proportion of the sample.

These two functions may be viewed as providing conservative

(Hardmin) and liberal CEasy^j^g^^) definitions of the peak

forces considered to be easy to pull. Inspection of Figures

5 through 8 reveals the very large difference between

the "liberal" and "conservative" estimates of easy to

pull. This difference reflects the large variability

in judgments of easy and hard both between individuals

and within a single individual.

15
It is recommended that the CPSC adopt a "conservative"

stand with regard to determining a force value for ease

of pull. This is suggested so that the greatest percentage

of the population could "easily" restart lawn mowers and

so that the incentive for defeating the dead-man control

would be reduced. In practice, this would mean determining

the peak force by using the Hardjy^i^ distribution for females

and selecting the sample percentage at 80 percent or greater

That is, based on Figures 5 and 7, the maximum values

for pulls from the handle position should be approximately

140 N (31 Ibf) and from the housing approximately 190

N (43 Ibf)

The lack of significant correlations between peak

force of pull and subject age, weight, and height serve

to illustrate that judgments about what is easy or hard

to pull are not directly related to the pullers' physical

characteristics. No measures of physical strength were

made in this study; however, it appears that pull judgments

in the present context cannot be predicated solely on

physical size, strength or age and are probably determined

by all of these plus other neuro -muscular cues in some

unknown combination.

This study does not, nor was it intended to, answer

the question of why one judgment is "easy" and another

"hard". Neither does it provide a definitive answer to

the question of what is easy to restart. The data generated

in the study do, however, provide a practical basis upon

which a policy decision regarding the upper limit for

"easy to restart" can be made.

16
References

U.S. Consumer Product Safety Commission. Product Profile:

Power Mowers , October 1976.

Consumers Union of United States, Inc. Proposed Safety

Standard for Power Lawn Mowers , Submitted to the

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

U.S. Consumer Product Safety Commission. Power Lawn Mowers,

Proposed Safety Standard and Extension of Time. Federal

Register, Vol. 42, No. 87 - Thursday, May 5, 1977,

23052-23072.

17

NBS-783 U.S. DEPARTMENT OF COMMEnCf

(2-701 APPENDIX A NATIONAL BURLAU Uf SrANDARDS

RESEARCH PARTICIPANT AGREEMENT

1, ( : M-.i ( .-iiici N...

1. Piim lu.il lnv<!'>liti.il<ii Val Pezoldt

2. Uivisinn. boi.lion 441.02

4, Local ion

}0( Ciaithnrsliiirti

|_ i 6. Exputiniont N.Miii) CoOo Olhur rsu. . ilvl Ease of Pull

6. OtiScriDtiun ol tiiPHrini<!nt The purpose of this research is to determine the ease with which power laun mower users can pull the starting cord on la^rm mower engines. It is being done

so that maximum force levels can be set for starting lavm mowers. Tlic participants

will perform a series of pulls on a simulated engine pull start and make

judgements about the ease or difficulty of the pulls

7. Risks CO Participant Risks to participants are similar to those encountered when pull -starting a lawn mower. They include (1) slips and falls while pulling the simulated laun mower cord; (2) muscle strains in the arm and back; and (3) cardiovascular traimia

due to overexertion. These will be minimized by employing a slip resistant surface

and securely anchoring the test device and by scheduling rest periods between

series of pulls. The simulator used will have no exposed moving parts and will

not use gasoline or any other flammable liquid.

8. Responsibilities of Participant

TTie participant will arrive promptly at the scheduled test times and, with

informed consent, will follow instructions regarding the pull task, ans\v'er questions

relating to age and lawn mower use, and allow the investigator to make weight and

height measurements.

9. Responsibilities of Investigator is)

Hie investigator will fully explain the test procedure, explain the purpose

of the test, ensure safe testing conditions and keep personal information confidential.

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

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

TERMINATION.

It. I hereby certify that my participation is voluntary and that I have read and accept the terms of this agreement.

Participant, or Parent or Guardian tSiynalurel Date

12. Principal Investieator tSignaturel

. / .

Date

13. Early Termmulion by (Signature! Date

US.COMM*NbS.OC 19

Appendix B

Number of Subjects and Number of Pulls Executed

At Each Pull Distance

Handle Simulator Data

Pull Distance (cm)

No. of Sub j ects 46 61 76 91 Total Pulls

Male 18 630 633 640 654^ 2557

Female 20 711 700 720 684 2815

Total 38 1341 1333 1360 1338 5372

Housing Simulator Data

Pull Distance (cm)

No. of Sub j ects 46 61 76 91 Total Pulls

Male 18 622 643 627 603^ 2495

Female 18 639 632 602 641 2514

Total 36 1261 1275 1229 1244^ 5009

One subject made 42 pulls.

17 subjects

35 subjects

21

Appendix CI Minimum Force Judged Hard and Maximum Force Judged

Easy for Subjects Using Handle Simulator (in Newtons)

Pull Distance (cm) Pull Distance (cm)

Subject No. 46 61 76 91 46 61 76 91

(Female)

1 276 231 187 209 343 298 276 370

2 165 165 142 142 276 187 187 254

3 165 187 142 142 209 231 209 209

4 165 254 187 209 231 343 254 276

5 187 165 142 165 231 254 231 231

6 209 254 231 231 254 343 276 276

7 142 142 120 142 165 187 165 187

8 142 120 142 120 165 142 165 187

9 320 187 231 298 343 387 320 343

10 165 142 254 209 231 231 276 276

11 165 165 209 187 231 231 231 209

12 187 187 165 231 276 231 209 254

13 231 142 254 231 254 209 298 298

14 165 231 187 209 231 276 254 231

15 120 142 142 165 165 165 165 187

16 142 165 165 142 187 209 187 187

17 120 98 98 98 142 120 165 142

18 142 231 187 209 209 254 231 254

19 165 165 165 165 231 209 231 231

20 120 120 120 142 231 142 165 165

(Male)

21 187 209 276 231 254 298 320 320

22 209 231 276 254 387 320 320 409

23 209 187 187 165 231 209 231 231

24 142 142 187 187 231 187 254 231

25 187 276 209 142 276 298 320 276

26 231 254 231 276 454 343 343 387

27 231 187 187 254 254 254 320 298

28 343 343 343 454 387 454 432 521

29 387 498 142 409 521 587 343 521

30 142 165 120 142 187 209 187 231

31 276 231 231 254 343 365 343 409

32 231 276 254 298 276 320 343 409

33 276 365 343 387 409 409 432

34 320 187 365 409 432 276 454 454

35 476 498 432 543 565 565 565

36 142 142 142 120 165 187 187 231

37 120 142 120 120 142 231 142 165

38 254 298 254 254 298 298 343 343

23

Appendix CII

Minimum Force Judged Hard and Maximum Forced Judged Easy

for Subjects Using Housing Simulator (in Newtons)

Hard^ij, Easy^a^

Pull Distance Pull Distance

Subject No. 46 61 76 91 46 61 76 91

(Female)

39 187 187 165 187 231 231 298 231

40 231 231 276 276 320 298 320 343

41 231 231 120 142 320 320 387 343

42 187 209 209 231 231 276 254 298

43 409 231 298 409 476 476 365 454

44 298 254 298 209 365 298 320 254

45 231 209 209 209 276 254 231 320

46 231 320 298 231 343 387 343 320

47 :f-87 187 231 254 254 231 276 343

48 298 298 298 276 454 387 409 387

49 365 387 365 343 454 432 432 387

50 365 387 454 454 409 476 454 476

51 298 320 231 276 365 365 320 320

52 298 276 298 254 343 320 343 298

53 387 454 454 276 498 521 498 476

54 187 142 231 231 231 276 276 276

55 343 209 298 320 409 454 365 387

56 409 187 276 409 498 387 432 476

(Male)

57 231 276 343 343 320 387 432 476

58 454 254 432 476 543 543 521 565

59 343 432 298 454 476 454

60 254 165 120 120 298 276 276 231

61 142 120 165 276 254 231 365 387

62 343 476 276 498 587 543 587 587

63 365 521 565 476 54' 610 587 654

64 298 254 298 231 40^ 365 365 387

65 365 298 454 343 476 476 543 432

66 387 231 387 343 521 365 454 432

67 409 432 409 432 521 409 565 521

68 276 254 254 231 343 365' 298 365

69 387 298 298 276 432 454 387 365

70 320 343 387 298 454 454 543 387

71 365 254 231 254 387 298 343 320

72 320 432 365 387 387 476 454 432

73 387 320 387 409 432 409 498 498

74 254 231 231 187 276 298 298 231

25
V
Appendix D

Analysis of Variance: Handle

Minimum Force Judged Hard (Hard^^^^^)

Source SS d£ MS F 2. Total 1 ,092,651 .85 151

Between 896,483 .33 37

Sex 210,537 .01 1 210,537 . 01 11.04 < .01

Error Between 685,946 .12 36 19,054 .06

Within 196,168 . 52 114

Distance 5,012 .41 3 1,670 .80 < 1 > .05

Distance x Sex 1,193 .50 3 397 .83 < 1 > .05

Error Within 189,962 .61 108 1,758 .91

Analysis o£ Variance: Handle

Ea;

MS

Maximum Force Judged Easy (Easy___,) max

Source SS d£

Total 1,534,013 .14 151

Between 1,380,915 .06 37

Sex 386,489 .75 1

Error Between 994,425 .31 36

Within 153,098 .08 114

Distance 11,344 .06 3

Distance x Sex 5,995 .60 3

Error Within 135,758 .41 108

27

386,489.75 13.99

27,622.92

3,781.35

1,998.53

1,257.02

3.01

1.59

01

05

05

Appendix E

Analysis of Variance: Housing

Minimum Force Judged Hard (Hard^j^^^j^)

Source SS df MS F E. Total 1 ,249,042 .69 143

Between 909,429 .81 35

Sex 83,665 .56 1 83,665 .56 3.44 > .05

Error Between 825 764 .25 34 24,287 .18

Within 339,612 .88 108

Distance 9,158 .46 3 3,052 .82 < 1 > .05

Distance x Sex 576 .85 3 192 .28 < 1 > .05

Error Within 329,877 .57 102 3,234 .09

Analysis of Variance: Housing

Maximum Force Judged Easy (Easyu^ax)

Source SS df MS F

Total 1,415,050 .21 143

Between 1,247,347 .79 35

Sex 193,105 .27 1 193,105 .27 6.23 < .05

Error Between 1,054,242 .33 34 >1,007 .13

Within 167,702 .42 108

Distance 1,990 .55 3 663 .52 < 1 > .05

Distance x Sex 4,657 .75 3 1,552 .58 < 1 > .05

Error Within 161,054 .12 102 1,578 .96

29

Appendix F

Duncan Multiple Range Test: Handle

Maximum Force Judged Easy (Eas/jj^^^^)

Distance (cm)

o£ Pull Mean(N)

76

272 .3

46

272.8

61

275 .9

91

293.2

Critical

Range

76

46

61

91

272 .3

272 .8

275 .9

293.2

20.9* 17.5

20.4* 16.7

17.3* 16.1

p < .05

31

Correlations Between

Appendix G

Hard^.^, Easy^ and Subject Variables

Handle

Correlates Female Male

Hard . - Age -,22 .22 mm ^ Hard . - Weight .02 .40 mm ° Hard„. - Height .10 .15 mm * Easy^^^ - Age .43 -.01

Easy^^^ - Weight -.19 .41 ^ max

Easy^^^ - Height .19 .30

Hard^. - Easy^^^ .85* .67* mm 'max

*p < .05

Housing

Correlates Female Male

Uard^^^ - Age .13 .26

Hardjjj^^ - Weight . 22 .68*

Hard^. - Height .20 .07 mm *

^^^^max ' '^^ '-^^

^^^^max " ^®ight .33 .41

Easy^^^ - Height .40 .20

Hard^. - Easy^^^ .55* .75* mm ^ max

*p < .05 33

Appendix H

Percentage of Double and Incomplete Pulls

Handle Simulator

Female

Male

All Subjects

Pull Distance (cm)

46 61 76 91

3.1 4.6 5.5 23.2

5.0 3.1 2.2 11. 8

4.1 3.8 3.8 17.7

Housing Simulator

Pull Distance (cm)

46 61 76 91

Female 0.8 1.1 1.9 34. 5

Male 0.5 0.6 1.9 14. 0

All Subjects 0.7 0.9 1.9 24.4

35

NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession

BIBLIOGRAPHIC DATA No. SHEET NBSIR 77-1298

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Power Lawn Mowers: Ease of Pull

5. Publication Date June 1977

6. Performing Organization Code

7. AUTHOR(S)

Pezoldt. V. J. and Persensky, J. J.

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADBr'ESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4412422

11. Contract/Grant No.

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

Consumer Product Safety Commission

5401 Westbard Ave.

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 {actual sutnmary of most sigpilicant information. If document includes a significant

bibliography or literature survey, mention it here.)

This Study was designed to provide objective information about

the subjective judgement of "easy to pull" as it relates to the effort

required to restart manual pull-start power lawn mowers. Seventy-four

lawn mower users performed a total of more than 10,000 pulls on two

simulated pull -start mechanisms. The peak forces applied in the pulls

were associated with the subjects' judgements about the ease or difficulty of the puils. These data were used to generate sample distributions of the maximum forces judged easy and the minimum forces

judged hard. While not providing a definitive answer to the question

of what is easy to restart, the data generated in the study provide

a practical basis upon which an upper force limit for easy to restart

can be based.

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

Consumer products; human factors; human performance; lawn mowers;

psychophysics, safety, standards.

18. AVAILABILITY [j^ Unlimited

1

' For Official Distribution. Do Not Release to NTIS

1

' Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20402. SD Cat. No. C13

IXJ Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

40

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

$4.00

USCOMM-DC 29042-P74