NBSIR 74-563 ^ )

Recommendations for a Safety

Standard for Home Playground

Equipment - Swing Sets

Bal M. Mahajan

Consumer Product Systems Section

Measurement Engineering Division

Interim Report

12/73 to 6/74

Prepared for Consumer Product Safety Commission

5401 Westbard Avenue

Bethesda, Maryland 20016

NBSIR 74-563

RECOMMENDATIONS FOR A SAFETY

STANDARD FOR HOME PLAYGROUND

EQUIPMENT - SWING SETS

Bal M. Mahajan

Consumer Product Systems Section

Measurement Engineering Division

Interim Report

12/73 to 6/74

Prepared for Consumer Product Safety Commission

5401 Westbard Avenue

Bethesda, Maryland 20015

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dem, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Oirector

Recommendations for a

Safety Standard For Home Playground Equipment — Swing Sets

Introduction

Injury data gathered by the National Electronic Injury

Surveillance System (NEISS) Indicate that more than 800,000

Injuries occur each year that Involve playground equipment and

require emergency-room treatment. Approximately 57^ of these

Injuries Involve home playground equipment [1] and [31. These

statistics strongly suggest the need for safety standards for

home equipment.

The objective of this study was to develop quality criteria

and test methods useful In formulating safety standards for home

equipment. The data In Tables 1 and 2 Indicate that the highest

Incident of mishaps (at least 6k%) Involved swing sets, and the

second highest Incidence (approximately l6%) of mishaps Involved

slides. Swing sets were therefore given priority attention over

other Items, and this report deals mainly with them.

The study program was divided Into the following five parts:

(1) Hazard Identification. Probable hazards associated with

swing sets were Identified, and categorized where feasible. (2)

Loading conditions representing child-hazard Interactions. These

conditions were estimated. (3) Data acquisition. Available

anthropometric and Injury threshold data were obtained from the

literature. Tests were conducted to determine relative degree of

hazard of certain functionally similar components having

different constructions, and the magnitudes of loads applied to

certain components of swing sets were calculated. (^) Quality

criteria and test methods. Insofar as the data gathered In parts

1 and 2 permit, criteria and test methods for establishing safety

standards for swing sets are suggested. (5) Additional studies.

Studies are suggested to provide the basis for establishing more

effective safety standards.
Hazard Identification

For the purpose of this study a hazard is defined as any

component or characteristic of a product which directly or

indirectly presents a risk of injury during normal use or

reasonably foreseeable abuse or misuse of the product. For

example, an exposed sharp point presents a direct risk of injury,

because a person coming in contact with a sharp point during

normal use of the product may receive a skin injury; the

accessibility of high frame members, such as the cross bar of an

A-frame or top bar of a swing set, presents an indirect risk of

injury because a child playing on the cross bar or top bar may

fall and be injured.

The hazards associated with swing sets may be placed in

three categories: Category A , the hazards which can be directly

attributed to any design and construction defects; Category B , the hazards which may result from improper installation and

maintenance; and Category C , the hazards which are associated

with functional components; the mishaps involving these

components are thought to result from human error rather than a

product defect.

In order to identify the hazards specifically, the summaries

of swing-set-related accident investigations reported in

references [2] and [3] were carefully studied. In addition,

proposed standards for children's home playground equipment [^]

and safety standards for children's toys [5] and [6] were also

reviewed.

Swing-set related mishaps, from reference [2], are presented

in Table 3 by specific hazard, along with the hazard category.

The brief description of swing-set related accidents obtained

from reference [3] are given in Table 4 along with the

responsible hazard and hazard category.

The data presented in Tables 3 and 4 represent too small a

sample to draw any statistically meaningful conclusions. These

data do indicate, however, that nearly half of the investigated

cases of mishaps involved Category A hazards, the other half

resulting from Category B and C hazards.

Description and categorization of some of the identifiable

hazards are given below.

Pinch Points are defined as any intersection of two

components moving relative to one another and having such

clearance as to permit pinching or entrapping children's fingers

or toes. For example, these would include chain links, a vertical tube or bar holder and the hinged tube or bar of a glider or lawn swing (Figures 1 and 2), and the Juncture where

the horizontal and vertical tubes of glider swings are hinged

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together (Figure 1). Pinch points are placed in hazard Category

A.

Holes and Slots present in any component of the swing set

with such dimensions as to permit entrapment of children's

fingers or toes would be considered hazardous (Figure 3). Holes

and slots are Category A hazards.

Protrusions are defined as: (1) any component protruding

from the main structure that presents a direct risk of injury

when contacted by someone. These include exposed bolts, nuts,

pipes, edges, points, open s-hooks, wire ends of chain links, and

sharp corners; (2) any component protruding from the main

structure which may not present a direct risk of injury when

contacted, but presents an indirect risk of injury by catching

some part of a child's clothing or shoes, for example, capped

protruding bolts (Figure 4). Protrusions are placed in hazard

Category A.

Swing Seats . Glider and single-occupancy swing seats, due

to their design and the material used in their construction,

present risk of injury when impacting a child. Impacts between

moving swings and children in the swing's path cannot be

prevented. However, the trauma resulting from these impacts can

be mitigated by changing the construction of the swing seats.

Poorly designed rigid seats are Category A hazards.

The swing seat presents a risk of fall; the individual

falling from the seat may be injured by striking the ground below

the swing set or by being hit by the moving swing. However, with

the exception of a baby falling from a baby swing without proper

restraints, falls may be attributed primarily to human error.

Falls from seats are assigned to hazard Category C.

Inadequate Spacing . (1) Horizontal. Swings situated too

close to each other or to the frame of the swing set present a

risk of collision between the hard metal frame and swing occupant

or between the occupants of the two adjacent swings. The problem

is worsened by the multi-direction mobility of the swings.

Inadequate horizontal spacing is placed in hazard Category A.

(2) Vertical. A swing seat that is too low presents the

risk of a child's feet or legs being caught between the swing

seat and the ground. A swing seat that is too high presents the

risk of falls, especially for a smaller child, when the child

attempts to mount or dismount the swing. The v/ide variations in

ages and sizes of children using swings makes it difficult to

standardize the vertical clearance. The lack of standard

installation procedures adds to the difficulty. Inadequate

clearance is a Category B hazard.

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Poor Strength . Poor strength of load-carrying components of

the swing set presents the risk of traumatic mishap, because the

component with inadequate strength will fail when the applied

load exceeds the component's capacity. Some load carrying

components of the swing sets are bolts, nuts, chains, hinges,

seats, frame, top bar, and various connecting tubes.

Deterioration of components that originally may have had adequate

strength when put in service often leaves them with inadequate

strength. In all accidents involving failure of some loadcarrying component the equipment was old and had deteriorated.

New equipment having poor strength presents a Category A hazard;

strength inadequacy resulting from improper maintenance of the

equipment is a Category B hazard.

Instability presents a risk of the swing set tipping over

and injuring children playing on it or near it. Instability

often results from inadequate anchoring. Proper anchoring is

left entirely to the consumer because, in most cases, anchoring

devices are sold as accessories and are not included with the

equipment. Therefore, instability is placed in hazard Category

A.

Accessibility of High Frame Members . The cross bar of the

A-frame and the top bar present indirect risks of injury during

reasonably foreseeable misuse of the product. Use of the cross

bar and top bar by children for climbing or gymnastics can lead

to traumatic falls.

The top bar is a necessary support for the swings in a

conventional set. A cross bar may be considered by some as a

design defect: first, because it probably encourages children to

use the cross bar for climbing, for gymnastics, and as a step for

climbing onto the top bar; second, because the cross bar adds

very little to the strength, stability, or durability of the

swing set if the set is properly anchored. Others may consider

the cross bar an essential part of the equipment intended for

elementary exercises, such as chin-ups, for building arm muscles

and for improving muscular coordination. Furthermore, the cross

bar is needed when the swing set is not permanently anchored.

Both the cross bar and top bar are placed in hazard Category C.

Trapeze Bar presents a risk of traumatic fall when the user

loses his grip. The trapeze bar, however, is a functional

component of the product, and the risk of fall is thought to be

due to human error. The trapeze bar is placed in hazard Category

C.

Swing Set Frame . The frame presents a risk of injury when

someone falls against it or runs into it, but it is a necessity

in a conventional swing set. The frame is placed in hazard

Category C.
Loading Conditions Representing Child-Hazard Interactions

Loading conditions which are of particular Interest for this

study Include child-hazard Interactions (that Is, a child's

Interaction with a hazardous component) and the magnitudes of

loads Involved.

The several possible ways In whch children can Interact with

a given hazard make It difficult to determine the appropriate

loading conditions. The problem Is further complicated by

variations In the children's ages and by the variations In

physical size and strength capabilities among children of the

same age group. Rough estimates of these conditions, however,

would be adequate for this study.

Many child hazard Interactions can be roughly represented by

the following two: (a) casual handling and (b) Impact.

Casual handling conditions represent Interactions Involving

two types of loading mechanisms: (1) Interactions In which the

applied load Is a single force acting In a direction

perpendicular to the area of contact between the interacting

object and the child's skin. Examples: situations where a

finger or toe is caught in a pinch point, and situations where

the child is pushing, gripping, sitting on, or stepping on a

protrusion. (2) Interactions in which the applied load has two

factors, a normal force between the skin tissue and the

interacting protrusion and the relative velocity between the skin

and the protrusion in a direction perpendicular to the force.

Examples: a child rubbing some part of his body against a

protrusion, and a child trying to free his finger caught in a

pinch point or entrapped in a slot.

The loads involved in casual handling can be estimated from

children's strength capability data. Such data, especially the

forces that children are capable of exerting when pulling,

pushing and gripping, are given in Table 5> and were obtained

from references [1^] and [15]. Forces involved in casual

handling may vary from 5 to 296 newtons* for children of 2 to 6

years. However, to determine whether or not an exposed edge

would cut the skin under casual handling conditions, a force of

89 Newtons (20 lb ) was used by Sorrells and Burger [13].

*SI Units are used throughout this report, however, for the

convenience of the readers the conversion factors from SI

to customery units are given in Table 4A, and the suggested

criteria are given in both units.

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Impact loading occurs In situations where a child hits a

component or where the child is hit by a moving component, such

as a swing seat. In impact loading the resulting impulse, the

peak value of impulsive force and contact area are the important

factors [12] (also see Table 8), The impulse and the peak

impulsive force primarily depend upon the striking momentum,

rigidity of the hazardous object, thickness and resilience of the

skin tissue, and properties of the backing material of the skin

such as bone, muscle, abdominal cavity, etc.

The impulse or peak impulsive force, being dependent on so

many factors, are difficult to determine. In Impacts where the

child hits the hazardous object, the data required for

determining even the striking momentum are almost non-existent;

therefore, the treatment of these impacts is postponed until the

required data become available. Impacts in which the moving

swing seat hi^s a child, however, were treated on a relative

basis. The relative magnitudes of peak impulsive forces to which

a child may be subjected when hit by differently constructed

swine- seats were measured. The results of such measurements are

given later.

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Data Acquisition

This part of the study deals with the Information necessary

to develop quality criteria and test methods for establishing

safety standards for swing sets, or for playground equipment In

general. Anthropometric data for children and Injury threshold

data were obtained from the literature. Tests were conducted to

determine the relative degree of hazard of certain functionally

similar components having different constructions. Magnitudes of

probable loads applied to certain swing set components were

calculated.

Anthropometric Data . The literature was searched to obtain

available data on physical measurements of children. The

measurements of particular Interest Include sizes of fingers and

toes, arm length, popliteal height, and weight. Some such data

may be found In references [7], [8], [9] and [10]. However, data

for three-year-old and younger children, and data on sizes of

toes for children of all ages are almost non-existent.

Reference [7] gives sizes of children's fingers In terms of

penetration of fingers In slots and holes. Some data from

reference [7] are presented In Table 6. The data concerning the

weight, arm length and popliteal height of children of various

ages, obtained mostly from reference [8], are presented In Table

7.

Injury Threshold Data . Data obtained from references [11]

and [12], for Injuries such as contusion, skin laceration and

puncture, and skull and zygomatic bone fracture are given In

Table 8.

Most of the Injury threshold data present In the literature,

having been derived from clinical reports of accidents and from

laboratory experiments with animals and cadavers, may be

questionable. Furthermore, very few of these data were derived

specifically for injuries to children. The available data,

however, must suffice until our knowledge of Injury mechanisms

and properties of human tissue is greatly Improved.

Relative Hazardousness of Functionally Similar Components

Having" Different Constructions . The construction (design,

geometry and material ) of certain hazardous components used in

swing sets, even to perform the same or similar functions,

differs from set to set as well as within the same set. These

functionally similar components include most of the Category A

hazards, such as bolts, nuts, hinged joints, swing seats, etc.

They would present different degrees of hazards in identical

child hazard interactions. This is so because, for given loading

-7-
conditions, the hazardousness (injury potential*) of an object is

critically influenced by its construction [11].

Some of the Category A hazards were tested to determine

their hazardousness and/or relative hazardousness. Test

procedures and results of the tests are given below.

Protrusions . Any exposed protrusion that would cut the skin

under casual handling conditions is considered sharp. The

criteria, test procedure, and the test device described by

Sorrells and Burger [13] for detecting exposed sharp edges were

considered adequate for determining whether or not an exposed

protrusion is sharp. Using the procedure and device of reference

[13], tests were conducted to determine if the protrusions found

on swing sets are sharp. The protrusions included exposed bolts,

nuts, pipes, and wire ends in chain links. The results are

presented in Table 9. It should be pointed out, however, that

protrusions which are not sharp under casual handling conditions

may present a risk of injury under other loading conditions.

Swing Seats . Hazardousness of a swing seat impacting a

person is related to the peak impulsive force and the contact

area. The factors affecting the impulse or the peak impulsive

force were discussed earlier. The contact area is dependent upon

seat thickness (Figure 5), the part of the child's body impacted,

the angle of impact and impact velocity.

The relative peak impulsive forces to which a child may be

subjected when hit by differently constructed seats were

measured. The seat to be tested was swung through a specified

sv/ing angle, 0^ , and made to Impact a force transducer fixed in

the apparatus** shown in Figure 6. The resulting impulse was fed

to an oscilloscope and the trace was photographed. Typical

oscillograms for various impacts are presented in Figures 7

through 12. Peak impulsive forces were read from the

photographs

.

*The injury potential of a hazardous object is defined as the type

and severity of injury the object is capable of inflicting on the

human body for any loading conditions [11].

**This particular apparatus was made available by the courtesy of

Mr. N. J. Calvano of NBS. A brief description of the apparatus

is given in Appendix A. The apparatus has been designed and used

for testing the effectiveness of face shields which are part of

helmets used by law enforcement officers. Any similar apparatus

however, would have been adequate for this purpose.

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The two gilder seats which were tested are shown in Figures

13 and 14, one made of sheet metal material and the other of

plastic. Pour different single-occupancy swing seats were

tested: (1) a seat made of wood; (2) a seat made of sheet metal;

(3) a plastic seat Just as It came from the manufacturer (Figure

15); and (4) the plastic seat modified by cutting some of the

ribs out to make It less rigid without altering the seat's

appearance or Its functional capabilities (Figure 16). Tests

were performed with empty seats using a swing angle of 20 degrees

for glider seats, 30 degrees for single-occupancy swing seats,

and with the force transducer so located that at the time of

impact the swing had maximum velocity.

The seat thickness was measured and recorded. Seat

thickness and peak impulsive force are given in Table 10, These

data indicate that the plastic glider seat is less hazardous than

the metallic glider seat, and that the modified plastic swing

seat is the least hazardous of the four swing seats tested.

However, the impacts used in the above tests are far from

real child-swing impacts, and these tests cannot measure the

impulse or the peak Impulsive force to which a child would be

subjected when impacted by a swing seat. Nevertheless, these

tests or other similar tests can be used to measure the relative

hazardousness of those variously constructed seats. Furthermore,

a swing seat that is least hazardous by these tests would

probably be least hazardous when hitting a child in real

situations. Therefore, su^h testing procedures would be adequate

for establishing safety standards. Further studies are needed

for developing an apparatus that would be suitable for testing

the relative hazardousness of seats and that could specified in

safety standards.

Loads Applied to Certain Swing Set Components . The

components meant for carrying loads are seats, chains or tubes

connecting swing sets to the top bar, supports at the top bar and

at the seats such as s-hooks, bolts, and hinges, and the cross

bar and the bolts connecting the cross bar to the frame.

Probable loads carried by some of these components are calculated

below.

The effective load carried by a moving swing depends upon

the weight of the child occupying the swing and the swing angle;

this is given by (assuming that the swing is a simple pendulum):

P = Mgn (3 cose - 2 cosSq) newtons (N), (1)

where M = weight of an occupant in kilograms

(kg) (for simplicity weight of seat is neglected)

g^i = 9.80665 metres per second squared (m/s ),
0 = angular deflection of the swing from the vertical,

00 = maximum angular deflection of the swing (or swing

angle)

.

The effective load F is maximum when 0=0, and is given by

F = Mg (3-2 cosG ) newtons (2)

max n o

Pjjjg^yis also the load to which top bar is subjected because of one

moving swing.

The maximum value of tension in each chain of a single-occupancy

swing (see Figure 1 for single occupancy swing) and the load

applied to each of the chain supports at the top bar is

approximately one-half of F^j^g^^^.

The maximum load to which the cross bar may be subjected

occurs when two or more children use it for various activities.

The effective load acting on any component due to shock, when a

child Jumps on the component, can be much greater than the weight

of the child.

The strength of load-carrying components may be tested

statically by loading the swing seats with static loads (P'st =Wgj^)

equal to Fmax multiplied by a safety factor. The singleoccupancy seats are often occupied by two children, but the

multiple occupancy swing (i.e. glider or lawn swing) is less apt

to be occupied by more children than the number for which it is

designed. Therefore, it is suggested that a safety factor

between 1.25 and 2 should be used for determining the static test

loads of single occupancy swing, and a safety factor between 1.25

and 1.5 for multiple occupancy swing. Sample calculations of

static test loads are given below.

For single occupancy swings, let us assume:

0Q = 90°

M = 30 kg (weight of an average 9-year-old)

Then, from equation 2, P^^^j^ = (90 )g^ = 882 newtons

and,

1103 IF St 11765 newtons

or 112.5 kg iMgt 1180 kg (3)

And for multiple occupancy swings, let us assume:

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M = 30 kg per occupant

Therefore, F^^^^^ = (48)gj^ = 471 newtons per occupant

and 588 ^Fg^/occupant <706 newtons

or 60 kg <_Mg^/occupant _<72 kg (4)

Assuming that the swing is a simple pendulum, the tipping

moment (T) acting on the swing set due to one single occupancy

swing is given by

T = P L sin (e- a)

where L is the projected length of the A-frame leg, and is

one-half the projected A-frame angle, both projections being

in a plane perpendicular to the top bar (Figure 17).

From equation 1,

For given values of M and L, T will be maximum when K is

maximum (eqn. 6). For given values of and a , the maximum

value of K and instantaneous position ©j^, at which this maximum

occurs can be calculated.

For example.

Let 00 = 90°. (The assumption of 9q = 90° would serve as a

safety factor for stability consideration although this swing

angle is probably not attainable by the riders of gliders and

lawn swings.) Then equation (7) becomes

K = 3 cos8 sin ( 8-a

)

which has two extremes in 8, of which the pertinent maximum is

for 8 > a now.

T = Mg^ L (3cos8-2coseQ) sin ( 0- a ) (5)

or T = Mg^ LK

where, K = ( 3cos8-2cos8o ) sin (8- o)

(6)

(7)

dK/d8 = 3 cos (28- a

)

at 8 = 8 , dK/d8 = 0, e>o. Then 0 = ( Tr/4) +( a/2).

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®m " /^^"^ °'/^)» and

Km^^ = 3 cos e sin (e„ -a)

max m m

= (3/2)(l-sln a)

and the maximum tipping moment due to a single occupancy swing

or due to one occupant of glider or lawn swing, is

^max = ^n^ ^3/2) (1-sina) (8)

The maximum tipping moment to which the swing set may be

subjected occurs when all the swinging units are in phase. This

tipping moment is counteracted by the moment of the weight of the

swing set and the reactions at the anchors, if the set is

anchored. When the tipping moment is larger in magnitude than

the counteracting moment, the swing set will tip over.

If the frame is not to tip, then the restraining moment Tp

must exceed the tipping moment. For an unanchored set,

Tp = Mf, g^ L sin a>^ Mg^L (3/2) (1-sin a)s

where is the mass of the frame, and s is the number of

occupants for which the set is designed.

Or M^(Ms) = (3/2)(l-sina)/ sin a (9)

is the minimum necessary ratio of frame mass to mass of swings

and occupants.

The stability of anchored as well as unanchored swing sets

can be tested by loading the top bar of the swing set v/ith a

single force P acting in a direction perpendicular to the top bar

and parallel to the ground, applied at a point in the vicinity of

the middle point of the top bar. The magnitude of the force P

can be calculated as follows:

force p , for representing one single occupancy swing,

^ trapeze bar, or for representing one occupant

of glider or lawn swing.

Pi = (MSn L Kj^ax = ^n (^^^ax ) sec a (10)

where y is the perpendicular distance from the top bar to the

ground (Figure 17).

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For determining the value of pj , it is suggested that the

value of Kjpg^jj used in equation 10 should be based upon ©q = 90°

and a = 25 (this value of a is usually found on most of the sets).

Using the suggested values of K^^ax* « ~ ^5° and M = 30 kg,

equation 10 may be rewritten as:

p-j^ = 281 newtons (11)

The magnitude of force P to be used for testing the stability is

given by

P = Ap-^ + Bp-^ + NCp^^ + NDp-,^= sp-^. (12)

where A = number of single occupancy swings in the set,

B = number of trapeze bars in the set,

C = number of gliders in the set,

D = number of lawn swings in the set,

n = number of occupants for which the glider

or the lawn-swing seat is designed, and

s = (A+B+Dn+Dn) = number of occupants for which the

set is designed.

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Suggested Quality Criteria and Test Methods

Based upon the Information presented In the previous section

the quality criteria and test methods, where needed, for some of

the Category A hazards are suggested below. It must be pointed

out, however, that the suggested criteria may be debatable,

primarily because subjective factors must be Included In the

decisions. Furthermore, because of the limited nature of the

needed Information, these criteria do not cover all of the

possible hazards associated with swing sets. Nevertheless, an

Interim safety standard based upon these criteria can be

established, and to the extent that It Is utilized would either

reduce the number of mishaps resulting from Category A hazards or

mitigate the trauma resulting from those mishaps which do occur.

Pinch Points . (a) The Juncture of two hinged components

moving relative to one another shall have clearance such as not

to admit a rod with a diameter equal to or greater than ^ mm

(0.16 Inch). (b) Chain links shall be so designed that a 5 mm

(0.20 Inch) diameter rod cannot be Inserted Into the gap between

the links when the chain Is unloaded, and when the chain Is

loaded If a 5 nim (0.20 Inch) diameter rod can be Inserted through

a link, then the link shall also admit a 15 mm (0.60 inch)

diameter rod Inserted separately.

Holes and Slots . The holes and slots shall be so designed

that If a 5 mm (0.20 Inch) diameter rod can be Inserted In them,

then they shall also admit a 15 mm (0. 60 Inch) diameter rod

Inserted separately.

Protrusions . (a) Protrusions such as exposed bolts, nuts, pipe

ends, s-hooks, edges, corners, wire ends of chain links, etc,

shall not be sharp according to the test and criteria of

reference [13]. (b) The protrusions shall be so designed that

they will not catch the clothing or shoes of children. (c) All

exposed bolt ends and tubes or pipes shall be capped or plugged

without creating a clothing or shoe catching hazard. The plugs

and caps shall not be removable by a pulling force of 178 N (kO

Ibf ). (Data in Table 5 show that a six year old child when

attempting to pull apart objects can exert a force of 100 N; the

older children probably can exert considerably larger force than

100 N).

Swing Seats . (a) All surfaces of swing seats shall be free from

protrusions, and all corners of seats shall be rounded. (b) Seat

thickness shall be 25 mm (1.0 inch) or greater. (c) Glider

seats under Impact conditions described in the previous section

(unloaded, swing angle 20°, and impacting the apparatus with

maximum velocity) shall not produce a peak impulsive force

greater than 900 N (200 Ibf). (d) Single-occupancy swing seats

under impact conditions described earlier (i.e. unloaded, swing

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angle of 30°, and impacting with maximum velocity), shall not

produce peak impulsive forces in excess of 900 N (200 Itf ),

Criteria (c) and (d) for the glider and single occupancy

swing seats given above are based upon very limited data. More

data are needed to design the apparatus for impact testing and

for specifying the criteria (peak impulsive force and seat

thickness), since the peak impulsive force depends upon the

effective mass of the impacted object.

Other criteria that can be used for swing seats Instead of

criteria (c) and (d) above, are specification of mass, rigidity

and thickness of seats.

Horizontal Spacing . Behavioral data, concerning children *s

behavior as associated with swing sets, is needed for specifying

the spacing limits. However, the following criteria are

suggested; (a) the separation between the swing set frame and

any swing or between any two swings shall be at least 0.36 metres

(14 inches).

Strength . Strength of the load-carrying components of swing sets

may be tested as follows. The suggested test loads are based

upon the average weight of a nine year old, which is

approximately 30 kilograms (66 Ib^^.

(a) Individual Swings . Individual swings shall be tested

one at a time by loading the swings for 10 minutes with a load

equal to the maximum value of M obtained from equations 3 or

4, whichever is applicable, without evidence of structural

failure of the swing seat and its supporting system. For

example, for a single-occuoancy swing and for a trapeze bar the

test load should be l80 kilograms (396 Ib^J, and for glider and

multiple occupancy swings the test load should be 72 kilograms

(158 Ibj^) per occupant.

(b) Top bar and frame . The top bar and frame of any

multiple suspended swing set shall be tested by simultaneously

loading each position, which may be normally occupied, for ten

minutes with loads equal to the minimum value of M obtained

from equations 3 and k, whichever is applicable, without any

evidence of structural failure of the top bar or of the frame.

For example, the test loads for single occupancy swings and

trapeze bar should be 112 kilograms (2^7 Ibm) per occupant and

for multiple occupancy swings the test load should be 60

kilograms (132 Ih jj) per occupant.

(c) Cross Bar of the A-frame . The cross bar of the A-frame

shall be loaded for ten minutes with a single load of I80

kilograms (396 Ih j^) , equally distributed over a distance of 80 mm

(3.1 inch) at the center of the cross bar, with no evidence of

failure of the cross bar at any location.

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The above mentioned procedures are thought to be adequate

for testing the strength of various components of new swing sets.

The strength can be tested by applying dead loads as indicated

above, or by applying the equivalent forces mechanically. The

strength inadequacy resulting from the deterioration of the

product could be equally hazardous. At present, however, no

practical methods are available to check for this hazard, other

than by educating consumers to perform periodic inspections.

Stability . With the swing set installed in accordance with

the written instructions provided by the manufacturer, the top

bar of the swing set shall be loaded with a force P acting in a

direction perpendicular to the top bar and parallel to the

ground, and applied at a point in the vicinity of the middle

point of the top bar; the magnitude of the force P shall be

obtained from equation 12; when so loaded for 10 minutes the

swing set shall not tip over.

-16-
Additional Studies

The information needed to establish safety standards for

home playground equipment is either not available or very limited

in scope. Studies to obtain the needed information and to

establish interim standards for other items of home playground

equipment (such as slides, climbing apparatus, merry-go-round,

etc.) are suggested below.

Case studies . An in-depth study should be made of accidents

related to home playground equipment. The study should include

investigation of such factors as the cause of the accident, the

product and the component of the product involved, the child's

activities leading to the mishap, age of the child involved, age

and conditions of the product involved, tjrpe and severity of

injury sustained by the individual involved, etc. Some case

studies have been conducted by the Consumer Product Safety

Commission. However, the number of cases investigated by the

Commission is too few to represent a statistically meaningful

sample, in light of the number of reported accidents.

Anthropometric studies . Studies should be undertaken to

generate data concerning physical measurement of children of all

ages. These data should include sizes of fingers and toes, arm

reach, popliteal height; weight; sizes of feet and hands;

effective masses and moments of inertia of various body parts,

etc.

Behavior studies . An investigation of children* s activities

associated with home playground equipment, especially during

unsupervised playing, should be undertaken. Data on the behavior

of children may help in establishing limits for misuse and/or

abusive use of the equipment. In addition, these data may

provide some assistance in finding means to minimize mishaps

resulting from Category C hazards; these data would also help in

planning consumer education programs.

Strength capability studies . Studies of the strength

capabilities oT children of all ages should be undertaken. These

investigations should determine the forces that children are

capable of exerting when pushing, pulling, and gripping, as well

as velocities and momenta which children are capable of

generating when hitting or kicking. Presently available

information is limited to the forces that children of ages 2 to 6

years are capable of exerting when pulling, pushing and gripping.

Effects of weathering and aging . Investigations of the

effects of weathering and aging on the strength of the equipment

and its various components should be undertaken. If feasible

these investigations should study strength reduction rate in

terms of percent strength reduced per one year of environmental

exposure, and strength preservation techniques. The results of

-17-
such studies by providing strength prevention techniques may be

of help in consumer education programs.

C riteria for other items of the equipment . Studies to

develop criteria for establishing interim standards for such

items of home playground equipment as slides, climbing apparatus,

merry-go-rounds, see-saws, etc., are being conducted at NBS as a

continuation of the project.
Concluding Remarks

It Is difficult at present to establish safety standards for

home playground equipment because most of the needed Information

Is either not available or very limited In scope. However, based

upon the suggested criteria. Interim standards for swing sets may

be developed. Promulgation of Interim standards would be

expected to reduce the number of mishaps due to design and

construction defects of the product or mitigate the trauma

resulting from these mishaps. The development of Interim

standards Is desirable primarily because approximately half of

the swing set related mishaps are due to design and construction

defects of the product. Furthermore, as the state of the art

advances, the Interim standards can be appropriately revised.

No means other than consumer education Is presently

available to control the remaining mishaps. Consumer education

may reduce these mishaps.

-19-
Appendix A

Brief Description of the Impact Test Apparatus

The apparatus which was used for measuring the magnitudes of

peak impulsive force resulting from the impact of differently

cons+-ructed swing seats is shown in Figure 8. The apparatus

consists of a force transducer fastened to an eopxy headform in

the vicinity of the location of the nose. The headform is

mounted on a slide which is restrained to move linearly in a

horizontal plane in the direction of the impact. The mass of the

headform-sled assembly is approximately 5 kg. The static force

requried to initiate motion of the headform-sled assembly is

about 9 N.

The force transducer used has the following specifications:

charge sensitivity c^9*2 pC/N

frequency response . = 4000 Hz

capacitance =* 191 pP

Maximum Allowable Load « ^4^50 N

-20-
References

1. McConnel, W. H. , et al., "Product Investigation Report Contract

No. FDA- 73-6. Public Playground Equipment," University of Iowa,

October 15, 1973

2. "Summary Analysis of Home Playground Equipment", Consumer Product

Safety Commission, January 7, 1973

3. "Staff Analysis of Playground Equipment Injuries", Bureau of Product

Safety, FDA, March 1973

4. "Proposed Voluntary Product Standards for Children's Home Playground

Equipment", submitted to the National Associations of Children's Home

Playground Equipment by United States Testing Company, Inc., 15th draft,

February 28, 1974

5. "Safety Standards for Toys", Porposed Voluntary Product Standard TS 215,

June 25, 1973, Office of Engineering Standards Services, NBS

6. "Technical Recommendation for Tricycle Safety", Bureau of Engineering

Sciences, Consumer Product Safety Commission, June 5, 1973

7. Stevenson, John, "Ivestigation of Penetration by Fingers Through Slots

and Holes", Underwriters' Laboratories, Inc., May 1970

8. Dreyfuss, Henry, "The Measure of Man", Whitney Library, Second Edition, 1967

9. Martin, W. E. , "Children's Body Measurements for Planning and Equiping

Schools, "Special Publication No. 4, HEW, 1955

10. Martin, W. E. , "The Functional Body Measurements of School Age Children",

A Joint Project of the U.S. Office of Education, The University of Michigan

and the National School Service Institute, 1954

11. Mahajan, B. M. , "Systematic Analysis of Mechanical Hazards from Toys",

NBS Report No. NBSIR 73-137, May 23, 1973

12. Mahajan, B. M. "Standards for Athletic Helmets — State of the Art and

Recommendations", NBS Report No. NBSIR 73-276, April 1974

13. Sorrells, J. R. and Berger, R. E. , "An Inspection Procedure For Detecting

Hazardous Edges", NBS Report No. NBSIR 74-428, April 1974

14. Brown, W. C, Buchanan, C. J. and Mandel,, J., "A Study of the Strength

Capabilities of Children Ages Two Through Six", NBS Report No. NBSIR

73-156, August 1973

15. Brown, W. C. , Buchanan, C. J., and Mandel, J., "A Study of Young Children's

Pull-Apart Strength [An Addendum to NBSIR 73-156 - A Study of the Strength

Capabilities of Children Ages Two Through Six]", NBS Report No. NBSIR 73-424

April 1974

-21-
I

i

i

I
Table 1. Incidence of Mishaps Involving Home Playground

Equipment By Specific Product

(Neiss Data, July 1972 to August 1973 [1]

Product No. of Cases Percentage

Climbing Apparatus 274 11,0

Seesaws ' 89 3.6

Slides 394 15.8

Swings 1593 64.1

Not Specified 137 5.5

Total 2487 100.0
Table 2. Incidence of Mishaps Involving Home Playground

Equipment By Specific Product Based on 76

Investigated NEISS Reported Cases [2]

Product No. of Cases Percentage

Climbing Apparatus 4 5.3

Merry-Go- Round 2 2.6

Seesaws 1 1.3

Slides 13 17.1

Swings 54 71.1

Trapeze 2 2.6

Total 76 100.0
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Table 4A. Conversion Factors from SI

Units to Customary Units

Quantity To Change From SI To" Customary Multiply By

Force Newton (N)

Length Millimetre (mm)

Length Centimetre (cm)

Length Metre (m)

Length Metre (m)

Mass Kilogram (kg)

Pressure Newton per square

metre (N/m^)

Pound (Ib^)

Inch (in)

Inch (in)

Inch (in)

Foot (ft)

Pound (lb ) ^ m^

Pounds per square

inch (psi)

0.225

0.039

0.394

3.937

0.328

2.205

0.145 X

10"
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Table 6. Penetration of Children's Fingers

in Holes (H) and Slots (S) [7]

Hole Diameter

Or Slot Width Finger Penetration in Centimetre

in (mm) 10th Percentile 50th Percentile 90th Percentile

1/8 (0.32) H - 0,00 0.05 0.07

S 0.09 0.10 0.16

3/16 (0.48) H 0.05 0.07 0.13

S 0.11 0.21 0.26

1/4 (0.64) H 0.07 0.18 0.30

S 0.40 0.72 1.00

5/16 (0.79) H 0.17 0.40 0.50

S 1.00 2.05 2.45

3/8 (0.95) H 0.35 0.82 1.30

S 2.42 2.78 3.60

7/16 (1.11) H 1.27 2.18 2.42

S 3.88 4.42 4.96

1/2 (1.27) H 2.99 3.89 4.40

S 4.30 5.01 5.95

3/4 (1.91) H 5.44 6.12 6.78

S 7.20 8.50 9.98
Table 7. 50th Percentile Values of Children's Weight,

Arm Length, and Popliteal Height [S]*

I ears tVCJ-gllL , H Popliteal Height, m Arm Length , m Female Male Female Male Female Male

AH 1 A 7 17 2 0. 416 0.424 0 .254 n 9i4

1

U . Z41

nD 19 5 0. 450 0.455 0 .254 u . z / o

O z u . o 21 8 0. 480 0.478 0 .278 u . z y

7 24 0 0. 510 0.518 0 .292 U . OUD

8 25.8 26 3 0. 536 0.546 0 .317 0.330

9 28.5 29. 3 0. 564 0.569 0 .330 0.343

10 ; 31.7 32. 2 0. 592 0.594 0 .330 0.355

11 35.8 34, 8 0. 622 0.617 0 .366 0.355

12 40.3 39. 0 0. 652 0.642 0 .373 0.368

13 45.3 43. 5 0. 676 0.668 0 .381 0.394
10 o

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u J 03 CO uu
Table 9. Results of Tests Performed to Determine Whether

Or Not the Certain Protrusions Found on a Swing

Set Were Sharp

Protrusion Tested Test Results Remarks

Sharp Not Sharp

Bolts Tests were performed with the

exposed threaded surface of the

bolts.

Square Nuts

Hexagonal Nuts

Pipe holding the

glider seat

Wire ends at the

chain links.

10

1 One nut had sharp burrs created

by tightening.

Old rusted equipment, exposed

pipe.

New swing set, chain not rusted,
<u x:

^->

OJ

030)

10

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Figure 3. Finger entrapping slot.
Figure 4. Shoe string catching protrusion.
>•

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IX,
Figure 6. Apparatus used for determining the relative

hazardousness of swing seats in impact loading.
Time - 5 ms/div.

Figure 7. Typical force- time record of impulse resulting from

the impact of metal glider seat with the apparatus

shown in figure 6.
Time - 5 ms/div.

Figure 8. Typical force- time record of impulse resulting from

the impact of the plastic glider seat with the

apparatus shown in figure 8.
Time - 2 ms/div.

Figure 9. Typical force-time record of impulse resulting from

the impact of wooden single occupancy swing seat with

the apparatus shown in figure 8.
Time - 2 ms/div.

Figure 10. Typical force- time record of impulse resulting from

the impact of metal single occupancy swing seat with

the apparatus shown in figure 8.
Time - 2 ms/div.

Figure 11. Typical force- time record of impulse resulting from

the impact of plastic single occupancy swing seat

with the apparatus shown in figure 8.
Time - 2 ms/div.

Figure 12. Typical force-time record of impulse resulting from

the impact of the modified plastic single occupancy

swing seat with the apparatus shown in figure 8.
Figure 13. Metal glider seat.
Figure 14. Plastic glider seat.
Figure 15. Bottom surface of the new plastic single

occupancy swing seat.
Notice the ribs from

one side of the seat

have been cut.

gure 16. Modification made in the plastic single occupancy

swing seat shown in figure 17.


NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. I'lJhl.U A I ION OR KI.POR r NO,

NBSIR 74-563

2. Gov 't Acci ssion No.

3, Kc'f ipicnt 's Ai i ession No.

4. TITLl£ AND SUin ITLIi Recommendations for a Safety

Standard For Hone Playp-round Equipnent - Sv/lnp; Sets

5. Publication Dati- Spntpmbpr* 107/1

6. Perfornrting Organisation Code

7. AunioR(S) ^g^i f;^^ Mahajan 8. Performing Org.m. Report No. NBSIR 74-563

9. PERFORMING ORGANIZATION NAMF AND ADDRKSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. F'rojei t/Task/Vl'ork Unit No.

11. Contract/Grant No.

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

Consumer Product Safety Commission

5^101 V/estbard Avenue

Bethesda, Maryland 20016

13. Type of Rcf>ort ^ Period

.Covered Interim

12/7^ to

14. sponsoring Agt-ncy Code

15. SbPPLK.MENTARY NOTES

16. .ABST RACT (A 200-word or less factual summary of most ai^ificant information. If documvnt includes a siAnificant bibliography or literature survey, mention it here.)

Summaries of swing set related accident investigations were examined in

order to identify the hazards associated with swing sets. Approximately

half of the sv/ing set related mishaps were directly attributed to the

design and construction defects of the products. T'he other half of the

recorded mishaps results from improper installation, deterioration or

misuse of the product.

Some necessary Information, such as anthropometric data, injury threshol.d

data, loading conditions representing child-ha^.ard interactions, and

loads applied to certain components to swing sets is presented.

Quality criteria and test methods are suggested, to establish safety

standards for reducing har.ards due to design and construction defects

of the equipment.

17. KEY WORDS (six to twelve entries; olph.tbolical order; capitalize only the first latter of the first key v.ord unless a proper name; separated by yomicoions) Component J Criteria; dovelopmontj equlpm.ent; home playground; hazard; hazardousness ; identification; mishap; misuse;

product defect; safety standard; swing set; test method

18. AVAILABILITY Unlimited l». SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

For Official Distribution. Do Not Release to NTIS

UNCL ASSIFIED

51

I Order From Sup. of Doc, U.S. Government

WashinKton. D.C. 20 102. SO Cat. N... ( H

Printing Office 20. SECURITY CLASS

(THIS PAGE)

22. Price

1 ^ OfdT From National Technical Inform.ition Sprinefield, Virginia 22151

Service (NTIS)

UNCLASSIFIED

'USCOMM-OC 29042-P74