NATIONAL BUREAU OF STANDARDS REPORT

A STUDY OF SEAT BELT BUCKLE RELEASE METHODS

by

A, B. Castle, Jr.

Occupant Restraint Systems Section

Office of Vehicle Systems Section

NBS PROJECT

4080402 January, 1971

NBS REPORT

10387

IMPORTANT NOTICE NATIONAL BUREAU OF STAN

for use within the Government. Be and review. For this reason, the p whole or in part, is not authorize Bureau ot Standards, Washington,

the Report has been specifically pr

Approved for public release by the

b

, Director of the National Institute of y Standards and Technology (NIST) h on October 9, 2015.

accounting documents intended

bjected to additional evaluation

sting of this Report, either in

Office of the Director, National he Government agency for which

ies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

A Study of Seat Belt Buckle Release Methods

by A. B. Castle, Jr.

I. INTRODUCTION

The Occupant Restraint Systems Section of the Office of Vehicle

Systems Research has obtained a number of seat belt assemblies which

represent a cross section of those used in 1969 vehicles sold in

this country. This group of assemblies, both foreign and domestic,

provides examples of a large variety of buckle release methods.

These samples and others obtained over the past few years have

been evaluated and analyzed for the purpose of determining whether or

not there should be a single standardized method of buckle release

prescribed in the Federal Seat Belt Standard and, if so, what that

release method should be.

II. BACKGROUND

The present requirements of Motor Vehicle Safety Standard No.

209 do not specify the use of any particular type of device to secure

a person in a seat belt assembly. The general requirements section

of the Standard specify that there shall be no burrs or sharp edges

which can contact a person, clothing, or webbing, and that buckles

be readily accessible to the occupant for his easy and rapid release.

The Standard further specifies that all buckle release mechanisms shall

be designed to minimize the possibility of accidental release. The accidental release requirement was of primary importance in the evaluation

of existing designs.

1
1 \
Personnel of the Occupant Restraint Systems Section have been,

and are now, taking an active part in several Society of Automotive

Engineers Committees which are concerned with seat belts. The question of standardization of a single release method has been discussed

by these committees for several years and, basically, two positions

have been presented. The first position, supported by the Occupant

Restraint Systems Section personnel and others in the past, was that

a single method of operation of a release system should not be prescribed in a standard. The adoption of such a requirement would have

had the tendency to stagnate a relatively new area of engineering

design and discourage innovation which could produce a more effective

system.

The second position, presented from a user’s standpoint, was that

a single release system should be specified in a standard. The main

reasons for this approach was that users would always be faced with

the same type of release operation in an emergency, and seat belt

usage would be greater if the public were not confused with a variety

of devices. It was also recognized that a large percentage of the

public is not mechanically oriented and certainly some existing systems

appeared complicated.

III. DESIGN REVIEW

A design review of a cross section of seat belt assemblies, representative of those sold in this country, was conducted to determine

what existing features of release devices which were detrimental to or

beneficial in improving their safety and gain greater public acceptance.

2

All of the samples reviewed presented a means of quick release and in

many cases, some offered small advantages over others.

The widely used lever or lift cover buckle presents several disadvantages in normal usage. The action required to effect a release

(see Fig. 1 and 2) varies from either a push or pull, depending on

which hand is used and in both cases the direction of the action required tends to keep the tongue and buckle engaged. When the lever

or lift cover pivot is located so that the release action tends to

separate the mating parts, the fingers or fingernails are then exposed

to possible damage as the assembly separates (Fig. 3). Some models

of a lever type buckle require an excessive amount of lever travel in

order to effect a release and this may be difficult to accomplish in

an emergency, especially if there are any restrictions on body movement as a result of a collision. The position of the lever shown in

Fig. 4 (almost 90°) is the minimum distance that the lever must travel

in order to effect a release.

A serious disadvantage in panic release of a lever type is incurred when changing positions from one side of the vehicle to the

other, say, from passenger to driver. The releasing action for the

same hand is reversed from a pull to a push or vice-versa, or the

buckle must be released with the opposite hand to maintain the same

action.

Some of the more unique systems reviewed, not common in this

country, presented a variety of release techniques which contain

several disadvantages. The first and most important of these is that

the release lever is extremely vunerable to accidental release through

3
J I 1 1 1 11
inadvertent contact with the occupant's hands, arms, or clothing (see

Fig. 5). The lever of this buckle is also openly exposed to damage by

a closing door or folding seat which may cause it to be inoperable or

unsafe. This buckle assembly, including the tongue, is over six inches

in length and presents the user with a large metallic object which

must come in contact with his body. Such a buckle is uncomfortable to

a user and would increase the possibility of injury to the abdomen or

skeleton in a collision.

Another unique system designed to be mounted rigidly between two

bucket seats (see Fig. 6) is susceptible to accidental release by contact with an occupant's extremities. This particular model, with the

sharp release levers protruding, also exposes an occupant to possible

injury from the hardware during collision. An advantage of this type

of system is that the assembly is mounted in a fixed position which

makes one hand operation extremely easy and keeps all the hardware off

the body.

Several systems reviewed present a user with either no obvious

operation point or a misleading operating point. Although the two

systems shown in Figs. 7 and 8 operate well, there is no indication,

through design or labeling, to direct a person where or how to effect

a release. In Fig. 7 the release mechanism is operated by applying a

force on the knob in the direction indicated by the arrows and the assembly shown in Fig. 8 is also released when a force is applied on the

bar in the direction indicated by an arrow. The buckle in Fig. 9 is

actually a lever release (liftcover) type but through ornamentation

and cover design this buckle could easily appear to the user to be a

4
LM
pushbutton type.

IV. DESIGN CRITERION

In an effort to determine whether a single type of release method

should be specified by a federal standard, a study and evaluation of the

various seat belt release systems was conducted. This study was

directed towards a determination of what advantages and disadvantages

were offered by the existing systems of vehicles sold in this country.

The study indicated that any single type of release mechanism adopted

should include as many of the following desirable features as possible.

(a) Any release device must meet the present applicable requirements of MVSS 209.

(b) The operation should be easily and quickly accomplished

with either hand by the same relative motion. (See Figs.

10 and 11)

.

(c) The direction of the force required to effect a release

should not tend to keep the assembly together when the

latching mechanism is opened. (See Fig. 1).

(d) The area of the release mechanism where the release effort

is to be applied should have sufficient protection to prevent accidental operation of the device by the arms, hands,

and clothing. (See Figs. 5 and 6).

(e) The direction of the forces on the release mechanism, due

to the vehicle frontal impacts (deceleration) should

tend to keep the latching device closed.

(f) The operation of the release mechanism shall require one

5
I I I I 1 I 1 I I I I I 1 I I 1 I 1 1 II 1 1 11
and only one continuous motion.

(g) The release device shall be protected so as to minimize

the possibility of objects, such as buttons and cufflinks, getting into the mechanism.

(h) In normal usage a person's body or clothing should not

interfere with the operation required to release the t

assembly

.

(i) The operation of a release mechanism should be accomplished

without the fingers or fingernails being exposed to damage

when the assembly separates. (See Fig. 3).

(j ) The design of any release device should obviously direct

a user to the operating area and the cover of any device

should not have any design or emblem which falsely implies

an operation point. (See Figs. 7, 8, 10 and 11).

(k) There shall be no projections of levers or handles which

could cause a serious injury to a belted or unbelted

vehicle occupant. (See Fig. 6).

V. RECOMMENDATIONS

Seat belt assemblies have been required as standard equipment in

all passenger vehicles sold in this country for several years. The

seat belt industry has now had sufficient time, with no restrictions

on release mechanism design, to develop a great number of systems.

Since no radically new designs have appeared for several years, the

Occupant Restraint Systems Section recommends that a single type of

release for all seat belt assemblies should be specified in a Federal

6

Standard.

i

As far as the public is concerned, the adoption of a single type

of release would benefit all, particularly those who are not mechanically

oriented, by exposing them to one and only one simple type of release

action.

After the examination and evaluation of the different existing

systems that have been and are now being used, the pushbutton type of

release appears to be the logical type to specify.

The reasons which led to selecting the pushbutton type operation

over other types are presented in three following categories.

(a) Service and reliability

The pushbutton buckle has been in use in this country

since late 1964. This period of time represents a

significant test period for such a system and the results

have been very satisfactory. In some isolated instances,

problems have arisen and the manufacturers have made

slight design modifications to eliminate these problems.

The system, as we know it today, has a very high level

of reliability and meets all of the existing Federal

specifications concerning release mechanisms.

(b) Desirable features

The pushbutton release presents the user with one single

operation which may be accomplished easily and quickly

in the same manner with either hand and thus allows the

device to be used on the inboard end of belts for either

side. This is in contrast to the single ended lever type

7

which requires release with the opposite hand when the

buckle is placed inboard on the opposite side of the

vehicle.

This type of system also complies with those design criteria set

forth in Section III as desirable. Several other types of release devices qualify partially with the above statements but none to the extent of the pushbutton type. Any release device which is selected to

be specified certainly should have a period of exposure to general

usage and not introduce some unique operational action.

(c) Usage and acceptance

Seat belts have been installed by the automobile manufacturer

as standard equipment in outboard front seats since 1964 and

in all seated positions since 1968. According to vehicle

population figures, approximately 70 percent of all passenger

vehicles on the road today have seat belts in the outboard

front seats and 35 percent have seat belts in all seated

positions. The pushbutton system was first introduced in 1964

and the use of this type of system has greatly increased each

year to the extent that in 1970 approximately 85 percent of all

original equipment seat belts used are pushbutton. The predicted figures for 1971 model vehicles show that 97 percent of

all seat belt assemblies installed as original equipment will

have a pushbutton type release. The increase in acceptance of

the pushbutton, both by the automotive industry and the using

public, presents a strong indication that if all seat belt

8

assemblies were equipped with this same type of convenient

release device, the percentage of people who would wear them

would also increase.

VI. ADDITIONAL REQUIREMENTS

In addition to a requirement that only a pushbutton type release

system be acceptable, the following items should be considered for inclusion in a Federal Standard which should improve the operation,

reliability and encourage more usage.

(a) Enclosure #1. The National Bureau of Standards Proposed

Revisions to Motor Vehicle Safety Standard 209 should be

adopted.

(b) The area in which the release force is to be applied

should be labeled "push". The color of the area should

also be in contrast with the cover and no insignia or

other design which could be confused with the pushbutton

should be allowed. (See Fig. 12).

VII. CONCLUSIONS

The pushbutton type release system employed in a large majority

of vehicles, both foreign and domestic, sold in this country should be

specified in a Federal Standard as the only acceptable means of releasing a seat belt assembly. The adoption of this and the preceding

recommendations would increase the usage of seat belts by the public

and add to the safety value provided by them.

9 .

10
1 

11
11

FIG.
ill
12

FIG.

3
1 I ! I I I Il I 1 J L I 1 I 1 1 .1—1
13

FIG.
J—I—I—

L

-I—I—I—v

i

i i i i i i i M I 1 l 1 I 11 1 1 I 11 I 1 1 I-
14

FIG.

5
1 1 1 1 I I 1 I 1 I I I I I I I 1 I M 1 1 11 1 I -LI llMii
15

FIG.

6
-I 1 1 1 1 1

mini rrTi i i i i i i i i i i i i i i i i i i
16
i i 1 i ; ' i i i i i i i i i i •
17

FIG.

8
I i i 1111 ; i m 1 i m i i 1 M 1 1 1 l—l
18

19

FIG.

10
huh
20

I I I I T I I I

FIG.

12

NBS PROPOSED REVISIONS TO MOTOR

VEHICLE SAFETY STANDARD NO. 209;

SEAT BELT ASSEMBLIES

Motor Vehicle Safety Standard No. 209 (32 FR 2415) incorporated by

reference the requirements of the Department of Commerce, National

Bureau of Standards, Standards for Seat Belts for Use in Motor Vehicles

(15 CFR Part 9; 31 FR 11528). The following revisions to this Standard

are recommended:

I. BUCKLE RELEASE

Discussion :

In tests conducted at NBS, buckle release or malfunction

occurred with a compressive force of as little as 275 pounds

when the force was applied in surface areas other than the

area directly over the pushbutton. It is recommended that a

new test procedure be required that would apply the present

force of 400 pounds in all areas of the surface of the buckle.

It is also recommended that this requirement be extended to

all types of buckles, including lever, squeeze, etc. This

new test will tend to eliminate buckle designs that are prone

to accidental damage which would reduce their load carrying

effectiveness. It also reduces the probability of buckle release during the initial phases of an accident. The Society

of Automotive Engineers is planning to adopt an identical requirement in the next revision of the SAE J4C Seat Belt Standard .

1
4-4—t—

; —i

\-
Revisions to Standard:

1. Rewrite paragraph 9.5 (d)(3) as follows: "A buckle of a Type 1

or a Type 2 seat belt assembly shall not release under a compressive force of 400 pounds or 180 kilograms applied as prescribed in paragraph 9.8 (d)(1) and shall be operable and meet

the applicable requirements of paragraph 9.6 upon removal of the

compressive force".

2. Paragraph 9.8 (d)(3). Delete the entire paragraph and substitute the following: "The buckle of a Type 1 or Type 2

seat belt assembly shall be subjected to a compressive force

of 400 pounds or 180 kilograms applied anywhere on the center

line and along lines at approximately 60 degrees to the center

line, with the point of intersection of these lines centered

over the release mecharaism, through a cylindrical bar 0.75 inch

or 2 centimeters in diameter and curved to a radius of 6 inches

or 15 centimeters. The bar shall be placed with the center

line of the bar directly above the lines through the center

line of the buckle and at approximately 60 degrees to it. The

buckle shall be engaged and a tensile force of 75 pounds or 34

kilograms shall be applied to the connected webbing during the

application of the compressive force. Buckles from three seat

belt assemblies will be tested to determine compliance with

paragraph 9.5 (d).

II. BUCKLE LATCH

Discussion:

During NBS dynamic tests conducted at Holloman Air Force Base,

2
tfci—J s. I

•I—I—I—!—I—!—I—!—I—!—I—I—I—I—I—I—I

I I I ! I I I ! I ! 1 I LJ L
using anthropometric dummies (Alderson Model F-50), two buckle

latch failures occurred. Static laboratory tests of buckle latches

were set up at the National Bureau of Standards, in which the tensile force was applied at 30 degrees to the transverse belt direction. These tests indicated that the strength of certain buckle

latch arrangements were reduced by as much as 50 percent to a

figure' of 1500 pounds total tensile strength. This failure is attributable to the rounding of the tongue of these designs which is

used to improve the ease of tongue insertion into the buckle.

Minor changes of the tongue and/or latch construction would eliminate this potential weakness.

Revisions to Staiidard:

1. Paragraph 9.5(g). Add the arabic numeral 1 in parenthesis

following the paragraph designation as "Paragraph 9.5(g)(1)".

2. Add a second paragraph to paragraph 9.5(g) as follows: "(2)

The buckle of a Type 1 or a Type 2 seat belt assembly when

tested by the procedure specified in paragraph 9.8(g)(2) shall

not fail and shall be operable and meet the applicable requirements of paragraph 9.5(d)(1). The load applied to the Type 1

and Type 2 seat belt assembly shall be 2,000 pounds or 910

kilograms and 1250 pounds or 570 kilograms, respectively."

3. Add the paragraph symbol "(1)" following the words "Buckle

Latch".

4. Paragraph 9.8(g)(2). Add the following test: "The buckles

and webbing from three seat belt assemblies shall be tested

3
J—I—I—I—I—I—I—

1— —i

-I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—

k
in the following manner: The testing machine shall conform

to the requirements specified in paragraph 9.7(b). A split

drum grip equivalent to that of Figure 1 shall be used in one

head of the machine to grip the webbing of the buckle. A

gripping device similar to that one in Figure 9 is used to

hold the tongue to the other head of the machine so that it

is loaded at a 30 degree angle in the transverse plane of

the webbing. The rate of grip separation shall be between

2 and 5 inches per minute or 5 and 10 centimeters per minute.

The seat belt assembly shall be loaded until the values

specified in paragraph 9.5(g)(2) are applied.

5. Add Figure 9 as shown in attached sketch.

III. REDUCTION OF BUCKLE RELEASE FORCE

Discussion :

A reduction of buckle release force is desirable so long as the

other security aspects of the seat belt assembly are maintained.

It is recommended that the 150 pound loop load for the 30 pounds

release force be increased to 200 pounds. This is equivalent to

a reduced release load of 22.5 pounds with a loop load of 150

pounds. Only the pushbutton type of design will be affected to

any degree by this recommended reduction in release force. Two

major manufacturers of seat belt assemblies already have prototypes

of buckles which will meet this new requirement.

Revision to Standard:

Paragraph 9.3(d)(1). Change 150 + 10 lbs. to 200 + 10 lbs. and

4
1 . : f

1 I

1 : I— —rt-*—I! 1 iJ 4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ! ! 1 1 1 1 ! 1 1 1 1 1 4
change 68+4 kilograms to 91+5 kilograms.

IV. EMERGENCY LOCKING RETRACTORS

Discussion :

The emergency locking retractor accomplishes tvo objectives:

1. Stowing the webbing when not in use

2. Permitting relatively free motion while wearing the webbing

from the retractor.

In the few production models of this device to date, many can

and do lock at very low g levels, such as 0.2 gravity, due to

manufacturing tolerances. Locking at this very low critical

level makes it difficult to don a seat belt assembly and discourages the use of emergency locking retractors (inertia reels).

It is recommended that a lower limit of 0.3 gravity be specified

for non-locking of the retractor.

A recent development in emergency locking retractors is an arrange

ment in which the first few feet of webbing extraction does not

involve any locking and enhances ease of donning this type of

seat belt assembly. It is recommended that a statement be made

in the standard which would permit this desirable alternative

type of design.

Revisions to Standard:

1. Paragraph 9.5(j). Add in the first sentence after "0.5

gravity or 5 meters per second per second" the following

"but shall not lock when exposed to 0.3 gravity or 3 meters

per second per second or less, etc." At the end of this paragraph, add the words "These requirements need only apply for

5
i- —I—I—I—i—I—I—I—I—I—f
I

and only one continuous motion.

(g) The release device shall be protected so as to minimize

the possibility of objects, such as buttons and cufflinks, getting into the mechanism.

(h) In normal usage a person’s body or clothing should not

interfere with the operation required to release the

assembly

.

(i) The operation of a release mechanism should be accomplished

without the fingers or fingernails being exposed to damage

when the assembly separates. (See Fig. 3).

(j ) The design of any release device should obviously direct

a user to the operating area and the cover of any device

should not have any design or emblem which falsely implies

an operation point. (See Figs. 7, 8, 10 and 11).

(k) There shall be no projections of levers or handles which

could cause a serious injury to a belted or unbelted

vehicle occupant. (See Fig. 6).

V. RECOMMENDATIONS

Seat belt assemblies have been required as standard equipment in

all passenger vehicles sold in this country for several years. The

seat belt industry has now had sufficient time, with no restrictions

on release mechanism design, to develop a great number of systems.

Since no radically new designs have appeared for several years, the

Occupant Restraint Systems Section recommends that a single type of

release for all seat belt assemblies should be specified in a Federal

6
I I I I 'I I I I -!
required, additional anchorages for Type 3 seat belt assemblies

should not be necessary. To encourage the use of Type 3 assemblies,

it is recommended that the anchorages already required be used

for their installation. It is also recommended that retainers

for Type 3 seat belts should use the Type 1 and Type 2 seat belt

anchorages already specified.

Revisions to Standard:

1. Add to paragraph 9.3(f)(1) the following: "The attachment

mechanism of the Type 3 assembly shall be designed to utilize

the Type 1 and Type 2 seat belts and anchorages."

2. Paragraph 9.3(h). Seat belt retainer. Add to the end of the

paragraph the following sentence: "The retainer shall be

designed to utilize the Type 1 and Type 2 seat belts and anchorages .

VII. EDITORIAL CHANGES

Discussion :

The Figure 3 label was inadvertently left out. The addition of

the word "exposed" in the notation A is recommended so that the

words will agree with the text where this figure is referenced.

Revisions to Standard:

1. Label the figure on page 11537 "Figure 3".

2. Change notation A in this figure to read: "A - 2 full

threads exposed."

7
J I 1 1 1 u _] I
WEBBING

BUCKLE

BUCKLE TEST FIXTURE

FIGURE 9
-ir-UJ- 4—I—I—I—I—I—It—M-
RECOMMENDED ABRASION TEST OF SEAT BELT

ASSEMBLY LENGTH ADJUSTMENT DEVICES

9.6 Requirements for assembly performance

(d) The webbing in a seat belt assembly, after being subjected

to abrasion as specified in paragraph 9.9(d) shall have a

breaking strength not less than 75 percent of the strength

before abrasion when measured by the procedure specified

in paragraph 9.7(b). At the completion of the abrasion

cycling the assemblies shall meet the requirements set

forth in paragraphs 9.5(e) and 9.5(f).

9

9.9 Test procedure for assembly performance

(d) Resistance to abrasion . Three seat belt assemblies shall

be tested for resistance to abrasion at the buckle or any

manual adjusting device normally used to adjust the size of

the assembly. The webbing of the assembly to be used in this

test shall be exposed for at least four hours to an atmosphere having relative humidity of 65+2 percent and a temperature of 21+2 degrees Celsius or 70+3 degrees Fahrenheit. The webbing shall be cycled through the buckle or

manual adjusting device as shown schematically in Figure 9.

The anchor end of the webbing (A) shall be attached to a

weight (B) which has a mass of 3.0 + 0.1 pounds or 1.4 + 0.05

kilograms. The webbing shall pass through the buckle (C) and

the other end (D) attached to a reciprocating device so as

to form an angle of 8 + 2 degrees with the hinge stop (E)

.

The reciprocating device shall be operated for 2500 cycles

at a rate of 18+2 cycles per minute with a stroke length

of 6 to 8 inches. The abraded webbing shall be tested for

breaking strength by the procedure described in paragraph 9.7

(b) and each valve shall not be less than 4500 pounds but

the median values for the breaking strengths determined on

abraded and unabraded specimens shall be used to calculate

the percentage of breaking strength retained.

10
|^J i 1—| 1 1 1

i 1 I -4—I

1 1 1 1

. 1 .

1 1 '

1

1 1

'

1 *

i i r i t I < . r~r i

' i i — i t i i ^
SHORTENING STROKE „

PIVOT BUCKLE (C)

r A i —

HINGE STOP (E)

• NO TENSION

\

WEBBING (A)

3 LB. WEIGHT (B)

1 1
I | I 1 I —I 1 1 1 \