EP2773232B1 - Belt closure and chin strap of a protective helmet equipped therewith - Google Patents

Description

The invention relates to a buckle for releasably connecting two strap ends, with a housing which is connectable or connected to one of the strap ends, and with a clipped into the housing connector which is connectable or connected to the other strap end (10b).

Moreover, the invention relates to a chin strap of a protective helmet equipped with such a belt buckle.

A closure of this type and a chin strap equipped with it are from the document DE 20 2007 017 686 U1 known, which will be discussed below.

According to the European standard FprEN 397, final draft June 2011, is meant by a chin strap a belt running under the chin, which improves the hold of a helmet. The chin strap attachment, so a device for attaching the chin strap to the helmet, closes according to this standard z. B. components with which are attached for this purpose at the ends of the chin strap, and the part of the helmet shell or the carrying device to which the chin strap is attached. Furthermore, according to this standard, the helmet shell or the carrying device must be equipped with a chin strap or with devices for its attachment. Each chinstrap supplied with the helmet must be attached either to the helmet shell or to the carrying device.

The European standard FprEN 12492: 2011 (D) is based on the consideration that the chin strap brings with it the danger that the helmet gets stuck and thereby causes a strangulation risk for the helmet wearer. The standard therefore requires that, for a given pulling force applied to the chinstrap, the chinstrap or its attachment (s) or generally the carrying means must (must) fail to release the helmet held by the chinstrap. Therefore, during the safety testing of protective helmets, the chin strap is subjected to a prescribed test. For this purpose, a chin strap, which is a pine model containing two rigid cylindrical rollers of specific diameter, is used on a tester. On the pine reproduction is a known changing Stress applied and then the displacement of the pine simulation is measured. The stress is to increase linearly at a speed of (500 ± 50) N / min until the pine simulation is triggered due to the failure of the support. These values concern the testing of mountaineering helmets. For industrial helmets covered by the first-mentioned FprEN 397 standard, the test jaw must be released with a force of at least 150 N and not more than 250 N only by giving (giving) the chin strap attachment (s). For the test, an industrial helmet is placed on a test head and a tensile force is exerted on the chinstrap. On the tester, which includes the appropriate test head, the test jaw is subjected to a tensile force of 150 N. This force is then increased by (20 ± 2) N / min until the test jaw is released only due to the failure of the fixture (s).

To the knowledge of the applicant, the chinstrap test has so far been handled laxly by the industry. However, there are indications that compliance with the Guidelines on chin straps will be more stringently reviewed in the future and will be given greater consideration by liability insurers in dealing with accidents involving a safety helmet. In the related art, particularly in the patent literature, no examples could be found which would illustrate how chinstrap fasteners or wearers of helmets are formed to yield at a given tensile stress on the chinstrap.

The document WO 2005/058086 A1 shows a buckle for releasably connecting two strap ends, of which one end of the strap is attached to a housing and the other strap end to a clip-in connector. The plug has two projecting into the housing, flexibly formed struts, which are pressed by a means of spring biasing force against the struts outwardly pressing cross bolt to the outside. In the region of the transverse bolt, the struts also have outwardly facing thickenings which engage with correspondingly formed grooves in the housing when the plug is inserted into the housing as far as an end stop. On the outside of the cross bolt is surrounded by a flexible housing. The transverse pin and the flexible housing in fluid communication with the interior of the transverse pin are filled with an incompressible fluid. In order to release the cross pin, a lateral force is needed, whereby the lateral force must exceed the spring biasing force. The transverse force is caused by a contraction-sliding of the contact surfaces by a tensile force. The adjustment of the required shear force is thereby by the choice of Federvorspannkraft and made by the choice of the angle of the contact surfaces of the thickenings and the grooves.

The document US 2004/0194264 A1 shows a snap closure with a two-piece, spring-biased claw into which a corresponding strap end can engage with a ball head. One of the two claw parts is rotatably mounted relative to the other claw part by a bolt, wherein a spring means is arranged on an opposite portion of the claw and the two claw parts relative to each other acted upon such that it closes the ball head enclosing claw portion. If the first jaw part is acted on at a section, for example, by hand against the spring biasing force, the first jaw part is rotated about the bolt and thus moved from a set position.

The document DE 296 08 345 U1 relates to a helmet, in particular for occupational, fire and civil protection, which also has a chin strap with a closure. The same applies to the document DE 87 14 490 U1 , which relates to a protective helmet, which has a length-adjustable chin strap, which is provided with a closure which comprises an eyelet and a mushroom-like pin, which are engageable with each other.

The document already mentioned at the beginning DE 20 2007 017 686 U1 contains no indication as to how could give in at a certain tensile stress of the chin strap whose attachment or the carrying device. The sidebands of the chinstrap are connectable to each other by a so-called click-lock, which is widely used nowadays and in its basic structure eg from the document EP 0 815 761 B1 is known. In the click closure, a latching surface area is evenly distributed around the top, bottom and outside of two finger flaps disposed on the opposite narrow sides of the closure. By this uniform distribution a greater Einklink- and holding strength of the closure should be achieved. All this serves the purpose of preventing a release of the closure during high load. Another example of such a carrying device is in the document WO 2007/049048 A1 described where a belt buckle for releasably connecting two strap ends is shown. One of the strap ends is clipped into the other end of the strap, wherein the first strap end has flexible struts with a thickened end, which engage in a correspondingly formed opening of the second strap end.

There are other types of safety equipment where it is desirable for the equipment to yield or disengage from the person wearing it to protect the person from injury or overuse. These include, for example, dog collars, satchel straps or backpack straps, straps on work shoes, straps on ski clothing or equipment and the like. More. In addition, in the chin strap after the document already mentioned several times above DE 20 2007 017 686 U1 or the document EP 0 810 761 B1 The application and release of the chin strap difficult because the closure of this known chin strap has two finger flaps, which are arranged on the narrow longitudinal sides of the closure and must be operated simultaneously by the wearer of the protective helmet by pressure with two fingers, so that the closure at clicked plug at all can be opened. It is also disadvantageous that, in the case of a bearded person who wears the chin strap, the beard hairs and / or the skin on the cheek are pinched when the closure is closed. Furthermore, it is difficult to operate the known click-lock when wearing the chin strap or solving person wearing gloves.

In addition, in the known closure, the period in which the finger flaps operate trouble-free, limited because the finger flaps are molded parts on the plug, which must apply the required spring force at its junction with the plug, which for clicking the plug into the finger openings whose narrow longitudinal sides is needed. The time span is simply limited by material fatigue and aging. In addition, temperature changes may affect the functionality of the finger flaps of the known closure.

Finally, when acting on the chin strap diagonal pull a situation occur in which the closure by finger pressure can not be opened at all, because the diagonal pull can cause a finger flap is blocked so that it can not be depressed by finger pressure.

The above-described properties of the known closure mean that this closure is unsuitable for contributing to the standard of reducing the risk of strangulation for the helmet wearer. Therefore, in the use of the known chin strap actually its attachment (s) or generally the support device must fail, so that at a Strangulierungsgefahr by the chin strap held helmet is released. However, the state of the art does not provide assistance for a deliberate failure, as already explained above.

It is also critical that in the known chin strap, the failure of the attachment (s) or generally the support device can be done too soon, so even if there is no risk of strangulation. In a fall, this can be associated with the disadvantage that the person wearing the helmet loses it too early and hits his head when the helmet is already no longer upside down. It must therefore be ensured that the release of the helmet within the tolerance range for the prevention of Strangulierungsgefahr not too early. Also for the state of the art offers no help.

In addition, it would be necessary to be able to choose the limit of the load at which failure should occur, e.g. depending on the sex, weight or age of the person for whom the safety helmet or any other type of safety equipment is intended. Here is the invention to provide a remedy.

The object of the invention is therefore to overcome the problems outlined above and to achieve in a simple manner that the carrying or fastening device of a helmet or generally a belting gives in at a certain tensile stress of the chin strap or the strap ends or the person who him or she wears, releases.

This object is achieved on the basis of a closure of the type mentioned above in that the closure is designed as a safety closure that opens when acting on the strap ends or on the housing and the plug a certain tensile stress.

The inventive solution to form the closure as a safety closure that opens at a certain tensile stress ensures a simple and secure way to fail in a hard hat the carrying device to release the helmet, or a belt or belt equipment of any kind from its wearer to solve.

Further, the plug for clipping into the housing according to the invention a separate, arranged on one of its broad sides finger flap, which is hinged to a bolt at a front end of the plug. The finger flap, which is the broadside in essence occupies completely, provides for the finger operation a wide area and can be easily operated with one finger, and also with a glove. The opposite broad side of the closure housing, when the closure is in the closed position, will usually be facing the wearer of the harness, thus forming a pad for the closure and allowing its one-finger operation and at the same time the risk of entanglement of the skin and / or whiskers eliminated.

Further, according to the invention, the finger flap at its opposite end to its articulation point a stop surface, which is in an inserted into the housing plug on a stop surface in a formed in a broad side of the housing finger hole in abutment. The mutual contact of these two stop surfaces keeps the buckle in the closed position. The finger opening formed in a broad side of the housing is adapted to the large-scale design of the finger flap, which allows the operation of the closure by pressure with a finger.

Further, the two abutment surfaces according to the invention are inclined by an angle different from 0 ° against a perpendicular to the insertion direction of the plug forward.

The angle of inclination of the stop surfaces allows the choice of a specific tensile stress of the strap ends or of the housing and plug, at which the closure is to open. The angle of inclination of the abutment surfaces can simply be chosen so that the finger flap is pivoted inwardly upon reaching a certain amount of tensile stress in the finger opening of the housing into the plug, the abutment surface of the finger flap finally leaves the abutment surface in the finger opening, so that the Plug can be pulled out of the housing.

Advantageous embodiments of the invention form the subject of the dependent claims.

In a further embodiment of the belt fastener according to the invention, the finger flap is biased by a separate spring in the direction of a position in which it projects beyond the plug to the outside. In other words, the separate spring is a spring made separately from the plug, which can be made irrespective of the material and configuration of the plug in terms of material, structure and spring force and thus allows the independent choice of the size of a certain preload. The in turn allows the choice of a certain tensile force value for opening the safety closure.

In a further embodiment of the belt fastener according to the invention, the position in which the finger flap protrudes beyond the plug to the outside, by a projection on the finger flap, which is engageable on a projection on a body of the plug in Appendix set. In this embodiment of the invention, a certain starting position of the finger flap can be fixed, so that the opening of the closure begins as soon as the finger flap begins to leave this starting position, and in a repeatable manner.

In a further embodiment of the belt buckle according to the invention, the spring is a single or multi-leg spring, which surrounds the bolt and is supported with its free ends on the finger flap and on the body of the plug. Such a single or multi-leg spring is structurally particularly useful, because the levers with which their legs act on the finger flap and the body of the plug, in a small space have a large length. This makes it possible to obtain a high force, which can be achieved with another spring, e.g. a helical compression spring which is inserted transversely to the insertion direction of the plug between the finger flap and the body of the plug, would not be readily achievable. By varying the spring parameters can be determined in a simple manner, a certain tensile force at which the buckle should open according to the invention.

It should be noted, however, that instead of a leg spring any other spring, z. B. simply a V-shaped leaf spring made of spring steel could be used. Steel springs basically have the advantage that they tend less with changing temperature to change their spring force, in contrast to springs made of plastic od. Like.

In a further embodiment of the belt closure according to the invention, the inclination angle is 20 °. In fact, a suitable value of the inclination angle is in a range ranging from a value of less than 20 ° to a value of more than 20 °. The value of 20 ° has been determined by experiments. The experiments have also shown that an inclination angle of 10 ° may also be suitable, but requires a very high tensile stress at which the closure actually opens. The variation of the angle of inclination allows a fine adjustment to achieve the required by the standard tensile force value, in which a carrying or fastening means of the chin strap a safety helmet or in general a belting equipment has failed. This fine adjustment makes it possible to take into account the effective friction between the stop surfaces in achieving a desired tensile force value, also taking into account the fact that the friction can also be changed by moisture when a chin strap or a belting generally with rainwater or simply with Sweat comes into contact.

In a further embodiment of the belt fastener according to the invention, the two stop surfaces in plan view, ie in each case viewing the finger flap and the housing from the top convex or concave curved, in such a way that the stop surface of the finger flap has a radius of curvature which is smaller than the radius of curvature of the stop surface of the finger opening. This results in a line contact between the two abutment surfaces, which usually takes place in the middle, but migrates when acting on the strap ends diagonal train depending on the pulling direction in one or the other direction to the outside. Reducing contact between the abutment surfaces for line contact reduces the effect of friction much less. In addition, the line contact allows to choose the spring force as large as possible, so that the effect of friction in relation to the spring force is additionally reduced accordingly. If the two stop surfaces were curved complementary to each other, would result in a much higher friction. In the prior art, which is described at the outset, this is desirable because the known click-closure should be increasingly protected against opening with increasing load. In the prior art, therefore, the inclination of the mutually contacting abutment surfaces of finger flap and housing is not obliquely forward inclined against the insertion direction, but obliquely inclined backwards against the insertion direction.

Fig. 1

in einer Längsschnittansicht eine bevorzugte Ausführungsform eines erfindungsgemäßen Gurtverschlusses, der zwei Gurtriemen miteinander verbindet und in geschlossenem Zustand dargestellt ist,

Fig. 2

den Gurtverschluss nach Fig. 1 in einer etwas perspektivischen Seitenansicht,

Fig. 3

in einer etwa gleichen perspektivischen, aber zusätzlich im Längsschnitt gezeigten Darstellung den Gurtverschluss nach Fig. 2,

Fig. 4

den Gurtverschluss nach Fig. 1 etwa in Draufsicht,

Fig. 5

als eine Einzelheit den Stecker des Gurtverschlusses nach Fig. 1 in einer Draufsicht,

Fig. 6

als eine Einzelheit das Gehäuse des Gurtverschlusses nach Fig. 1 in einer perspektivischen Darstellung und

Fig. 7

als eine Einzelheit den Stecker des Gurtverschlusses nach Fig. 1 in einer perspektivischen Darstellung.

Embodiments of the invention will be described in more detail below with reference to the drawings. It shows:

Fig. 1

in a longitudinal sectional view of a preferred embodiment of a belt fastener according to the invention, which connects two belt straps together and is shown in the closed state,

Fig. 2

the belt buckle after Fig. 1 in a somewhat perspective side view,

Fig. 3

in a roughly same perspective, but also shown in longitudinal section illustration of the buckle after Fig. 2 .

Fig. 4

the belt buckle after Fig. 1 approximately in plan view,

Fig. 5

as a detail the plug of the buckle after Fig. 1 in a plan view,

Fig. 6

as a detail, the housing of the buckle after Fig. 1 in a perspective view and

Fig. 7

as a detail the plug of the buckle after Fig. 1 in a perspective view.

In the Fig. 1 to 7 is a total of 20 designated Gurtverschluss with two attached ends 10a, 10b of a belt or belt 10 shown. The belt 10 may be formed as a part of a carrying device of a protective helmet, both of which are not shown. The belt fastener 20 comprises a housing 30 and a plug 40 which can be inserted into the housing 30 Fig. 1 showing the belt 10 with the buckle 20 in a longitudinal sectional view, the plug 40 is inserted into the housing 30 and there clicked or snapped in the manner described in more detail below. Fig. 2 shows the strap 10 and the buckle 20 after Fig. 1 in a somewhat perspective side view. Fig. 4 shows the strap 10 and the buckle 20 after Fig. 1 approximately in plan view. On the other hand, in each case as a detail in a plan view of the plug 40 of the buckle 20 after Fig. 1 in Fig. 5 and in a perspective view of the housing 30 of the buckle 20 after Fig. 1 in Fig. 6 shown. Fig. 3 shows a view like in Fig. 2 , but additionally in longitudinal section. Fig. 7 shows as a detail the plug 40 after Fig. 1 in a perspective view.

As shown in the Fig. 1 and 5 the plug 40 has one on one of its broad sides (in Fig. 5 it is the viewer facing broadside) arranged finger flap 44. The finger flap 44 is hinged to a bolt 48 at a front end 42 of the plug 40, as shown in the Fig. 1 . 3 and 7 can be seen. The bolt 48 is rotatably mounted in two bearing blocks, which are integrally formed on a body 49 of the plug 40, but not shown in the drawings. For the bolt 48 in the bearing blocks simply have a press fit. The finger flap 44 is a separate one Component, ie a separate component from the plug 40, which has been manufactured separately and then by means of the bolt 48 with the body 49 of the plug 40 has been pivotally connected. An insertion direction, ie, the direction in which the plug 40 is inserted into the housing 30, is in FIG Fig. 1 indicated by an arrow 46. The plug 40 is in Fig. 1 inserted from right to left in the housing 30, which is why the front end 42 of the plug 40 whose in Fig. 1 left end is shown. The interior of the housing 30 is adapted to the outer shape of the plug 40 so that the inserted plug 40 in the housing has no significant play.

Arranged around the pin 48 is a spring 50 which, in the embodiment of the buckle 20 described herein and shown in the drawings, is a leg spring designed to tend the free ends of its legs 51, 52 to move away from each other. The spring 50 surrounds the bolt 48 and is supported with the free ends of their legs 51, 52 on the finger flap 44 and on the body 49 of the plug 40 on projections 41 a and 41 b from, as best in Fig. 1 can be seen. In this way, the finger flap 44 is biased by the spring 50 in the direction of a position in which the finger flap 44 on the plug 40, more precisely on the body 49 of the plug 40 projects outwardly biased. This position of the finger flap 44 is in the Fig. 1 . 3 and 7 shown. In the slightly perspective longitudinal section in Fig. 3 It can be seen that the spring 50 is a double leg spring. A double leg spring develops more power than a single leg spring. The position in which the finger flap 44 projects beyond the plug 40 or its body 49 to the outside, as shown in FIG Fig. 1 by a projection 45 on the finger flap 44, which is on a projection 43 on the body 49 of the plug 40 in abutment set. By the mutual abutment of the projections 43 and 45, the pivoting of the leg spring 50 is limited by the bolt 48 in the counterclockwise direction. By choosing the material of the spring 50 and the number of legs, the force is selectable, with which the spring presses the finger flap 44 against the projection 43.

As spring 50, any spring can be selected, which provides the necessary voltage for the opening force. Therefore, instead of the torsion spring, it could be a compression spring, a leaf spring, resilient plastic elements, carbon leaf springs, carbon coil springs, or any other type of spring. In the embodiment described here, the double leg spring is made of high-alloy steel and therefore essentially changes its properties in a temperature range from -30 to + 40 ° C. Not. The finger flap 44, like the plug 40, has been made as a separate injection molded part. Both have then been joined together with the aid of the bolt 48, including the spring 50. This design of the spring 50 provides over the aforementioned temperature range more constant properties than a plastic spring, which is integrally formed on a plastic plug and connected to this only by a plastic bridge as in the above-described prior art. Although a compression spring could be arranged transversely to the insertion direction 46 between the body 49 of the plug 40 and the finger flap 44 instead of a leg spring, the spring 50 used in the embodiment shown, however, acts on the finger flap via a long lever arm, which provides for a force amplification.

The finger flap 44 has at its opposite to its pivot point on the bolt 48 a stop surface 47 which is when inserted into the housing 30 plug 40 on a stop surface 67 in a formed in a broad side of the housing 30 finger opening 60 in Appendix, as in Fig. 1 is shown. Finger opening 60 is best in FIG Fig. 6 can be seen in which the housing 30 is shown in perspective as a detail. The two stop surfaces 47, 67 are inclined towards the insertion direction 46 of the plug 40 obliquely forward. In the embodiment shown, the two abutment surfaces 47, 67 with a lying in the longitudinal cross-sectional plane of the shutter 20 Lot 70 on an in-plane arrow line, which symbolizes the insertion 46 of the plug 40, each having an inclination angle α of about 20 °. The inclination angle α is selected depending on the desired force. The larger the angle α is made, the smaller the tensile force that must be exerted on the strap ends 10a, 10b to automatically open the buckle 20.

If the buckle 20, however, to be opened by hand, is pressed by the finger opening 60 through with a finger on the finger flap 44 and thereby against the force of the spring 50 from the in Fig. 1 shown pressed position in the connector 40 and the buckle opened manually.

The housing 30 and the plug 40 have in cross-section, that is substantially transversely to the insertion direction 46 substantially the shape of a flat rectangle and are formed so that the finger opening 60 and the finger flap 44 each extend in one of the longitudinal sides of its associated rectangle , In other words, the finger flap 44 is arranged in one of the broad sides of the body 49 of the plug 40, and the finger opening 60 is disposed in one of the broad sides of the housing 30, as is the case Fig. 5 and 6 can be easily recognized.

The two stop surfaces 47, 67 are in plan view, ie when viewed in Fig. 1 convexly or concavely curved from above. The stop surface 47 of the finger flap 44 has a radius of curvature R ₂ , which is smaller than a radius of curvature R _{1 of} the stop surface 67 of the finger opening 60. The radii of curvature R ₁ and R ₂ are in the Fig. 6 or 7 indicated. Because of this construction, the stop surfaces 47, 67, if they are as in the Fig. 1 and 2 to 4 abut each other, not one another in surface contact, but mutually in line contact. When the tensile stress on the strap ends 10a, 10b is applied in line with the insertion direction 46, the line of contact lies in a transverse median plane of the finger flap 44 and the housing 30, in which the insertion direction 46 and the solder 70 are located. When the tensile stress is applied obliquely to the insertion direction 46, the contact line moves in Fig. 4 up or down, depending on the direction of pull.

Since the buckle 20 is designed as a safety closure, which is to open at a certain tensile stress of the belt 10, the angle α is chosen so that at a certain tensile stress, which is exerted on the belt 10, the force with which the stop surfaces 47 , 67 are pressed against each other due to the force component, which then acts on the finger flap 44 perpendicular to the insertion direction 46 and strives to move the finger flap 44 into the plug 40, finally becomes so large that the finger flap 44 in the plug 40 moved in. Once the abutment surfaces 47 and 67 have come free from each other, the finger flap 44 so far enough to their in Fig. 1 right end in the finger opening 60 has moved into the plug 40, the plug 40 is pulled out of the housing 30 due to the still exerted on the belt 10 tensile stress. The buckle 20 thus opens automatically at a certain tensile stress of the belt 10, which is why he is referred to here as a safety closure.

The chinstrap of a protective helmet (not shown in each case) can be provided with the belt buckle according to the invention. In this case, the attachment points of the chin strap on the inside of the helmet or on the carrying device of the same need not be modified so that at a certain tensile stress of the chin strap of the helmet is released, because this release is carried out solely by opening the belt buckle of the invention.

LIST OF REFERENCE NUMBERS

10

belt

10a

strap

10b

strap

20

buckle

30

casing

40

plug

41a

head Start

41b

head Start

42

front end

43

head Start

44

finger flap

45

head Start

46

insertion

47

stop surface

48

bolt

49

body

50

feather

51

leg

52

leg

60

finger opening

67

stop surface

70

solder

α

tilt angle

Claims (7)

1. A belt fastener (20) for detachably connecting two belt ends (10a, 10b) comprising a housing (30) connectable or connected to one of the belt ends (10a, 10b) and a plug (40) which can be clipped into the housing (30) and is connectable or connected to the other belt end (10b),
   wherein the fastener (20) is formed as a safety fastener which opens if a specific tensile load acts on the belt ends (10a, 10b) or on the housing (30) and the plug (40),
   characterised in that
   the plug (40) to be clipped into the housing (30) comprises a separate finger flap (44) which is located on one of its wide sides and hinged to a bolt (48) at a front end (42) of the plug (40), in that
   the finger flap (44) comprises a stop surface (47) at its end opposing its attachment point, which stop surface (47) abuts to a stop surface (67) in a finger opening (60) formed in of a wide side of the housing (30) when the plug (40) is inserted in the housing (30), and in that
   the two stop surfaces (47, 67) are tilted forward by an angle (α) not equal to 0° with respect to a perpendicular (70) to the insertion direction (46) of the plug (40).
2. The belt fastener according to claim 1, characterised in that the finger flap (44) is preloaded by a separate spring (50) in the direction of a position in which the finger flap (44) protrudes towards the outside beyond the plug (40).
3. The belt fastener according to claim 2, characterised in that the position in which the finger flap (44) protrudes towards the outside beyond the plug (40) is defined by a protrusion (45) on the finger flap (44) which can be brought in abutment on a protrusion (43) on a body (49) of the plug (40).
4. The belt fastener according to one of the claims 1 to 3, characterised in that the spring (50) is a one or more leg spring surrounding the bolt (48) and supported on the finger flap (44) and on the body (49) of the plug (40) at its free ends.
5. The belt fastener according to claim 1, characterised in that the angle of inclination (α) is 20°.
6. The belt fastener according to one of the claims 1 to 5, characterised in that the two stop surfaces (47, 67) are convex or concave and curved so that the stop surface (47) of the finger flap (44) has a radius of curvature (R₂) which is smaller than the radius of curvature (R₁) of the stop surface (67) of the finger opening (60).
7. A chin strap of a protective helmet, characterised by a belt fastener (20) according to one of the preceding claims.

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