AU697769B2

AU697769B2 - A static line shock absorber

Info

Publication number: AU697769B2
Application number: AU28328/95A
Authority: AU
Inventors: Ian David Travers
Original assignee: Moxham Industrial Pty Ltd
Current assignee: Moxham Industrial Pty Ltd
Priority date: 1994-08-02
Filing date: 1995-08-02
Publication date: 1998-10-15
Anticipated expiration: 2015-08-02
Also published as: AU2832895A

Description

II ;I -1- P/00/0011 Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

C C C I C r VE t C C I 0 tc

CO

CC

Name of Applicant: Actual Inventor: Address for service MOXHAM INDUSTRIAL PTY LTD IAN DAVID TRAVERS

-:I

i 8:I r: i I i j ~9 i~.i in Australia: CARTER SMITH BEADLE 2 Railway Parade Camberwell Victoria 3124 Australia Invention Title: A STATIC LINE SHOCK ABSORBER Details of Associated Provisional Application: PM 7231 filed 2 August 1994 The following statement is a full description of this invention, including the best method of performing it known to us 2- TITLE A STATIC LINE SHOCK ABSORBER This invention relates to a shock absorber and in particular to an energy absorbing device for absorbing the shock load acting on the anchorage of a static line when the free fall of a person is arrested.

In a fall protection system for persons working at great heights, a static line is usually stretched between two anchorage points above the level of the person and a lanyard extends fiom the static line to a harness worn by the person. The lanyard is able to slide along the static line in response to horizontal movement of the person, but is designed to provide a connection between the worker and the static line. Thus if the worker looses his footing and falls the lanyard becomes the sole means of support for the worker. In these situations, the falling body builds up kinetic energy until any slack in the lanyard is taken up. The arresting force on the body then becomes a shock load acting on the anchorage points of the static line.

To reduce this shock load, a shock or energy absorbing device is installed into V 15 either the static line or the lanyard. The purpose of the shock or energy absorbing device is to absorb the shock load and minimise any reaction forces acting on the static line supports through the static line. Such devices are generally'designed for single use and are replaced after having arrested a single fall.

Prior art devices such as that disclosed in US Patent No. 5,143,187 include chains formed into a U-shape in which the adjacent ends of the U are joined together by strength components which absorb kinetic energy as each strength component IC II is sequentially tensioned and broken. To prevent the mechanism from fouling during operation, the U-shaped chain is enclosed within a cylindrical housing and each end of the U-shaped chain is connected to attachment pins which protrude through the top and bottom of the housing. Since the shock absorbing element is a series of discrete strength components, as each strength component is broken there is a corresponding increase and decrease in the tension in the static line. Thus the energy of the shock load on the static line is not absorbed smoothly creating an undesirable pulsating effect on the static line supports. i Another form of energy absorbing device used in the prior art is a tear BC:JHr15704 2 August, 1995 t ss -3webbing as disclosed in U.S. Patent No. 5,202,177 in the name of Spanset Inter AG.

In such a device, two fabric straps are superimposed face to face along a partial length and woven or sewn together by a binding yarn. Pulling forces applied to the unbound strap ends are transferred to the contact region resulting in a progressive tearing (separation) of the strap connection while an irreversible energy absorption takes place. While such devices are adequate for a full protection system, over a period of time, exposure to the weather leads to deterioration in the strap fabric and regular periodic inspections of the device are required. Furthermore, each stitch of the binding yarn joining the two fabric straps is basically a separate strength component, the breaking of each stitch causing a pulse in the tension of the static line.

It is an object of the invention to provide a shock or energy absorbing device for a static line which is inexpensive to produce, is reasonably durable when exposed :o 15 to the elements of the weather and provides continuous absorption of energy.

o 15 Accordingly, the invention provides a shock or energy absorber for a static line including an energy absorbing rigid elongate member having a line of weakness o:o. formed therein extending partially along the length of the member,;' aid member having attachment means on either side of the line of weakness for securing the static line such that a shock load transmitted through the static line will be absorbed by the plate tearing along the line of weakness.

When a shock load due to a person falling is applied to the static line, the Skinetic energy of the falling person is absorbed by the rigid elongate member tearing along the line of weakness. This results in a lengthening of the static line and a decrease in the tension in the static line at the supports. Thus the physical material properties of the line of weakness determines the length of the tear in the elongate member and the amount of energy absorbed. This affects the length of the static line which in turn affects the tension in the static line and the force applied at the supports.

The line of weakness in the elongate member is preferably a small groove formed in at least one side of the elongate member and possibly both sides so that SBCJH:#5704 2 August, 1995

U[

i il -4the elongate member is thinner along the line of weakness. Due to this thinner line of metal, the elongate member is more likely to tear along this line.

To reduce the incidence of the elongate member continuing to tear beyond the line of weakness due to a continuing load in the static line, a tear arrest hole may be provided in the plate at the end of the line of weakness. This provides a mechanical stop to the propagation of the tear.

Clearly, if a sufficiently large load is applied to the static line, the elongate member will tear beyond the tear arrest hole and may even tear the shock or energy absorber into two pieces. As the static line is joined together via the shock absorber, separation of the shock absorber into two pieces has disastrous consequences.

As an additional safety feature, a preferred form of the invention includes a elongate restraint, the ends of which are secured to the elongate member on either side of the line of weakness. The restraint may be positioned and is of such a length that it only becomes tensioned once the elongate member is torn down to the tear arrest hole and prevents the shock absorber being torn into two pieces. The restraint may be designed to have sufficient strength to accommodate a safety factor S.over and above the anticipated design load of the elongate member.

The applicants have also found that due to the rigidity of the elongate member, as the tear resulting from the applied shock load progresses down the elongate member, the moment forces caused by tension in the line as applied at the attachment points about the point of tearing, increases resulting in the plate being easier to tear.

To alleviate the additional problem caused by the moment forces generated, the invention further provides for the line of weakness to be designed so that the resistance to tearing or the force required to tear the elongate member along the line of weakness increases the greater the distance from the attachment points of the static line. One means of increasing the tearing resistance or force required is to i decrease the depth of the groove or grooves of the line of weakness the greater the distance along the elongate member from the attachment points of the static line.

BC:JH:#15704.RSI 11 Augus, 1998 L.J ,7ffO 4a This decrease in the depth of the groove or grooves can be accomplished in discrete steps or continuously over the entire or partial length of the groove or grooves.

Alternatively, the line of weakness can be a series of holes in the elongate member with the distance between the holes progressively increasing the greater the distance from the attachment points to provide the increasing tearing resistance to the elongate member.

The features, objects and advantages of the present invention will become *r r *r t Sm C S C C C C C C CC oC C

*CS

*r S V(AL4..

1 CJH:#15704YSI II August, 1998 r t'!T I 1 1 N -A

I

~ii.~im~ii~ ln ~iiiiiln ii_ Mr~i*i1 11iiniiinii_____ more apparent from the following description of the preferred embodiment and accompanying drawings, in which:- Fig. 1 is a perspective view of an embodiment of the invention; Fig. 2 is a side view of the embodiment of Fig. 1; Fig. 3 is a front view of the embodiment of Fig. 2; Fig. 4 is a perspective view of the rigid member shown in Fig. 1; Fig. 5 is a sectional view at section A-A of Fig. 4; Fig. 6 is a sectional view at section B-B of Fig. 4; Fig. 7 is a sectional view at section C-C of Fig. 4; Fig. 8 is a sectional view at section D-D of Fig. 4; Fig. 9 is a perspective view of a second embodiment of the rigid member shown in Fig. 1; Fig. 10 is a sectional view at section B-B of Fig. 9; Fig. 11 is a sectional view at section C-C of Fig. 9; and Fig. 12 is a sectional view at section D-D of Fig. 9.

Referring to the drawings, an embodiment of the shock or energy absorbing device 1 in accordance with the invention is shown.

The device includes a rigid elongate member 2 formed from a rigid material Ctr I such as a steel plate and having a line of weakness 4 formed therein. The line of weakness 4 is preferably a groove formed by etching or another method of (C decreasing the thickness of the elongate member known to persons skilled in the art such as by machining the groove into the member. While the line of weakness may also be a line of holes or indentations in the elongate member, such a line of SWeakness would be likely to cause a pulsating application of force at the static line supports.

The line of weakness 4 extends partially along the length of at least one side of the elongate member 2 and may end in a tear arresting hole 3. In a preferred form of the invention the line of weakness may take the form of grooves 12,13 formed in corresponding positions on both sides of the elongate member as shown in Fig. 10, 11, 12. The tear arresting hole 3 is a mechanical discontinuity in the i BC!JH:#I5704 2August, 1995

L

aj IL AMEN ~I Li~n -6line of weakness which prevents the propagation of a tear.

The elongate member 2 has attachment means shown as apertured flanges 5,6 formed on either side of the line of weakness. These attachment means are each attached to a static line, so that the shock or energy absorbing device 1 forms part of the static line and tension in the static line is transmitted through the shock or energy absorbing device. The ends of the static line are fixed to secure mountings, for example, on a building and a lanyard is attached to the static line and secured to the harness of the person relying on the static line as a fall protection system.

The lanyard is able to move relatively along the static line and defines the maximum distance which the person can move from the static line. When a shock load due to the person falling is applied to the static line, the kinetic energy of the falling person is absorbed by the rigid elongate member 2 tearing along the line weakness 4. The tearing of the elongate member increases the effective length of the static line and the angle of the static line from the horizontal is increased. Since the tension in the static line as it is applied to the static line supports is inversely proportional to sin E where e is the angle of the static line from the horizontal, C CC Oo the tension in the line decreases as the effective length of the static line increases.

9 CC Clearly the amount of energy which is absorbed by the shock or energy v C absorbing device 1 and the length of the tear in the elongate member is determined by the material properties of the elongate member along the line of weakness and permissible length along which the shock absorbing device is designed to tear.

As discussed above, the permissible length which the shock absorbing device is designed to tear is determined by the position of the tear arresting hole. The S diameter of the tear ariesting hole 3 is preferably greater than the width of the line of weakness 4 but not wide enough to significantly weaken the structure of elongate member. In this way, a tear in the elongate member 2 will generally stop at the tear arresting hole 3 unless the load on the static line is large enough to, recommence Sthe tear on the opposite side of the arresting hole. In this situation there is a .significant risk of the elongate member tearing into two pieces with potentially BCH:#1704 2 August, 1995

C

i -7o cc *r C toi

C

C. c .e As an additional safety feature to safeguard against situations where a shock load greater than the design load of the shock absorbing device is applied to the static line, an elongate restraint 10 may be provided. The ends of the elongate restraint 10 which is typically a metal chain, are secured to the elongate member at fixing points on either side of the line of weakness. The fixing points are shown as U-shaped bolts 7,8 inserted into holes on the elongate member on respective sides of the line of weakness and fastened in position with nuts 9.

The restraint 10 may be positioned so that it is tensioned when the elongate member is torn down to the arrest hole. The breaking strength of the chain thus provides a safety factor over the design load of the shock absorbing device.

However, for design purposes, it is preferable that the depth of the groove 14 or grooves 12, 13, material properties of the elongate member and the thickness of the elongate member determine the design strength of the shock absorbing device and that the restraint only act as a safeguard in cases where the elongate member is 15 torn into two pieces. Therefore, for simplicity of design, the restraint will preferably only become tensioned after the elongate member has been completely torn into two pieces.

As the elongate member 2 tears under the application of the shock load, the inventors have also found that as a consequence of the rigidity of the elongate member, tensile forces in the static line are not transferred in a straight line to the point of tearing of the elongate member. Consequently, moment forces caused by the tension in the line as applied at the attachment flanges 5,6 about the point of the elongate member which is tearing, are created. As the tear progresses further down the elongate member, these moment forces increase resulting in the elongate member being easier to tear.

To overcome this problem, the inventors have found it advantageous to decrease the depth of the groove 14 or grooves 12, 13, as the distance from the attachment points increases. This decrease in the depth of the groove 14 or grooves 12, 13 can be accomplished in discrete steps as shown in Figures 4-8 or continuously over the entire or partial length of the groove. As the depth of the groove 14 or

I

BC-JH:#I5704 2 August 1995

I

-8grooves 12, 13 is decreased, the thickness of the metal in the line of weakness 4 increases thereby increasing the force required to tear the elongate member. This results in the groove 14 or grooves 12, 13 being shallowest near the tear arresting hole.

As movement between the elongate restraint relative to the elongate member will result in unnecessary and irritating metal on metal noises, especially as the static line with the shock absorbing device in position moves, a sheath 11 is provided on the shock absorbing device to immobilise the restraint on the elongate member.

The sheath 11 preferably has an aperture in its lower end so that water will not build up around the metal components and result in corrosion to the elongate member, thus shortening the life of the shock absorbing device.

In alternative forms of the invention (not shown), the line of weakness may be formed by a line of holes in the elongate member 2. The spacing between the C% "holes can vary depending on the required energy absorbing capacity of the shock S 15 absorbing device. Thus to account for moment factors, the spacing between the S: holes increases the greater the distance from that attachment points of the static line.

In such an embodiment, the size of the tear arresting hole will be much greater than the last hole in the line of weakness, thereby requiring a much larger force to recommence the tear beyond tear arresting hole. While such an arrangement of holes for the line of weakness will cause a pulsing effect on the static line supports *as the elongate member tears, the feature of a decreasing depth groove or grooves %foc is incorporated in this embodiment to overcome the potential problem of moment forces increasing as the tear length increases. To provide further strength to the line o *of weakness as the distance from the attachment points increases, the spacing between the holes may be increased.

BCIH:1S704 2 Augus, 9 i ~i i i fi r

Claims

4- C Ce C c 4- 4-4 C 4- THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. An energy absorber for a static support line including an energy absorbing rigid elongate member having a line of weakness formed therein extending partially along the length of the member, said elongate member having attachment means on 5 either side of the line of weakness for securing the static line such that a shock load transmitted through the static line to the attachment means will be absorbed by said elongatea member tearing along the line of weakness, said line of weakness being formed such that the force required to tear said member along said line of weakness increases the greater the distance along said elongate member from the attachment 10 means. 2. The energy absorber in accordance with claim 1 wherein said line of weakness is a small groove formed in at least one side of the elongate member. 3. The energy absorber in accordance with claim 2 wherein the line of weakness is a small groove formed in both sides of the elongate member. 15 4. The energy absorber in accordance with either claim 1 or claim 2 wherein a tear arrest hole is provided at the end of the line of weakness.
5. The energy absorber in accordance with claim 3 wherein the diameter of the tear arrest hole is wider than the line of weakness.
6. The energy absorber in accordance with claim 1 or claim 2 wherein the depth of the groove in at least one side of the elongate member decreases the greater the distance from the attachment means of the static line.
7. The energy absorber in accordance with claim 6 wherein the decrease in the depth of the groove in the at least one side of the elongate member occurs in discrete steps. c 2 August, 1995 BCJH:#15704MRSI 11 August, 1998
8. The energy absorber in accordance with claim 6 wherein the decrease in the depth of the groove in the at least one side of the elongate member occurs continuously over at least part of the entire length of the groove.
9. The energy absorber in accordance with any one of the preceding claims further including an elongate restraint having ends secured to the elongate member on respective sides of the line of weakness. The energy absorber of claim 9 wherein the elongate restraint is a chain having ends secured to said elongate member on respective sides of the line of weakness.
11. The energy absorber in accordance with claim 1 wherein the line of weakness is a line of holes in the elongate member.
12. The energy absorber in accordance with claim 11 wherein the spaci~c between the holes increases the greater the distance from the attachment points of C r, 1 the static line. CC C 0 C 15 13. An energy absorber substantially as hereinbefore described with reference to one or more of the accompany drawings. DATED: 26 August, 1998 CARTER SMITH BEADLE Patent Attorneys for the Applicant: MOXARAM INDUSTRIAL PTY LTD 26 August, 1998 Patent Attorneys for the Applicant STO: The Commissioner of Patents Fee: $280.00 Our Ref #15704 ABSTRACT A shock energy absorber for a static line is disclosed including an energy absorbing rigid elongate member having a line of weakness extending partially along the length of the member. The elongate member has attachment means on either side of the line of weakness for securing the static line such that a shock load transmitted through the static line is absorbed by the plate tearing along the line of weakness The line of weakness is preferably in the form of a small groove formed in at least one side of the elongqte member The depth of the groove preferably discloses the greater the distance from the attachment points. A tear arresting hole is provided to interrupt propagation of a tear along the line of weakness. CC Sccc ce r r C, f' t* t f c CS ,C