GB2451799A

GB2451799A - A fall arrest system

Info

Publication number: GB2451799A
Application number: GB0709435A
Authority: GB
Inventors: Karl Jones
Original assignee: Latchways PLC
Current assignee: Latchways PLC
Priority date: 2007-05-16
Filing date: 2007-05-16
Publication date: 2009-02-18
Anticipated expiration: 2027-05-16
Also published as: WO2008139127A1; GB2451799B; GB0709435D0

Abstract

A safely device in a fall arrest system comprises: a body 13, an attachment means 14 for attaching the safety device to a support structure, a drum 12 mounted for rotation in the body, a safety line 11 wound on the drum and an attachment point 17 for attaching a user personal safety equipment to the free end of the safely line. A speed-sensitive clutch 15 is connected to the drum to halt the rotation of the drum in response to unwinding of the line 11 above a threshold speed. The safety device also includes a linear energy absorber 18, connecting the free end of the safety line to the user personal safety equipment. The linear energy absorber 18 responds, when the speed-sensitive clutch is engaged, to an applied load along the safety line greater than a threshold value by deploying and absorbing energy so that the second attachment means moves away from the free end of the safety line. The linear energy absorber 18 may be of the type which absorbs energy by plastic deformation.

Description

Safety Device This invention relates to an improved safety device and particularly to an improved safety device for use in a fall arrest system.

Fall arrest systems are used to prevent personnel working at height from suffering injury or death due to falls. Fall arrest systems are also commonly referred to as height safety systems or fall prevention systems.

One common form of fall arrest system employs a safety block 1, as shown in Figure 1. The safety block 1 comprises a safety line or cable 2 wound around a drum 3 mounted for rotation within a casing 4. The casing 4 includes attachment means 5 for attaching the safety block to a fixed support structure (not shown). The drum 3 is biassed by a tensioning and re-spooling device 6 in a direction of rotation acting to tension the safety line 2 and wind it onto the drum 3. The drum 3 is selectively connected to a brake 8 through a speed sensitive clutch 9, the speed sensitive clutch 9 being arranged to allow free rotation of the drum 3 at low speeds of rotation and to engage the drum 3 to the brake 8 at high speeds of rotation above an activation speed. The brake 8 comprises a pair of opposed friction discs 8a and 8b loaded into contact with one another, one disc 8a being fixed to the casing 4 and the other disc 8b being arranged to rotate together with the drum 3 when the clutch 9 is engaged.

As shown in Figure 2, in use the safety block 1 is attached to a fixed support structure above a region in which a user to be protected is working. The user wears a personal safety harness and attaches the end of the safety line 2 to the harness. The user can then move around the region below the safety block, including ascending and descending any structures within the region, as necessary. As the user moves, the tensioning and spooling mechanism 6 allows the drum 3 to rotate to pay out the safety line 2 as required to allow the movement and also causes the drum 3 to rotate to reel in the safety line 2 as required so that there is no slack in the safety line 2.

Normal movement of the user will result only in slow rotation of the drum 3 at speeds below the activation speed of the clutch 9. If the user falls, the safety line 2 will be pulled out and the drum 3 rotated at a rapidly accelerating speed until the speed of the drum 3 reaches the activation speed of the speed sensitive clutch 9. The speed sensitive clutch 8 will then engage the drum 3 with the brake 8. The energy of the user's fall is then absorbed by friction in the brake 8 until the fall is arrested, and rotation of the drum 3 is stopped.

However, there are a number of problems with systems of this type. It is generally desirable to apply a high braking force to the safety line to bring the users fall to a stop as quickly as possible, in order to minimise the distance fallen by the user. It will be understood that the further the user falls the greater the risk that the user will strike the ground or some other obstacle before the fall is stopped, increasing the risk or injury or death. Further, as the distance fallen by the user gets larger, the total amount of energy which must be absorbed by the brake is increased, requiring a larger and more robust brake. However, if the brake force is too high the loads experienced by the user as their fall is arrested can become high enough to injure the user or cause damage or failure of the users safety harness. Further, fall accidents are relatively infrequent so the safety blocks are usually exposed to the elements for long periods between being operated to arrest a fall. As a result, due to weathering of the brake components, fully reliable operation of the safety block cannot be guaranteed.

In an attempt to reduce the risk of user injury by applied loads when a fall arrest occurs some personal harnesses include an energy absorber of the rip-out type as part of the harness.

Energy absorbers of this type comprise layers of fabric stitched together and are arranged so that when a tension load above a threshold is applied to the lanyard the stitches tear, allowing the fabric to deploy, so absorbing energy.

Devices of this type can provide some reduction in loads experienced by a user in a fall arrest situation. However, in practice an energy absorber of this type able to absorb sufficient energy to be effective in limiting the load experienced by a user in a fall arrest situation is relatively heavy and bulky. This weight and bulk of the energy absorber is generally undesirable because it has to be carried around as part of the user equipment, but is further particularly a problem for height safety equipment because increases in the weight of the user carried height safety equipment such as the harness and lanyard and increases in their bulk, which make them more awkward to work with, tends to results in an increase in personal failing to use height safety equipment and risking death and injury.

In practice it has been found that regardless of attempts to educate personal in the use of height safety equipment and to police and enforce policies of compulsory use of height safety equipment it is very difficult to ensure that height safety equipment is used if personnel find the equipment heavy or difficult to work with.

The present invention was made in an attempt to overcome these problems, at least in part.

The invention provides a safety device suitable for use in a fall arrest system, and comprising: a body, a first attachment means for attaching the safety device to a support structure, a drum mounted for rotation relative to the body, a safety line wound on the drum and having a free end, a second attachment means for attaching a user personal safety equipment to the free end of the safety line, a speed sensitive clutch connected to the drum, and a linear energy absorber connecting the free end of the safety line to the user personal safety equipment, in which the speed sensitive clutch is adapted to respond to rotation of the drum relative to the body in a direction tending to unwind the safety line from the drum and above a predetermined speed by engaging to resist further rotation of the drum in said direction relative to the body, and the linear energy absorber is adapted to respond, when the speed sensitive clutch is engaged, to an applied load along the safety line greater than a threshold value by deploying and absorbing energy so that the second attachment means moves away from the free end of the safety line.

As explained above, fall safety devices of the overhead safety block type must remain in place, often outdoors and exposed to the elements, for long periods of time before use because in practice user falls requiring arrest are relatively rare. The use of linear energy absorbers which absorb energy by plastic deformation according to the present invention provides the advantage that the operation and properties of the safety device, and in particular the tensile load at which the energy absorbers begin absorbing energy, and the amount of energy absorbed, are very stable even when they have been subjected to weathering over long periods. The load required to plastically deform materials is based on the bulk properties of the material so that linear energy absorbers of the plastic deformation type are inherently less prone to changes in the properties due to contamination and weathering than other types of energy absorbers which are generally dependent upon surface properties of the materials.

Accordingly, the present invention allows more reliably safe and repeatable arresting of a user's fall, so improving safety.

In the present invention the linear energy absorbers are all supported by the safety block and do not need to be carried by the user. Accordingly the sytsem is more convenient for the user, decreasing the probability of users avoiding use of the system.

Further, linear energy absorbers which absorb energy by plastic deformation are very efficient, absorbing very large amounts of energy relative to their size and weight. Accordingly, the invention provides the advantage that the weight and bulk of the safety device can be minimised.

Preferred embodiments of the invention will now be described by way of example only with reference to the accompanying diagrammatic figures, in which: Figure 1 shows a known safety block; Figure 2 shows a height safety system including the safety block of figure 1; Figure 3 shows a safety device according to a first embodiment of the invention; Figure 4a shows a cross sectional view from a first direction of a linear energy absorber of figure 3; Figure 4b shows a view of the linear energy absorber of figure 4a from a direction perpendicular to the first; Figure 5 shows a safety device according to a second embodiment of the invention; and Figure 6 shows a safety device according to a third embodiment of the invention.

A first embodiment of a safety device according to the invention is shown in figures 3 and 4.

The safety device comprises a safety block 10 having a safety line or cable 11 wound around a drum 12 which is mounted for rotation within a body 13. Similarly to a known conventional safety block, the safety block 10 includes attachment means 14 for attaching the safety block to a fixed support structure (not shown).

The drum 12 is selectively connected by a speed sensitive clutch mechanism 15 to a brake 19.

The speed sensitive clutch 15 is arranged to allow free rotation of the drum 12 in a direction unwinding the safety line 11 from the drum 12 a low speeds of rotation below a threshold speed and to engage the brake 19 to the drum 12 in response to rotation of the drum 12 in a direction unwinding the safety line 11 at or above the threshold speed.

It is only necessary for the speed selective clutch 15 to respond to rotation of the drum 12 at or above the threshold speed in a direction tending to unwind the safety line 11 from the drum. In practice, some known designs of speed sensitive clutch will respond to rotation of the drum 12 at or above the threshold speed in either direction, but this is not essential.

The drum 12 is biassed by a tensioning and re-spooling device 16. The tensioning and re-spooling device 16 biasses the drum 12 in a directional of rotation acting to tension the safety line 11 and wind it onto the drum 12. When a length of the safety line 11 is paid out from the drum 12 the re-spooling device 16 applies a small torque to the drum 12 in a direction which tends to rewind the safety line 11 back onto the drum 12. One preferred type of re-spooling device is a coil spring of the clock spring type. However, other suitable re-spooling devices are known, so this will not be described in detail herein.

As a safety precaution, it is preferred that the opposite (top) end of the safety line 11 is secured to the drum 12 so that the safety line 11 cannot be released from the safety block 10, even when the safety line Ii is fully unwound.

The free end of the safety line Ills connected to a personnel safety harness attachment point 17 through an energy absorber 18. The linear energy absorber 18 is a linear energy absorber of the type which absorbs energy by plastic deformation of part of the energy absorber.

The personal safety attachment point 17 is typically an eye or a similar attachment loop allowing a users personal safety equipment to be secured to the safety device. Typically this securing is carried out by a caribineer on the end of a safety lanyard connected to a users personal safety harness being attached to the attachment point 17.

Such an attachment arrangement using an attachment loop and carabineer is convenient and in practice is the most common arrangement used in fall arrest systems. However, other secure attachment arrangements may be used in the present invention.

The linear energy absorber 18 has a predetermined deployment threshold. That is, the linear energy absorber 18 does not respond to applied tensile loads below the deployment threshold, but responds to applied tensile loads above the deployment threshold by deploying and increasing in length while resisting the applied tensile load and so absorbing energy. Thus, the linear energy absorber 18 is arranged to connect the attachment point 17 to the free end of the safety line 11 rigidly with a fixed distance between them while the tensile load between the attachment 17 and the safety line 11 is below the predetermined deployment threshold load of the linear energy absorber 18. If the tensile load between the attachment 17 and the safety line 11 exceeds this tensile load, the energy absorber 18 will respond by deploying, so allowing the attachment point 17 to move away from the free end of the safety line 11 and absorbing energy.

A particularly preferred design of linear absorber 18 is shown in more detail in figures 4A and 4B. This linear energy absorber 18 is of the type which absorbs energy by passing a strip of plastically deformable material from a coil store through deforming means which cause the strip material to be plastically deformed as it moves.

The linear energy absorber 18 comprises a stainless steel strip I 8a connected at a first end 1 8b to the attachment point 17. The other end 18c of the stainless steel strip 18a is formed into a coil store 1 8d and has an end stop 1 8e. Deforming means I 8f is attached to the free end of the safety line 11 with the stainless steel strip 1 8a passing through the deforming means I 8f between the first end I 8b and the coil store 1 8d. The deforming means I 8f preferably comprises a series of curved surfaces I 8g in contact with the stainless steel strip I 8a and arranged so that the stainless steel strip I 8a undergoes plastic deformation as it passes through the deforming means 1Sf. However, alternative arrangements such as using pins or rollers to deform the stainless steep strip could be used.

The end stop 18e is provided as a safety precaution. If all of the stainless steep strip 1 8a is deployed so that the linear energy absorber 18 reaches is the end of its deployment, the end stop 1 Se will stop further deployment and prevent the stainless steel strip ISa from being released from the deforming means 1 8f. As a result, the attachment means 17 cannot become released from the safety line 11.

In a preferred arrangement, the energy absorber 18 includes a "U" shaped frame or yoke having two parallel side plates 18k connected by a base plate 1 8h. The deforrning means I 8f extends between and is supported by the two side plates 18k and the coil store I 8d is retained between the side plates 18k. The safety line 11 is attached to the base plate 18h of the yoke.

In use the safety block 10 is suspended from a fixed supporting structure (not shown) using the attachment point 14 above a region in which a user will be working, a required length of safety line 11 is deployed from the drum 12 and the personal safety line of the user is attached to the attachment point 17 at the free end of the safety line 11. These steps can be carried out in the convenient order, as required to set up the system.

The user can then move around a work area as desired. The safety line 11 will be deployed from the drum 12 as required by the users movements and the re-spooling mechanism 16 will automatically rewind any excess or slack safety line 11 back onto the drum 12. The threshold speed of the speed sensitive clutch 15 is set high enough that the threshold speed will not be reached by the drum 12 during normal movement of the user, so that the drum 12 can rotate freely and movement and work by the user is not interfered with.

If the user falls, the safety line 11 will be pulled out from the drum 12 at increasing speed until the speed of rotation of the drum 12 reaches the threshold speed of the speed sensitive clutch 15. The speed sensitive clutch 15 will then engage the drum 12 to brake 19. The brake 19 will then absorb the energy of the falling user, ending the fall and bringing rotation of the drum 12 to a stop.

When the speed sensitive clutch iS has engaged the drum 12 to the brake 19 the load along the safety line 11, in the event of a fall the load produced the momentum of the falling user and the braking force applied by the brake 19, is applied to the linear energy absorber 18. If this load is above the deployment load of the linear energy absorber 18, the linear energy absorber 18 will begin deployment and the stainless steel strip 18 will be deployed from the coil store 18c through the deployment means 18f. As a result, the attachment point 17 will move downwards away from the end of the safety line 11 as the linear energy absorber 18 deploys and absorbs energy. Accordingly, the linear energy absorber 18 limits the tensile load along the safety line 11 to which the user and the brake 16 and other parts of the safety block are subjected.

When the use?s fall has been arrested, the user remains suspended from the safety block 10 by the safety line 11 until the user is recovered, or is able to recover themselves.

If the load on the safety line 11 is less then the deployment load of the linear energy absorber 18, the linear energy absorber 18 will not deploy and will behave as a rigid body. This could occur, for example, if the user tugs on the safety line 11 to test the speed sensitive clutch 15.

The set value of the deployment load of which the linear energy absorber 18 begins deployment can be selected as required in a particular use. The deployment load should be significantly greater than the anticipated weigh of the user and their carried equipment in order to ensure that the safety device properly arrests the fall of the user.

Preferably, the speed sensitive clutch 15 is arranged so that when the speed sensitive clutch 15 has engaged the drum 12 to the brake 19, it will then remain engaged until the tension on the safety line ills reduced to zero or a very low value. This ensures that after a fall has been arrested the drum 12 remains locked, so preventing further falls or uncontrolled decent. It is particularly preferred that the speed sensitive clutch 15 is arranged so that when it has engaged the drum 12 to the brake 19, it can only be unengaged by movement of the drum 12 in the direction winding the safety line 11 back onto the drum 12. This means that it is necessary to reduce the load on the safety line 11 to a sufficiently low level that the re-spooling device 16 can move the drum 12 back in the rewinding direction in order to disengage the speed sensitive clutch 15 from the brake 19.

A second embodiment of the safety device according to the present invention is shown in figure 5. The safety device according to the second embodiment is similar to the safety device according to the first embodiment.

In the safety device according to the second embodiment the safety block 20 comprises a body 23 supporting a drum 22 around which a safety line 21 is wound. The drum 22 is provided with a re-spooling device 26 and the safety block can be attached to a fixed supporting structure (not shown) by attachment means 24 similarly to the first embodiment.

A speed sensitive clutch 25 is arranged to respond to rotation of the drum 22 in a direction deploying the safety line 21 from the drum 22 above a predetermined speed by locking the drum 22 to the body 23 and so stopping rotation of the drum 22 and further deployment of the safety line. Accordingly, the safety device according to the second embodiment is distinguished from the first embodiment mainly by the lack of a brake.

In use, when the user falls and the speed of rotation of the drum 22 reaches the threshold speed of the speed sensitive clutch 25, the speed sensitive clutch 25 will then lock the drum 22 to the body 23, stopping further rotation of the drum 22 in the paying out direction. A load along the safety line 11, in the event of a fall the load due to the weight of the falling user, is then applied to the linear energy absorber 18.

When the applied load is above the deployment load of the linear energy absorber 18, the linear energy absorber 18 will begin deployment as described above regarding the first embodiment.

As the energy absorber 18 deploys, it absorbs energy and so slows and finally stops the falling user. When the user's fall has been arrested, the user will remain suspended from the safety block 20 by the safety line 21 until the user is recovered, or is able to recover themselves.

In the second embodiment, all of the energy of the user's fall must be absorbed by the linear energy absorber 28. Accordingly, the linear energy absorber 28 in the second embodiment will require a larger energy absorbing capacity than the linear energy absorber 18 in the first embodiment. However, the general operating principles of the linear energy absorber will remain the same.

The third embodiment of the invention is shown in figure 6. The safety device according to the third embodiment is substantially similar to the device according to the second embodiment and the same reference numbers are used for common parts.

In the third embodiment a further second linear energy absorber 30 is arranged between the body 23 of the safety block 20 and the attachment point 24 connecting the safety device to a fixed structure (not shown).

The second linear energy absorber is similar in its general principles of operation to the first linear energy absorber 28. The two linear energy absorbers 28 and 30 may be similar in structure or different, as convenient.

In practice it may be preferred for the second linear energy absorber 30 to be set to have a slightly higher deployment load value than the first linear energy absorber 28 at the free end of the safety line 21, in order to allow for the extra static load applied to the second linear energy absorber 30 by the weight of the rest of the safety device.

In the safety device in the third embodiment, in the event of a fall, when the speed sensitive clutch 25 has locked the drum 22, both of the linear energy absorbers 28 and 30 will begin deploent. Accordingly, the body 23 of the safety block 30 will move downwards away from the attachment means 24 due to deployment of the second linear energy absorber 30 and the attachment point 27 will move downwards away from the end of the safety line 21 due to deployment of the first linear energy absorber 28. Thus, the safety device allows the energy of * the falling user to be absorbed by both of the two linear energy absorbers 28 and 30.

Accordingly, the weight and bulk of the first linear energy absorber 28 at the free end of the safety line 21 can be reduced.

Further, the deployment load of a linear energy absorber of the plastic deformation type is determined by the dimensions and material of it's components and not upon the loads applied to the components, as in a friction type device. As a result, it is expected that the second and third embodiments of the invention will be even more advantageous than the first embodiment.

The deployment of the linear energy absorber at the free end of the safety line in all of the embodiments will result in a permanent linear extension of the absorber which will be easily visible even from a distance, so that it is immediately apparent that a fall arrest event has occurred and that appropriate checking and replacement of parts should be carried out.

Optionally, parts of the or each linear energy absorber exposed during deployment have a colour contrasted to an outer casing of the energy absorber to ensure that even a small amount of deployment is easily visible.

In all of the embodiments of the invention, it usually preferred to use a linear energy absorber of the constant force type which has essentially constant deployment load required to continue deployment of the energy absorber across the fall arrangement of the deployment. That is, in the energy absorber described in the embodiments, the deployment load required to deploy the stainless steel strip from the coil store through the deforming means is constant along the full length of the strip. This arrangement is usually preferred because if the linear energy absorber is arranged so that this constant load is the maximum load which can be safely applied to the user during a fall arrest event, the amount of energy absorbed is maximised and the duration and length of fall of the user is minimised. However, linear energy absorbers having a variable deployment load could be used if preferred in particular applications.

* The speed sensitive clutch is preferably a clutch of the rocking pawl type. However, a centrifugal clutch may also be used.

In the embodiments the use of a safety line or cable wound around the drum is referred to.

This is not essential and other forms of elongate support such as a webbing strap could be used instead.

The above description refers to a height safety device for arresting a fall by a person. This is the most common application of a height safety system. However, the present invention can also be used in a height safety system to arrest falls by objects, for example equipment being used or moved at height, in which case the object should be regarded as the user.

The embodiments described above are examples only and are not exhaustive. The skilled person will be able to provide alternatives within the scope of the present invention as defined by the attached claims.

Claims

* CLAIMS 1. A safety device suitable for use in a fall arrest system, and comprising: a body, a first attachment means for attaching the safety device to a support structure, a drum mounted for rotation relative to the body, a safety line wound on the drum and having a free end, a second attachment means for attaching a user personal safety equipment to the free end of the safety line, a speed sensitive clutch connected to the drum, and a linear energy absorber connecting the free end of the safety line to the user personal safety equipment, in which the speed sensitive clutch is adapted to respond to rotation of the drum relative to the body in a direction tending to unwind the safety line from the drum and above a predetermined speed by engaging to resist further rotation of the drum in said direction relative to the body, and the linear energy absorber is adapted to respond, when the speed sensitive clutch is engaged, to an applied load along the safety line greater than a threshold value by deploying and absorbing energy so that the second attachment means moves away from the free end of the safety line.
2. A safety device according to claim 1, in which the linear energy absorber comprises a plastically deformable element which is plastically deformed to absorb energy when the linear energy absorber deploys.
3. A safety device according to claim 2, in which the plastically deformable element is an elongate member which is plastically deformed by passing through a deforming means when the linear energy absorber deploys.
4. A safety device according to claim 3, in which the elongate member is a strip or a round bar.
5. A safety device according to claim 3 or claim 4, in which the elongate member is stainless steel.
6. A safety device according to any preceding claim, in which the energy absorber includes an end stop preventing release of the second attachment means from the free end of the safety line.
7. A safety device according to any preceding claim, in which the speed sensitive clutch is arranged so that when the speed sensitive clutch has been engaged the load on the safety line must be reduced to zero to disengage the speed sensitive clutch.
8. A safety device according to any preceding claim, and further comprising a rewinding means adapted to bias the drum to rotate relative to the body in a direction tending to wind the safety line onto the drum.
9. A safety device according to claim 7, in which the speed sensitive clutch is arranged so that when the speed sensitive clutch has been engaged the drum must be rotated by the rewinding means in a direction tending to wind the safety line onto the drum in order to disengage the speed sensitive clutch.
10. A safety device according to any preceding claim, in which the speed sensitive clutch engages with a brake to resist rotation of the drum.
11. A safety device according to any one of claims I to 9, in which the speed sensitive clutch locks the drum relative to the body to resist rotation of the drum.
12. A safety device according to claim 11, and further comprising a second linear energy absorber connecting the body to the attachment means and adapted to respond, when the speed sensitive clutch has locked the drum, to an applied load along the safety line greater than a threshold value by deploying and absorbing energy so that the first attachment means moves away from the body.
13. A safety device substantially as shown in or as described with reference to Figures 3 to 6 of the accompanying drawings.