Speed responsive coupling device especially for fall arrest...

Fire escape – ladder – or scaffold – With storage device for supple escape – Reel

Reexamination Certificate

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Reexamination Certificate

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06279682

ABSTRACT:

This invention relates to a speed responsive coupling device for locking a first member with respect to a second against relative rotation in at least one direction.
This invention was made with particular reference to the use of such devices in fall arrest apparatus comprising an anchor member and a rotatable drum on which is wound a safety line, and although such devices may be used in various other kinds of apparatus, the invention will be described with special reference to its use in connection with fall arrest apparatus.
Such fall arrest apparatus is required for protecting a worker who is exposed to risk of injury due to a fall from an elevated position at a work place which may typically be a construction site for example of a high-rise building. The anchor member is made fast to a high point on a fixed structure and the safety line wound onto the drum is attached to a body harness with which the worker is equipped. The safety line may be paid out by unwinding from the drum in order to enable the worker to carry out his designated tasks without undue restriction. However, the length of maximum pay-out is often limited so as to limit risk in the event of a fall; having regard to such considerations a maximum pay-out length of say 6 meters can be regarded as typical. It is usual that line payout of this order be controlled by a spring which urges rotation of the drum relative to the anchor member in order to re-wind the safety line.
There is clearly a requirement to brake or prevent any sudden and rapid payout of the safety line in the event of a fall by the worker. This is accomplished by the use of a speed responsive coupling device which in effect locks the drum to the anchor member when the speed of rotation of the drum reaches a threshold value. In order that the arrest of the worker's fall be not too abrupt—which might in itself be the cause of injury—it is usual to associate the coupling device with a brake which acts to slow rotation of the drum in relation to the anchor member, though some other form of shock absorber may be used, or reliance may be had on any inherent elasticity of the safety line.
Speed responsive coupling devices for this and other purposes have been known for many years and have hitherto often comprised pawl and ratchet mechanisms which are brought into locking engagement by means of the centrifugal effect. Such known centrifugal clutch mechanisms commonly comprise at least one spring loaded pivotal pawl carried to rotate with the drum, and when the speed of rotation of the drum reaches a rate corresponding to that occasioned by the fall of a worker attached to a safety line wound onto the drum, a part of the pawl flies out under centrifugal force and this is effective to bring an operative nose of the pawl into engagement with a fixed stop which is usually in the form of a ratchet tooth.
We have found that there is a problem in regard to the reliability of the response of known speed responsive centrifugal clutches to the onset of sudden and rapid payout of safety line as is occasioned by a fall. Such response may be affected by the orientation or attitude of the clutch and drum assembly, and it may also be affected by temperature or by the effects of dirt or corrosion: it will be appreciated that such apparatus is commonly used out of doors. The pawls of centrifugal clutch mechanisms are often restrained by leaf springs and these may become stiffer in very cold weather and thus less reliable just when the risk of a worker falling may be greatest because of possible ice formation at the work place and because of reduced dexterity normally associated with cold weather. There is also a risk that ice may form within the clutch mechanism to prevent its operation.
It is an object of this invention to provide a speed responsive coupling device for locking a first member with respect to a second against relative rotation in at least one direction in which at least some of these disadvantages are alleviated.
In particular it is desired to avoid making use of the centrifugal effect in the operation of the speed responsive coupling device provided according to this invention.
According to this invention, there is provided a speed responsive coupling device for locking a first member with respect to a second against relative rotation in at least one direction, which device comprises a ratchet wheel carried by said first member which is engageable by a pawl carried by the second member, characterised in that said pawl is arranged as a mechanical toggle switch which is biased to flip between a “free” condition in which such relative rotation is permitted and a “lock” condition in which the pawl engages the ratchet wheel to disallow such relative rotation, said pawl being linked to a rocker which is so shaped and disposed that when the pawl is in the free condition, such relative rotation causes the rocker to rock and that when the speed of such relative rotation exceeds a threshold value, the strength of the rocking motion imparted causes the rocker to flip the pawl to the lock condition.
Such a device can be of very simple construction, and we have found that devices according to the invention also tend to be more reliable in operation than coupling devices of comparable cost based on the use of a centrifugal clutch.
In particular, having especial regard to the use of such a device in fall arrest apparatus, the reliability of operation of such a device is substantially independent of its orientation or attitude and it is not significantly affected by temperature variations over the range to be expected in acceptable human working environments. In particular, it may be noted that the pawl members of a centrifugal clutch will, except on the occasion of a fall which one would hope to be a rare event, remain stationary with respect to neighbouring parts of the clutch and are thus susceptible to partial seizing for a variety of reasons or to icing up in cold weather: in a device according to the invention, however, the rocker will be continuously moved by rotation of the drum as safety line is paid out or recovered during normal working, and this movement of the rocker and the linked movement of the pawl will reduce the possibility of partial siezure.
It will be noted that in the speed responsive coupling device according to this invention, no use whatso ever is made of the centrifugal effect.
Instead, at the heart of this invention, lies the concept of using a ratchet engaging pawl with the pawl being arranged as a mechanical toggle switch. Such a switch is one in which the switching member, here the pawl, is spring biased to remain in one of two stable or quasi-stable conditions. Switching is effected by causing the switching member, the pawl, to pass through a metastable position between those two conditions, whereupon it will flip over to the other condition. In the instant case, the rocker which is linked to the pawl, and is preferably an integral part of the pawl, rocks as the first member, e.g. the drum on which safety line is wound, rotates. The arrangement is such that during such rotation at speeds below a threshold value, for example corresponding to line withdrawal at rates appropriate to accommodate normal movements of a worker, this rocking is insufficient to push the pawl into or past its metastable state, and thus the pawl remains in the free condition and rotation of the drum is permitted: in case of rotation at speeds above the threshold value, however, the rapidity of the rocking motion imparted gives such momentum and energy to the pawl and rocker that the pawl is carried into its metastable position and then flips over into the lock condition where it engages the ratchet to prevent further rotation.
It will be appreciated that the required rocking motion could be imparted to the rocker in a variety of ways. For example the rocker could be arranged to bear on a cam surface associated with the first member such as a drum. Preferably, the arrangement is such that during such relative rotation, the ratchet wheel bears directly against

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