The two parts are engaged in transmitting a hoisting force, and equipped with an indicator, which is permanently deformable by a pressure force caused by the transmission of the hoisting force onto it by the two parts of the hoisting device. The indicator, located between the first and second parts of the hoisting device, is designed to be deformed by a hoisting force value that is greater than the minimum limit force for the hoist. Preferably this force is greater than the working load of at least one part of the hoist. Therefore it then gives a visual indication of the application of a hoisting force above the minimum limit, and thus gives a warning of possible misuse of the equipment.
Fig 1 – Views and part sections of the first embodiment of the invention
Earlier devices
The inventor cites several earlier devices designed to perform a similar function. That in the patent US5452679 uses markings on both sides of a hook so that deformations of the hook can be seen more easily. In this case overload detection only takes place when there is visible, possible irreversible, deformation of the hook.
The device described in US4578941 relies on tensile force. This shows a hoist chain with a detection rod parallel to a chain link. This breaks if a certain tensile strain is exceeded. Berk says that the breaking of the detection rod is relatively uncontrolled, and it is possible for a permanent deformation of the chain to be caused by impact loading.
In GB-1.366.108 an overload is indicated by deformation of a round link, such as provided by a plastics body that breaks when the round link deforms under stress, but by then the link will have already deformed to a considerable extent.
Fig 2 – Side view of an embodiment of the invention combining the features of two other variations (see text)
The device disclosed by US-5.745.042 is based on an electrical circuit using strain gauges or a coil-based device. In order to prevent electrical overloading of the circuit or the strain gauges in the cases of a power-line contact or lightning, an isolation element is provided between two links, which should keep them electrically isolated from each other. There is an additional measuring circuit to check the condition of the insulator, which is not designed to break or deform permanently.
Patent US-4.409.841 shows a fatigue damage indicator including a thin plate between or integral to two members to which the stress is applied in shear. Crack propagation, aided and directed by slots in the plate, is designed to ensure that the plate becomes detached or substantially bent away.
Fig 3 – Side view and section through an overload indicator, such as for a hoisting eye, in a single embodiment
In US-4.578.941 a load-carrying link chain includes a fuse link with link plates connected by a fuse bar of tensile strength less than that of the rest of the load-carrying chain. The fuse bar is designed to break on overload but is positioned and arranged to carry normal tensile loads. A safety link connects the link plates in an offset position but parallel to the line of tension. The tensile strength of this link is greater than that of the fuse bar and at least or equal to that of the rest of the chain. It can therefore hold the load when the fuse bar breaks when an overload occurs.
Other relevant state-of-the-art devices use wear indicators, such as EP1232991, which has exposed wear parts and an indicator to give an idea of the remaining life of the hoisting device.
Patent DE4408562 describes cams in a hoisting device to prevent the load sliding on the hoisting element.
Why?
The invention is aimed at solving some of the problems with existing devices, as above, whilst providing a simple, safe or controlled overload indication for a hoisting device. Another aim is to provide a warning of incorrect usage, useful to a user and to a supplier.
It provides a first hoist part and a second hoist part that engage with each other to transmit a hoisting force, and provided with an indicator part, various embodiments of which are described in the patent. The indicator is permanently deformable by a pressure force applied by the transmitted hoisting force, and so is located between these two hoist parts. The point of deformation and indication is when the transmitted hoisting force is greater than a designed minimum limit force, which should be greater than the hoisting force representing the working load limit. This action also provides indication of possible incorrect use without actual or permanent damage occurring in the hoist part.
The inventor claims that deformation by pressure force proves to be easier to adjust and easier to control. Such an indicator can also be used at places on the hoist part other than the usual ones, making possible a multiplicity of new designs. The device is also claimed to be accurate without disrupting the hoisting process.
The deformation range of the indicator device is designed to be discrete in order to provide clear indication of an overload situation. The overload should be detected independently of the visible deformation of the hoist part, since that is the very occurrence that needs to be prevented. For a controllable detection of overload, use is made of pressure force leading to plastic deformation of the indicator part, instead of fracturing it or using deformation by tensile strain.
Varied operation
Fig 1 shows a hoist part (1 – a chain link) with an indicator part (2) circumferentially extending as a narrow raised rim along its inner circumference. This indicator part is shown as a closed ring (6) with supports (7) at regular intervals (Fig 1c, an enlargement from Fig 1a). The indicator part 2 co-operates with an indicator part 3 that is provided on the second part of the hoist device (4 – the upper chain link). This is a narrow raised rim along the inner circumference of 4. Thus the hoisting force applied to hoist part 1 is transmitted via these indicator part to the hoist part 4.
Fig 4 – Side view of an indicator part provided with a crumple zone
If the hoisting force applied is greater than or equal to a set design value on which both hoist parts operate, the surface pressure between the two indicator parts ensures plastic deformation of indicator part 2. Figs 1c and 1d show details of indicator part 2, which can be placed into part 1 as a separately insertable part clamped in by, for example, pressing.
The circumferential ring 6 is made of material that does not wear under the influence of the circumferential cam or rim (3) on part 4. In this example, deformation parts (7) are provide on the ring 6, designed so that if the force transmitted from the ring is greater than a set value, the material in the deformation parts will deform plastically. This deformation will be immediately visible for visual detection of overload. The deformation section 7 may also be a full ring, but separate parts with gaps make visual inspection simpler. The patent presents a wide range of embodiments of the invention, with diagrams, to show their operation in detail. Only examples can be shown here.
Hoist eye applications
Fig 2, here, shows an embodiment in which the first indicator part (32) is an integral soft part of the hoist part 31 (link ring), and the second indicator part (33) is a hardened part of hoist part 34. That is, is this case it is not formed as a protruding part or rim.
Fig 3 shows an embodiment in section of the invention’s breaking element, based on tensile strain, relating to a single embodiment for a hoisting eye (105) instead of a double one (on two legs) as described earlier in the patent application. The fracture elements (119) are formed as rings locally weakened by the circumferential removal of material with remaining lips loaded under shear load. As shown, the set of cover parts (111 and 112) is integrally formed at one end of each leg of the eye (110) or in a single link. The other end (113) has the ring (119), which is mounted as a removable and replaceable part by means of a nut (120), for example. The end 113 extends in the axial direction within the cover part 118. The main ring fracture element comprises concentric rings connected by breaking parts. If these breaking parts fracture the diameter of the fracture element will decrease. Following this the end(s) 113 can move axially within the cover or sleeve part 118. Thus a part of the end 113, originally inside the cover 118, becomes visible.
The cover parts 111 and 118 can mutually form a space between them in which the edge of the ring is accommodated so that the axial movement of end 113 is prevented until the fracture parts break, so providing the required visual indication. As shown in Fig 4 the embodiment differs from Fig 1 in that the first indicator part 2 is now embedded in the hoist part 1 as a cam or rim, which extends along the inner circumference of hoist part 1.
Here the indicator part is a crumple zone between the hoist part 1 and the indicator part 3 of hoist part 2. The rim 2, which is at an angle with respect to the main indicator part provides a relatively small contact surface, this giving a high, point-concentrated load, which is clearly defined.
In the final embodiment disclosed in the patent application (not illustrated) the indicator part again extends over the circumference of the hoist part 1, and is insertable into the interior of the chain link, but in this case being held by held between blocking parts.
Another difference is that the indicator part 3 has two sharp cutting edges. There is a small radius contact surface between these cutting edges to prevent damage during normal use. In use this contact surface will exert a pressure on lips running around indicator part 2, and the lips will start to bend when a hoisting force is exerted. If the load force passes the design limit the cutting edges will come into contact with the surface of the lips, eventually cutting through their surface and then the indicator part will deform very rapidly. The first deformation will probably be elastic.
In this last embodiment the main advantage is that the indicator part will have a very sharp response, with a visual indication as soon as the load goes above the design level. The cutting edges are first protected by the contact surface and will only contact the indicator surface when the load passes a certain level. The surface properties, including local hardening, ensure that the cutting edges cut through this surface when the load is above the set level. The lips are engineered to bend completely once the cut is made through the surface layer, so giving a visual indication of overload.
About the patent
This article is an edited version of WIPO international patent publication no. WO/2008007961, published on January 17 2008, and based on Netherlands patent 2000139 of July 11 2006. The WIPO application was filed on July 11 2007. The inventor, Peter Paul Alexander Berk, is a member of Partners voor Strategie en Management Zwolle BV, the applicant.
Disclaimer
This edited version of the patent and may omit legally or technically important text. To see the full patent go to www.hoistmagazine.com/patents
