GB2099161A

GB2099161A - Load measurement

Info

Publication number: GB2099161A
Application number: GB8208105A
Authority: GB
Original assignee: OHRWELL GORAN; OHRWELL HAKAN
Current assignee: OHRWELL GORAN; OHRWELL HAKAN
Priority date: 1981-03-20
Filing date: 1982-03-19
Publication date: 1982-12-01
Also published as: SE8101790L; DE3210047C2; DE3210047A1

Abstract

A transmitter element, for use in connection with a lifting device to generate an electric signal corresponding to the load being handled comprises a tensile link (1) to which strain gauges are attached. At least one safety link (3) is disposed in parallel with the tensile link (preferably two safety links, one on each side of the tensile link) and is designed to absorb greater loads than the tensile link. In normal operation the tensile link will absorb the entire load acting on the transmitter element. However, the safety link will take over at least a portion of the load acting on the tensile link if the latter is ruptured or abnormally elongated. The transmitter element can be used in a lifting device with a stationary pulley wheel to support one end of the pulley axle, whereas the opposite end is pivotally supported in the frame of the lifting device. <IMAGE>

Description

SPECIFICATION Transmitter element for indicating a load and a lifting device comprising such element The present invention relates to a transmitter element which is to be used in a lifting device in order to generate an electrical signal corresponding to the load handled by the lifting device. Such a transmitter element is generally provided with strain gauges and is subjected to a tensile strain corresponding to the load.

Transmitter elements of the kind mentioned above are previously known and used in connection with different types of lifting devices such as telfers, cranes travelling cranes and similar. Such arrangements are known for example from SE-A 7310533-0 and 7502923-1, FR-A 2234557 and US-A 3827514.

There are also transmitting elements intended for weighing purposes or as protection against overload comprising a link-shaped element which is to be used as a splice element in a wire or cable or to support a pulley wheel which is subjected to tensile stresses corresponding to the actual load. These transmitter links must be designed to withstand tensile stresses with a good margin relative to the maximum permitted load to be handled by the lifting device. It is common to operate with safety factors in the region of three or four. This means of course that the deformation of the material arising through tensile stresses in the links will be insignificant and utterly uncertain, specifically when the actual load handled by the lifting device is relatively light and in the lower range of the capacity of said device.If the tensile link is to be used as an overload protective device the situation referred to above will of course not have any greater influence but if the link is intended to form an integral part of a weighing system it will naturally mean a considerable drawback.

The load detecting elements described in the above mentioned Swedish specifications are in themselves simple strain gauges which are easy to integrate in a lifting device where wires or cables are used for the lifting operation. Such a cable load gauge is also described in US-A 2795136, comprising two cable bearing members in the shape of rollers or pads at opposite ends of the device and a central portion with a tightening device which deforms or bends the cable intermediate the pads or rollers, said tightening device including a link which is subjected to tensile stresses when the cable is subjected to load. These kinds of previously known transmitter elements cannot, however, be used in applications other than pure overload protective equipment and/or for weighing purposes where there is no need for high accuracy.

An object of the present invention is to provide an element with a structure which is both simple and cheap to manufacture and easy to install on site in travelling cranes and other types of lifting devices and which simultaneously permits weighing with high accuracy.

According to the invention there is provided a transmitter element for use in a lifting device in order to generate an electric signal corresponding to the load being handled, the transmitter element comprising a tensile link which is provided with a strain gauge and which is subjected to tensile stress corresponding to the load handled by the lifting device, and at least one safety element arranged in parallel with the tensile link and being capable of withstanding a higher load than the tensile link, and in which the tensile link in normal operation is arranged to absorb the entire load acting on the transmitter element, whereas the safety element in case of rupture or abnormal elongations of the tensile link is arranged to take over at least a portion of the load acting on the tensile link.

The element according to the invention can either form an integral part of an electronic weighing system or serve as a pure overload protective device, breaking the energy supply to the lifting device when the maximum permitted overload is exceeded.

Since there is no need to design the tensile link for loads which substantially exceed the loads to be measured or handled, the deformation in the material will be comparatively high and therefore will be detected with high accuracy by means of the strain gauges.

A further object of the invention is to provide for the simple installation of an electronic transmitter element in a lifting device comprising at least one pulley wheel which is supported by a frame portion in which the pulley shaft is pivotally mounted, in order to generate an electric signal corresponding to the load acting on the pulley shaft. Preferably, therefore one end of the pulley shaft is pivotally supported in the frame portion, whereas its opposite end is pivotally supported in the electronic transmitter element, said element comprising a tensile link which is provided with strain gauges, one end of said link being pivotally connected to the frame portion, whereas the opposite end is supporting the pul leyshaft.

The invention may be carried into practice in various ways and two embodiments will now be described by way of example with reference to the accompanying drawings in which like reference characters refer to like parts in the several Figures of the drawings, and in which: Figure 1 is a cross sectional view through a transmitter element according to the invention, Figure 2 is a side view of the transmitter element according to Figure 1, Figure 3 is a perspective view from below of a travelling crane and an enlarged detail in the crane arrangement showing the spliced portion with a transmitting device according to the invention, Figure 4 is a cross sectional view of another embodiment of the invention in which the transmitter element supports one end of a pulley shaft and Figure 5 is a perspective view from below of a telfer line with a pulley shaft one end of which is supported by an arrangement according to Figure 4 as shown on the enlarged detail on the Figure.

The transmitter element according to Figures 1 and 2 comprises a tensile link to which strain gauges 2 have been attached to generate an electric signal corresponding to the tensile stress to which the link is subjected. Two security links 3 and 4 are located on each side of the tensile link 1 and all links are provided with bores at their ends, said bores being disposed right opposite one another in order to form two openings 5 and 6 in the transmitting element.

The tensile link 1 is pivotably mounted in the upper opening 5 in the two safety links by means of a sleeve 7 passing through the bores. The two safety links 3 and 4 are rigidly connected with one another by means of a plate 12 which is welded to the peripheries of the links.

Spacing elements 8 and 9 are located on each side of the tensile link in order to maintain a correct position between the safety links 3 and 4. Asupport ring 10 is arranged in the lower opening 6 ofthe bore in the tensile link and this support ring is so shaped that the inner effective length d, of the tensile link is less than the corresponding inner effective lengths d2 of the safety links. The support ring 10 is provided with seats for elastic packings 11 facing the two safety links 3 and 4to prevent dirt and moisture from penetrating into the strain gauges 2 which are glued to the tensile link 1.

Since the inner effective length d, of the tensile element is less than the corresponding inner effective lengths d2 of the safety links the support ring 10 will normally absorb the entire tensile load on the transmitter element 1. Due to the magnitude of the load the support ring 10 will consequently be displaced in the longitudinal direction between the two safety links and only if the tensile link is ruptured or obtains an inner effective length d, which is equal to the effective lengths d2 of the safety links will the latter take over at least a portion of the load acting on the tensile link.

The effective length d, of the tensile link and those of the safety links should be so adapted to one another that the maximum permitted elongation of the tensile link is well below the yield point of the material. This means that after an instantaneous overload of the transmitter element to an amount which exceeds the tensile strength of the tensile link several times the latter can resume its normal function without having been destroyed and replaced. In this respect the transmitter element according to the invention differs from hitherto known structures where an occasional overload above the yield point of the material will entail the rejection ofthe entire transmitter element.

In the embodiment shown in Figure 1 the sleeve 7 is shaped with a gradually increasing inner diameter from the centre and towards the end of the sleeve.

The contact surface against the shaft passing through the sleeve will consequently be spot-shaped which means that the shaft can be somewhat tilted relative to the transmitter element without any bending moments being introduced in the tensile link.

It may also be suitable to fixedly attach the tensile link 1 relative to the safety links 3 and 4 by means of a pin 28 passing through the link in order to prevent the tensile link from being rotated around the sleeve 7 and get stuck in a position where the safety link at normal load will shunt away part ofthe load to be measured bythetensile link.

One of the safety links 3 is provided with a bore for bushing 13 for an electric cable 14 which is connected to the strain gauges 2. The cable 14 is in a conventional manner connected to an electronic circuit, such as that shown in the prior art referred to in the opening part of the description, in which the signal from the transmitter is converted to a signal which either controls a relay breaking the current supply to the lifting device or to a read-out display showing the actual load handled by the device. The electronic circuit also comprises potentiometers for balancing the weighing equipment and for calibrating the overload protective device and/or the weighing device.

Figure 3 shows a specific use of the transmitter element according to the invention in a traverse crane. One end of the element 15 is consequently connected to the fixed portion of a cable 16 which is connected to the frame portion of a transverse crane, in this case a lateral beam 17, whereas the other end of the element 15 is connected to that part of the cable which is running through the hook block 18.

Since the transmitter is connected to the electronic circuit through the cable 14 the element must not take part in the lifting movement. As appears from the enlarged detail of Figure 3 the cable 16 is connected to the element 15 by means of shackles 19 which are introduced into loops 20 on the cable. In this embodiment the tensile strength in the transmitter element will consequently correspond to one fourth of the load handled by the lifting device.

Figure 4 shows another embodiment of a lifting device in which the transmitter element according to the invention is used to support one end 25 of an axle 22 supporting a pulley wheel 21. The opposite end ofthe pulley axle is pivotally supported in a frame portion 23 and that end 25 which is supported by the pulley wheel is passed with clearance through a hole in the frame portion 24 and is then received in the lower opening 6 of a transmitter element 15 according to the invention. The transmitter element is in its turn pivotally connected by means of a shaft 27 through the upper opening 5 to a supporting structure 26 which is rigidly connected to the frame structure 24 by means of welding or bolts. Thus, half ofthe load acting on the pulley wheel 21 will be detected by the transmitter element 15 and it will thus be possible to measure the load handled by the crane.

Figure 5 shows a specific appliance of the weighing structure according to Figure 4 in a telfer line where the arrangement of the transmitter element 15 to the frame portion 24 supporting the pulley wheel 21 can be seen in the enlarged detail in the Figure. This specific embodiment has certain advantages over the arrangement shown in Figure 3 since it will not harmfully affect the lifting height of the hook block which is the case in the embodiment shown on Figure 3. Moreover, it is simpler to apply an overload and/or measuring device according to Figure 4to a crane structure due to the fact that the only detail which has to be replaced in the crane is the original pulley shaft, and this could be done without dismantling the wheel from the frame. The supporting structure with the transmitter element can then easily be welded to the outside of the frame. In the embodiment according to Figure 3 the cable must be spliced and the cable ends provided with loops to be connected with the intermediate transmitter element. This is a more complicated and time consuming operation.

Claims

1. A transmitter element for use in a lifting device in order to generate an electric signal corresponding to the load being handled, the transmitter element comprising a tensile link which is provided with a strain gauge and which is subjected to tensile stress corresponding to the load handled by the lifting device, and at least one safety element arranged in parallel with the tensile link and being capable of withstanding a higher load then the tensile link, and in which the tensile link in normal operation is arranged to absorb the entire load acting on the transmitter element, whereas the safety element in case of rupture or abnormal elongations of the tensile link is arranged to take over at least a portion of the load acting on the tensile link.

2. A transmitter element as claimed in Claim 1, in which the safety element comprises two safety links which are arranged in parallel with the tensile link, on each side of the tensile link, all of the links being provided with a bore at each end, the bores being so located that the inner effective length of the tensile link is less than the corresponding effective inner lengths of the safety links.

3. A transmitter element as claimed in Claim 2, in which both safety links are rigidly connected with one another, and that one end of the tensile link is rotatably journalled in the two safety links by means of a sleeve being disposed in one bore of the element.

4. A transmitter element as claimed in any preceding claim in which the inner effective lengths of the tensile link and the safety links are so arranged that the maximum permitted elongation of the tensile link is below the yield point of the material.

5. A transmitter element as claimed in any of claims 2 to 4 in which the tensile link is provided with an intermediate portion between the two bores to which portion strain gauges are fixedly attached.

6. A transmitter element constructed and arranged substantially as herein specifically described with reference to and as shown in Figures 1 and 2 of the accompanying drawings.

7. A lifting device comprising at least one pulley wheel supported by a frame portion in which the pulley shaft is pivotally supported, an electronic transmitter element being connected to the pulley shaft in order to generate a signal corresponding to the load acting on said shaft, and in which one end of the pulley shaft is pivotally supported in the frame portion, whereas its opposite end is pivotally supported in the electronic transmitter element, the element comprising a tensile link which is provided with strain gauges, one end of the link being pivotally connected to the frame portion whereas the opposite end supports the pulley shaft.

8. A lifting device as claimed in Claim 7 in which two safety links are disposed on each side ofthe tensile link, each of the links being provided with a bore in each end, the bores being so located that the inner effective length of the tensile link is less than the inner effective lengths of the safety links.

9. A lifting device constructed and arranged substantially as herein specifically described with reference to and as shown in Figures 1, 2 and 3 or Figures 4 and 5 of the accompanying drawings.